gvpr - Online in the Cloud

PROGRAM:

NAME

gvpr - graph pattern scanning and processing language

SYNOPSIS

gvpr [-icnqV?] [ -o outfile ] [ -a args ] [ 'prog' | -f progfile ] [ files ]

DESCRIPTION

gvpr (previously known as gpr) is a graph stream editor inspired by awk. It copies input
graphs to its output, possibly transforming their structure and attributes, creating new
graphs, or printing arbitrary information. The graph model is that provided by
libcgraph(3). In particular, gvpr reads and writes graphs using the dot language.

Basically, gvpr traverses each input graph, denoted by $G, visiting each node and edge,
matching it with the predicate‐action rules supplied in the input program. The rules are
evaluated in order. For each predicate evaluating to true, the corresponding action is
performed. During the traversal, the current node or edge being visited is denoted by $.

For each input graph, there is a target subgraph, denoted by $T, initially empty and used
to accumulate chosen entities, and an output graph, $O, used for final processing and then
written to output. By default, the output graph is the target graph. The output graph
can be set in the program or, in a limited sense, on the command line.

OPTIONS

The following options are supported:

-a args
The string args is split into whitespace‐separated tokens, with the individual
tokens available as strings in the gvpr program as ARGV[0],...,ARGV[ARGC-1].
Whitespace characters within single or double quoted substrings, or preceded by a
backslash, are ignored as separators. In general, a backslash character turns off
any special meaning of the following character. Note that the tokens derived from
multiple -a flags are concatenated.

-c Use the source graph as the output graph.

-i Derive the node‐induced subgraph extension of the output graph in the context of
its root graph.

-o outfile
Causes the output stream to be written to the specified file; by default, output is
written to stdout.

-f progfile
Use the contents of the specified file as the program to execute on the input. If
progfile contains a slash character, the name is taken as the pathname of the file.
Otherwise, gvpr will use the directories specified in the environment variable
GVPRPATH to look for the file. If -f is not given, gvpr will use the first non‐
option argument as the program.

-q Turns off warning messages.

-n Turns off graph read-ahead. By default, the variable $NG is set to the next graph
to be processed. This requires a read of the next graph before processing the
current graph, which may block if the next graph is only generated in response to
some action pertaining to the processing of the current graph.

-V Causes the program to print version information and exit.

-? Causes the program to print usage information and exit.

OPERANDS

The following operand is supported:

files Names of files containing 1 or more graphs in the dot language. If no -f option
is given, the first name is removed from the list and used as the input program.
If the list of files is empty, stdin will be used.

PROGRAMS

A gvpr program consists of a list of predicate‐action clauses, having one of the forms:

BEGIN { action }

BEG_G { action }

N [ predicate ] { action }

E [ predicate ] { action }

END_G { action }

END { action }

A program can contain at most one of each of the BEGIN, END_G and END clauses. There can
be any number of BEG_G, N and E statements, the first applied to graphs, the second to
nodes, the third to edges. These are separated into blocks, a block consisting of an
optional BEG_G statement and all N and E statements up to the next BEG_G statement, if
any. The top‐level semantics of a gvpr program are:

Evaluate the BEGIN clause, if any.
For each input graph G {
For each block {
Set G as the current graph and current object.
Evaluate the BEG_G clause, if any.
For each node and edge in G {
Set the node or edge as the current object.
Evaluate the N or E clauses, as appropriate.
}
}
Set G as the current object.
Evaluate the END_G clause, if any.
}
Evaluate the END clause, if any.

The actions of the BEGIN, BEG_G, END_G and END clauses are performed when the clauses are
evaluated. For N or E clauses, either the predicate or action may be omitted. If there
is no predicate with an action, the action is performed on every node or edge, as
appropriate. If there is no action and the predicate evaluates to true, the associated
node or edge is added to the target graph.

The blocks are evaluated in the order in which they occur. Within a block, the N clauses
(E clauses, respectively) are evaluated in the order in which the occur. Note, though,
that within a block, N or E clauses may be interlaced, depending on the traversal order.

Predicates and actions are sequences of statements in the C dialect supported by the
expr(3) library. The only difference between predicates and actions is that the former
must have a type that may interpreted as either true or false. Here the usual C
convention is followed, in which a non‐zero value is considered true. This would include
non‐empty strings and non‐empty references to nodes, edges, etc. However, if a string can
be converted to an integer, this value is used.

In addition to the usual C base types (void, int, char, float, long, unsigned and double),
gvpr provides string as a synonym for char*, and the graph‐based types node_t, edge_t,
graph_t and obj_t. The obj_t type can be viewed as a supertype of the other 3 concrete
types; the correct base type is maintained dynamically. Besides these base types, the
only other supported type expressions are (associative) arrays.

Constants follow C syntax, but strings may be quoted with either "..." or '...'. gvpr
accepts C++ comments as well as cpp‐type comments. For the latter, if a line begins with
a '#' character, the rest of the line is ignored.

A statement can be a declaration of a function, a variable or an array, or an executable
statement. For declarations, there is a single scope. Array declarations have the form:

type array [ type0 ]

where type0 is optional. If it is supplied, the parser will enforce that all array
subscripts have the specified type. If it is not supplied, objects of all types can be
used as subscripts. As in C, variables and arrays must be declared. In particular, an
undeclared variable will be interpreted as the name of an attribute of a node, edge or
graph, depending on the context.

Executable statements can be one of the following:

{ [ statement ... ] }
expression // commonly var = expression
if( expression ) statement [ else statement ]
for( expression ; expression ; expression ) statement
for( array [ var ]) statement
forr( array [ var ]) statement
while( expression ) statement
switch( expression ) case statements
break [ expression ]
continue [ expression ]
return [ expression ]
Items in brackets are optional.

In the second form of the for statement and the forr statement, the variable var is set to
each value used as an index in the specified array and then the associated statement is
evaluated. For numeric and string indices, the indices are returned in increasing
(decreasing) numeric or lexicographic order for for (forr, respectively). This can be used
for sorting.

Function definitions can only appear in the BEGIN clause.

Expressions include the usual C expressions. String comparisons using == and != treat the
right hand operand as a pattern for the purpose of regular expression matching. Patterns
use ksh(1) file match pattern syntax. (For simple string equality, use the strcmp
function.

gvpr will attempt to use an expression as a string or numeric value as appropriate. Both
C-like casts and function templates will cause conversions to be performed, if possible.

Expressions of graphical type (i.e., graph_t, node_t, edge_t, obj_t) may be followed by a
field reference in the form of .name. The resulting value is the value of the attribute
named name of the given object. In addition, in certain contexts an undeclared,
unmodified identifier is taken to be an attribute name. Specifically, such identifiers
denote attributes of the current node or edge, respectively, in N and E clauses, and the
current graph in BEG_G and END_G clauses.

As usual in the libcgraph(3) model, attributes are string‐valued. In addition, gvpr
supports certain pseudo‐attributes of graph objects, not necessarily string‐valued. These
reflect intrinsic properties of the graph objects and cannot be set by the user.

head : node_t
the head of an edge.

tail : node_t
the tail of an edge.

name : string
the name of an edge, node or graph. The name of an edge has the form "<tail‐
name><edge‐op><head‐name>[<key>]", where <edge‐op> is "->" or "--" depending on
whether the graph is directed or not. The bracket part [<key>] only appears if the
edge has a non‐trivial key.

indegree : int
the indegree of a node.

outdegree : int
the outdegree of a node.

degree : int
the degree of a node.

root : graph_t
the root graph of an object. The root of a root graph is itself.

parent : graph_t
the parent graph of a subgraph. The parent of a root graph is NULL

n_edges : int
the number of edges in the graph

n_nodes : int
the number of nodes in the graph

directed : int
true (non‐zero) if the graph is directed

strict : int
true (non‐zero) if the graph is strict

BUILT‐IN FUNCTIONS

The following functions are built into gvpr. Those functions returning references to graph
objects return NULL in case of failure.

Graphs and subgraph
graph(s : string, t : string) : graph_t
creates a graph whose name is s and whose type is specified by the string t.
Ignoring case, the characters U, D, S, N have the interpretation undirected,
directed, strict, and non‐strict, respectively. If t is empty, a directed, non‐
strict graph is generated.

subg(g : graph_t, s : string) : graph_t
creates a subgraph in graph g with name s. If the subgraph already exists, it is
returned.

isSubg(g : graph_t, s : string) : graph_t
returns the subgraph in graph g with name s, if it exists, or NULL otherwise.

fstsubg(g : graph_t) : graph_t
returns the first subgraph in graph g, or NULL if none exists.

nxtsubg(sg : graph_t) : graph_t
returns the next subgraph after sg, or NULL.

isDirect(g : graph_t) : int
returns true if and only if g is directed.

isStrict(g : graph_t) : int
returns true if and only if g is strict.

nNodes(g : graph_t) : int
returns the number of nodes in g.

nEdges(g : graph_t) : int
returns the number of edges in g.

Nodes
node(sg : graph_t, s : string) : node_t
creates a node in graph g of name s. If such a node already exists, it is returned.

subnode(sg : graph_t, n : node_t) : node_t
inserts the node n into the subgraph g. Returns the node.

fstnode(g : graph_t) : node_t
returns the first node in graph g, or NULL if none exists.

nxtnode(n : node_t) : node_t
returns the next node after n in the root graph, or NULL.

nxtnode_sg(sg : graph_t, n : node_t) : node_t
returns the next node after n in sg, or NULL.

isNode(sg : graph_t, s : string) : node_t
looks for a node in (sub)graph sg of name s. If such a node exists, it is returned.
Otherwise, NULL is returned.

isSubnode(sg : graph_t, n : node_t) : int
returns non-zero if node n is in (sub)graph sg, or zero otherwise.

indegreeOf(sg : graph_t, n : node_t) : int
returns the indegree of node n in (sub)graph sg.

outdegreeOf(sg : graph_t, n : node_t) : int
returns the outdegree of node n in (sub)graph sg.

degreeOf(sg : graph_t, n : node_t) : int
returns the degree of node n in (sub)graph sg.

Edges
edge(t : node_t, h : node_t, s : string) : edge_t
creates an edge with tail node t, head node h and name s in the root graph. If the
graph is undirected, the distinction between head and tail nodes is unimportant.
If such an edge already exists, it is returned.

edge_sg(sg : graph_t, t : node_t, h : node_t, s : string) : edge_t
creates an edge with tail node t, head node h and name s in (sub)graph sg (and all
parent graphs). If the graph is undirected, the distinction between head and tail
nodes is unimportant. If such an edge already exists, it is returned.

subedge(g : graph_t, e : edge_t) : edge_t
inserts the edge e into the subgraph g. Returns the edge.

isEdge(t : node_t, h : node_t, s : string) : edge_t
looks for an edge with tail node t, head node h and name s. If the graph is
undirected, the distinction between head and tail nodes is unimportant. If such an
edge exists, it is returned. Otherwise, NULL is returned.

isEdge_sg(sg : graph_t, t : node_t, h : node_t, s : string) : edge_t
looks for an edge with tail node t, head node h and name s in (sub)graph sg. If the
graph is undirected, the distinction between head and tail nodes is unimportant.
If such an edge exists, it is returned. Otherwise, NULL is returned.

isSubedge(g : graph_t, e : edge_t) : int
returns non-zero if edge e is in (sub)graph sg, or zero otherwise.

fstout(n : node_t) : edge_t
returns the first outedge of node n in the root graph.

fstout_sg(sg : graph_t, n : node_t) : edge_t
returns the first outedge of node n in (sub)graph sg.

nxtout(e : edge_t) : edge_t
returns the next outedge after e in the root graph.

nxtout_sg(sg : graph_t, e : edge_t) : edge_t
returns the next outedge after e in graph sg.

fstin(n : node_t) : edge_t
returns the first inedge of node n in the root graph.

fstin_sg(sg : graph_t, n : node_t) : edge_t
returns the first inedge of node n in graph sg.

nxtin(e : edge_t) : edge_t
returns the next inedge after e in the root graph.

nxtin_sg(sg : graph_t, e : edge_t) : edge_t
returns the next inedge after e in graph sg.

fstedge(n : node_t) : edge_t
returns the first edge of node n in the root graph.

fstedge_sg(sg : graph_t, n : node_t) : edge_t
returns the first edge of node n in graph sg.

nxtedge(e : edge_t, node_t) : edge_t
returns the next edge after e in the root graph.

nxtedge_sg(sg : graph_t, e : edge_t, node_t) : edge_t
returns the next edge after e in the graph sg.

opp(e : edge_t, node_t) : node_t
returns the node on the edge e not equal to n. Returns NULL if n is not a node of
e. This can be useful when using fstedge and nxtedge to enumerate the neighbors of
n.

Graph I/O
write(g : graph_t) : void
prints g in dot format onto the output stream.

writeG(g : graph_t, fname : string) : void
prints g in dot format into the file fname.

fwriteG(g : graph_t, fd : int) : void
prints g in dot format onto the open stream denoted by the integer fd.

readG(fname : string) : graph_t
returns a graph read from the file fname. The graph should be in dot format. If no
graph can be read, NULL is returned.

freadG(fd : int) : graph_t
returns the next graph read from the open stream fd. Returns NULL at end of file.

Graph miscellany
delete(g : graph_t, x : obj_t) : void
deletes object x from graph g. If g is NULL, the function uses the root graph of
x. If x is a graph or subgraph, it is closed unless x is locked.

isIn(g : graph_t, x : obj_t) : int
returns true if x is in subgraph g.

cloneG(g : graph_t, s : string) : graph_t
creates a clone of graph g with name of s. If s is "", the created graph has the
same name as g.

clone(g : graph_t, x : obj_t) : obj_t
creates a clone of object x in graph g. In particular, the new object has the same
name/value attributes and structure as the original object. If an object with the
same key as x already exists, its attributes are overlaid by those of x and the
object is returned. If an edge is cloned, both endpoints are implicitly cloned.
If a graph is cloned, all nodes, edges and subgraphs are implicitly cloned. If x
is a graph, g may be NULL, in which case the cloned object will be a new root
graph. In this case, the call is equivalent to cloneG(x,"").

copy(g : graph_t, x : obj_t) : obj_t
creates a copy of object x in graph g, where the new object has the same name/value
attributes as the original object. If an object with the same key as x already
exists, its attributes are overlaid by those of x and the object is returned. Note
that this is a shallow copy. If x is a graph, none of its nodes, edges or subgraphs
are copied into the new graph. If x is an edge, the endpoints are created if
necessary, but they are not cloned. If x is a graph, g may be NULL, in which case
the cloned object will be a new root graph.

copyA(src : obj_t, tgt : obj_t) : int
copies the attributes of object src to object tgt, overwriting any attribute values
tgt may initially have.

induce(g : graph_t) : void
extends g to its node‐induced subgraph extension in its root graph.

hasAttr(src : obj_t, name : string) : int
returns non-zero if object src has an attribute whose name is name. It returns 0
otherwise.

isAttr(g : graph_t, kind : string, name : string) : int
returns non-zero if an attribute name has been defined in g for objects of the
given kind. For nodes, edges, and graphs, kind should be "N", "E", and "G",
respectively. It returns 0 otherwise.

aget(src : obj_t, name : string) : string
returns the value of attribute name in object src. This is useful for those cases
when name conflicts with one of the keywords such as "head" or "root". If the
attribute has not been declared in the graph, the function will initialize it with
a default value of "". To avoid this, one should use the hasAttr or isAttr function
to check that the attribute exists.

aset(src : obj_t, name : string, value : string) : int
sets the value of attribute name in object src to value. Returns 0 on success,
non‐zero on failure. See aget above.

getDflt(g : graph_t, kind : string, name : string) : string
returns the default value of attribute name in objects in g of the given kind. For
nodes, edges, and graphs, kind should be "N", "E", and "G", respectively. If the
attribute has not been declared in the graph, the function will initialize it with
a default value of "". To avoid this, one should use the isAttr function to check
that the attribute exists.

setDflt(g : graph_t, kind : string, name : string, value : string) : int
sets the default value of attribute name to value in objects in g of the given
kind. For nodes, edges, and graphs, kind should be "N", "E", and "G", respectively.
Returns 0 on success, non‐zero on failure. See getDflt above.

fstAttr(g : graph_t, kind : string) : string
returns the name of the first attribute of objects in g of the given kind. For
nodes, edges, and graphs, kind should be "N", "E", and "G", respectively. If there
are no attributes, the string "" is returned.

nxtAttr(g : graph_t, kind : string, name : string) : string
returns the name of the next attribute of objects in g of the given kind after the
attribute name. The argument name must be the name of an existing attribute; it
will typically be the return value of an previous call to fstAttr or nxtAttr. For
nodes, edges, and graphs, kind should be "N", "E", and "G", respectively. If there
are no attributes left, the string "" is returned.

compOf(g : graph_t, n : node_t) : graph_t
returns the connected component of the graph g containing node n, as a subgraph of
g. The subgraph only contains the nodes. One can use induce to add the edges. The
function fails and returns NULL if n is not in g. Connectivity is based on the
underlying undirected graph of g.

kindOf(obj : obj_t) : string
returns an indication of the type of obj. For nodes, edges, and graphs, it returns
"N", "E", and "G", respectively.

lock(g : graph_t, v : int) : int
implements graph locking on root graphs. If the integer v is positive, the graph is
set so that future calls to delete have no immediate effect. If v is zero, the
graph is unlocked. If there has been a call to delete the graph while it was
locked, the graph is closed. If v is negative, nothing is done. In all cases, the
previous lock value is returned.

Strings
sprintf(fmt : string, ...) : string
returns the string resulting from formatting the values of the expressions
occurring after fmt according to the printf(3) format fmt

gsub(str : string, pat : string) : string

gsub(str : string, pat : string, repl : string) : string
returns str with all substrings matching pat deleted or replaced by repl,
respectively.

sub(str : string, pat : string) : string

sub(str : string, pat : string, repl : string) : string
returns str with the leftmost substring matching pat deleted or replaced by repl,
respectively. The characters '^' and '$' may be used at the beginning and end,
respectively, of pat to anchor the pattern to the beginning or end of str.

substr(str : string, idx : int) : string

substr(str : string, idx : int, len : int) : string
returns the substring of str starting at position idx to the end of the string or
of length len, respectively. Indexing starts at 0. If idx is negative or idx is
greater than the length of str, a fatal error occurs. Similarly, in the second
case, if len is negative or idx + len is greater than the length of str, a fatal
error occurs.

strcmp(s1 : string, s2 : string) : int
provides the standard C function strcmp(3).

length(s : string) : int
returns the length of string s.

index(s : string, t : string) : int

rindex(s : string, t : string) : int
returns the index of the character in string s where the leftmost (rightmost) copy
of string t can be found, or -1 if t is not a substring of s.

match(s : string, p : string) : int
returns the index of the character in string s where the leftmost match of pattern
p can be found, or -1 if no substring of s matches p.

toupper(s : string) : string
returns a version of s with the alphabetic characters converted to upper-case.

tolower(s : string) : string
returns a version of s with the alphabetic characters converted to lower-case.

canon(s : string) : string
returns a version of s appropriate to be used as an identifier in a dot file.

html(g : graph_t, s : string) : string
returns a ``magic'' version of s as an HTML string. This will typically be used to
attach an HTML-like label to a graph object. Note that the returned string lives in
g. In particular, it will be freed when g is closed, and to act as an HTML string,
it has to be used with an object of g. In addition, note that the angle bracket
quotes should not be part of s. These will be added if g is written in concrete DOT
format.

ishtml(s : string) : int
returns non-zero if and only if s is an HTML string.

xOf(s : string) : string
returns the string "x" if s has the form "x,y", where both x and y are numeric.

yOf(s : string) : string
returns the string "y" if s has the form "x,y", where both x and y are numeric.

llOf(s : string) : string
returns the string "llx,lly" if s has the form "llx,lly,urx,ury", where all of llx,
lly, urx, and ury are numeric.

urOf(s)
urOf(s : string) : string returns the string "urx,ury" if s has the form
"llx,lly,urx,ury", where all of llx, lly, urx, and ury are numeric.

sscanf(s : string, fmt : string, ...) : int
scans the string s, extracting values according to the sscanf(3) format fmt. The
values are stored in the addresses following fmt, addresses having the form &v,
where v is some declared variable of the correct type. Returns the number of items
successfully scanned.

split(s : string, arr : array, seps : string) : int

split(s : string, arr : array) : int

tokens(s : string, arr : array, seps : string) : int

tokens(s : string, arr : array) : int
The split function breaks the string s into fields, while the tokens function
breaks the string into tokens. A field consists of all non-separator characters
between two separator characters or the beginning or end of the string. Thus, a
field may be the empty string. A token is a maximal, non-empty substring not
containing a separator character. The separator characters are those given in the
seps argument. If seps is not provided, the default value is " \t\n". The
functions return the number of fields or tokens.

The fields and tokens are stored in the argument array. The array must be string-
valued and, if an index type is specified, it must be int. The entries are indexed
by consecutive integers, starting at 0. Any values already stored in the array will
be either overwritten, or still be present after the function returns.

I/O
print(...) : void
print( expr, ... ) prints a string representation of each argument in turn onto
stdout, followed by a newline.

printf(fmt : string, ...) : int

printf(fd : int, fmt : string, ...) : int
prints the string resulting from formatting the values of the expressions following
fmt according to the printf(3) format fmt. Returns 0 on success. By default, it
prints on stdout. If the optional integer fd is given, output is written on the
open stream associated with fd.

scanf(fmt : string, ...) : int

scanf(fd : int, fmt : string, ...) : int
scans in values from an input stream according to the scanf(3) format fmt. The
values are stored in the addresses following fmt, addresses having the form &v,
where v is some declared variable of the correct type. By default, it reads from
stdin. If the optional integer fd is given, input is read from the open stream
associated with fd. Returns the number of items successfully scanned.

openF(s : string, t : string) : int
opens the file s as an I/O stream. The string argument t specifies how the file is
opened. The arguments are the same as for the C function fopen(3). It returns an
integer denoting the stream, or -1 on error.

As usual, streams 0, 1 and 2 are already open as stdin, stdout, and stderr,
respectively. Since gvpr may use stdin to read the input graphs, the user should
avoid using this stream.

closeF(fd : int) : int
closes the open stream denoted by the integer fd. Streams 0, 1 and 2 cannot be
closed. Returns 0 on success.

readL(fd : int) : string
returns the next line read from the input stream fd. It returns the empty string ""
on end of file. Note that the newline character is left in the returned string.

Math
exp(d : double) : double
returns e to the dth power.

log(d : double) : double
returns the natural log of d.

sqrt(d : double) : double
returns the square root of the double d.

pow(d : double, x : double) : double
returns d raised to the xth power.

cos(d : double) : double
returns the cosine of d.

sin(d : double) : double
returns the sine of d.

atan2(y : double, x : double) : double
returns the arctangent of y/x in the range -pi to pi.

MIN(y : double, x : double) : double
returns the minimum of y and x.

MAX(y : double, x : double) : double
returns the maximum of y and x.

Associative Arrays
# arr : int
returns the number of elements in the array arr.

idx in arr : int
returns 1 if a value has been set for index idx in the array arr. It returns 0
otherwise.

unset(v : array, idx) : int
removes the item indexed by idx. It returns 1 if the item existed, 0 otherwise.

unset(v : array) : void
re-initializes the array.

Miscellaneous
exit(v : int) : void
causes gvpr to exit with the exit code v.

system(cmd : string) : int
provides the standard C function system(3). It executes cmd in the user's shell
environment, and returns the exit status of the shell.

rand() : double
returns a pseudo‐random double between 0 and 1.

srand() : int

srand(v : int) : int
sets a seed for the random number generator. The optional argument gives the seed;
if it is omitted, the current time is used. The previous seed value is returned.
srand should be called before any calls to rand.

colorx(color : string, fmt : string) : string
translates a color from one format to another. The color argument should be a color
in one of the recognized string representations. The fmt value should be one of
"RGB", "RGBA", "HSV", or "HSVA". An empty string is returned on error.

BUILT‐IN VARIABLES

gvpr provides certain special, built‐in variables, whose values are set automatically by
gvpr depending on the context. Except as noted, the user cannot modify their values.

$ : obj_t
denotes the current object (node, edge, graph) depending on the context. It is not
available in BEGIN or END clauses.

$F : string
is the name of the current input file.

$G : graph_t
denotes the current graph being processed. It is not available in BEGIN or END
clauses.

$NG : graph_t
denotes the next graph to be processed. If $NG is NULL, the current graph $G is the
last graph. Note that if the input comes from stdin, the last graph cannot be
determined until the input pipe is closed. It is not available in BEGIN or END
clauses, or if the -n flag is used.

$O : graph_t
denotes the output graph. Before graph traversal, it is initialized to the target
graph. After traversal and any END_G actions, if it refers to a non‐empty graph,
that graph is printed onto the output stream. It is only valid in N, E and END_G
clauses. The output graph may be set by the user.

$T : graph_t
denotes the current target graph. It is a subgraph of $G and is available only in
N, E and END_G clauses.

$tgtname : string
denotes the name of the target graph. By default, it is set to "gvpr_result". If
used multiple times during the execution of gvpr, the name will be appended with an
integer. This variable may be set by the user.

$tvroot : node_t
indicates the starting node for a (directed or undirected) depth‐first or breadth‐
first traversal of the graph (cf. $tvtype below). The default value is NULL for
each input graph. After the traversal at the given root, if the value of $tvroot
has changed, a new traversal will begin with the new value of $tvroot. Also, set
$tvnext below.

$tvnext : node_t
indicates the next starting node for a (directed or undirected) depth‐first or
breadth‐first traversal of the graph (cf. $tvtype below). If a traversal finishes
and the $tvroot has not been reset but the $tvnext has been set but not used, this
node will be used as the next choice for $tvroot. The default value is NULL for
each input graph.

$tvedge : edge_t
For BFS and DFS traversals, this is set to the edge used to arrive at the current
node or edge. At the beginning of a traversal, or for other traversal types, the
value is NULL.

$tvtype : tvtype_t
indicates how gvpr traverses a graph. It can only take one of the constant values
with the previx "TV_" described below. TV_flat is the default.

In the underlying graph library cgraph(3), edges in undirected graphs are given an
arbitrary direction. This is used for traversals, such as TV_fwd, requiring
directed edges.

ARGC : int
denotes the number of arguments specified by the -a args command‐line argument.

ARGV : string array
denotes the array of arguments specified by the -a args command‐line argument. The
ith argument is given by ARGV[i].

BUILT‐IN CONSTANTS

There are several symbolic constants defined by gvpr.

NULL : obj_t
a null object reference, equivalent to 0.

TV_flat : tvtype_t
a simple, flat traversal, with graph objects visited in seemingly arbitrary order.

TV_ne : tvtype_t
a traversal which first visits all of the nodes, then all of the edges.

TV_en : tvtype_t
a traversal which first visits all of the edges, then all of the nodes.

TV_dfs : tvtype_t
TV_postdfs : tvtype_t
TV_prepostdfs : tvtype_t
a traversal of the graph using a depth‐first search on the underlying undirected
graph. To do the traversal, gvpr will check the value of $tvroot. If this has the
same value that it had previously (at the start, the previous value is initialized
to NULL.), gvpr will simply look for some unvisited node and traverse its connected
component. On the other hand, if $tvroot has changed, its connected component will
be toured, assuming it has not been previously visited or, if $tvroot is NULL, the
traversal will stop. Note that using TV_dfs and $tvroot, it is possible to create
an infinite loop.

By default, the traversal is done in pre-order. That is, a node is visited before
all of its unvisited edges. For TV_postdfs, all of a node's unvisited edges are
visited before the node. For TV_prepostdfs, a node is visited twice, before and
after all of its unvisited edges.

TV_fwd : tvtype_t
TV_postfwd : tvtype_t
TV_prepostfwd : tvtype_t
A traversal of the graph using a depth‐first search on the graph following only
forward arcs. The choice of roots for the traversal is the same as described for
TV_dfs above. The different order of visitation specified by TV_fwd, TV_postfwd
and TV_prepostfwd are the same as those specified by the analogous traversals
TV_dfs, TV_postdfs and TV_prepostdfs.

TV_rev : tvtype_t
TV_postrev : tvtype_t
TV_prepostrev : tvtype_t
A traversal of the graph using a depth‐first search on the graph following only
reverse arcs. The choice of roots for the traversal is the same as described for
TV_dfs above. The different order of visitation specified by TV_rev, TV_postrev
and TV_prepostrev are the same as those specified by the analogous traversals
TV_dfs, TV_postdfs and TV_prepostdfs.

TV_bfs : tvtype_t
A traversal of the graph using a breadth‐first search on the graph ignoring edge
directions. See the item on TV_dfs above for the role of $tvroot.

EXAMPLES

gvpr -i 'N[color=="blue"]' file.gv

Generate the node‐induced subgraph of all nodes with color blue.

gvpr -c 'N[color=="blue"]{color = "red"}' file.gv

Make all blue nodes red.

BEGIN { int n, e; int tot_n = 0; int tot_e = 0; }
BEG_G {
n = nNodes($G);
e = nEdges($G);
printf ("%d nodes %d edges %s\n", n, e, $G.name);
tot_n += n;
tot_e += e;
}
END { printf ("%d nodes %d edges total\n", tot_n, tot_e) }

Version of the program gc.

gvpr -c ""

Equivalent to nop.

BEG_G { graph_t g = graph ("merge", "S"); }
E {
node_t h = clone(g,$.head);
node_t t = clone(g,$.tail);
edge_t e = edge(t,h,"");
e.weight = e.weight + 1;
}
END_G { $O = g; }

Produces a strict version of the input graph, where the weight attribute of an edge
indicates how many edges from the input graph the edge represents.

BEGIN {node_t n; int deg[]}
E{deg[head]++; deg[tail]++; }
END_G {
for (deg[n]) {
printf ("deg[%s] = %d\n", n.name, deg[n]);
}
}

Computes the degrees of nodes with edges.

BEGIN {
int i, indent;
int seen[string];
void prInd (int cnt) {
for (i = 0; i < cnt; i++) printf (" ");
}
}
BEG_G {

$tvtype = TV_prepostfwd;
$tvroot = node($,ARGV[0]);
}
N {
if (seen[$.name]) indent--;
else {
prInd(indent);
print ($.name);
seen[$.name] = 1;
indent++;
}
}

Prints the depth-first traversal of the graph, starting with the node whose name is
ARGV[0], as an indented list.

ENVIRONMENT

GVPRPATH
Colon‐separated list of directories to be searched to find the file specified by
the -f option. gvpr has a default list built in. If GVPRPATH is not defined, the
default list is used. If GVPRPATH starts with colon, the list is formed by
appending GVPRPATH to the default list. If GVPRPATH ends with colon, the list is
formed by appending the default list to GVPRPATH. Otherwise, GVPRPATH is used for
the list.

On Windows systems, replace ``colon'' with ``semicolon'' in the previous paragraph.

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