NAME
gvpr - graph pattern scanning and processing language ( previously known as gpr )
SYNOPSIS
gvpr [-icV?] [ -o outfile ] [ -a args ] [ ’prog’ | -f progfile ] [ files ]
DESCRIPTION
gvpr 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 libagraph(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 GPRPATH to look for the file. If -f is not given, gvpr will use the first non-option argument as the program. | |
-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 } | |
Evaluate the BEGIN clause, if any. For each input graph G { 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.
Predicates and actions are sequences of statements in the C dialect supported by the libexpr(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 ’...’. In certain contexts, string values are interpreted as patterns for the purpose of regular expression matching. Patterns use ksh(1) file match pattern syntax. 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 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, the variable var is set to each value used as an index in the specified array and then the associated statement is evaluated. 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. gvpr will attempt to use an expression as a string or numeric value as appropriate.
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 libagraph(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. | |
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. If x is a graph, this indicates that g is the immediate parent graph of x. | |
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. | |
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. | |
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". Returns NULL on failure or if the attribute is not defined. | |
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. Returns NULL on failure or if the attribute is not defined. | |
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 setDflt above. | |
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 what kind of graph object is the argument. For nodes, edges, and graphs, it returns should be "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. | |
length(s : string) : int | |
returns the length of the string s. | |
index(s : string, t : string) : int | |
returns the index of the character in string s where the leftmost 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. | |
canon(s : string) : string | |
returns a version of s appropriate to be used as an identifier in a dot file. | |
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. | |
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. | |
Miscellaneous
exit() : void
exit(v : int) : void | |
causes gvpr to exit with the exit code v. v defaults to 0 if omitted. | |
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. | |
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. | |
$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 traversal of the graph (cf. $tvtype below). The default value is NULL for each input graph. | |
$tvtype : tvtype_t | |
indicates how gvpr traverses a graph. At present, it can only take one of six values: TV_flat, TV_dfs, TV_fwd, TV_ref, TV_bfs, TV_ne, and TV_en. TV_flat is the default. The meaning of these values is discussed below. | |
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 | |
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. | |
TV_fwd : tvtype_t | |
a traversal of the graph using a depth-first search on the graph following only forward arcs. In | |
TV_bfs : tvtype_t | |
a traversal of the graph using a bread-first search on the graph ignoring edge directions. See the item on TV_dfs above for the role of $tvroot. libagraph(3), edges in undirected graphs are given an arbitrary direction, which is used for this traversal. The choice of roots for the traversal is the same as described for TV_dfs above. | |
TV_rev : tvtype_t | |
a traversal of the graph using a depth-first search on the graph following only reverse arcs. In libagraph(3), edges in undirected graphs are given an arbitrary direction, which is used for this traversal. The choice of roots for the traversal is the same as described for TV_dfs above. | |
EXAMPLES
gvpr -i ’N[color=="blue"]’ file.dot
Generate the node-induced subgraph of all nodes with color blue.
gvpr -c ’N[color=="blue"]{color = "red"}’ file.dot
Make all blue nodes red.
BEGIN { int n, e; int tot_n = 0; int tot_e = 0; } BEG_G { n = nNodes($G); e = nEdges($G); printf ("%d nodes %d edges %s0, n, e, $G.name); tot_n += n; tot_e += e; } END { printf ("%d nodes %d edges total0, tot_n, tot_e) }
Version of the program gc.
gvpr -c ""
Equivalent to nop.
BEG_G { graph_t g = graph ("merge", "S"); } E { node_t h = clone(g,$.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] = %d0, n.name, deg[n]); } }
Computes the degrees of nodes with edges.
ENVIRONMENT
GPRPATH | |
Colon-separated list of directories to be searched to find the file specified by the -f option. | |
BUGS AND WARNINGS
When the program is given as a command line argument, the usual shell interpretation takes place, which may affect some of the special names in gvpr. To avoid this, it is best to wrap the program in single quotes.
As of 24 April 2008, gvpr switched to using a new, underlying graph library, which uses the simpler model that there is only one copy of a node, not one copy for each subgraph logically containing it. This means that iterators such as \Inxtnode\P cannot traverse a subgraph using just a node argument. For this reason, subgraph traversal requires new functions ending in "_sg", which also take a subgraph argument. The versions without that suffix will always traverse the root graph.
There is a single global scope, except for formal function parameters, and even these can interfere with the type system. Also, the extent of all variables is the entire life of the program. It might be preferable for scope to reflect the natural nesting of the clauses, or for the program to at least reset locally declared variables. For now, it is advisable to use distinct names for all variables.
If a function ends with a complex statement, such as an IF statement, with each branch doing a return, type checking may fail. Functions should use a return at the end.
The expr library does not support string values of (char*)0. This means we can’t distinguish between "" and (char*)0 edge keys. For the purposes of looking up and creating edges, we translate "" to be (char*)0, since this latter value is necessary in order to look up any edge with a matching head and tail.
Related to this, strings converted to integers act like char pointers, getting the value 0 or 1 depending on whether the string consists solely of zeroes or not. Thus, the ((int)"2") evaluates to 1.
The language inherits the usual C problems such as dangling references and the confusion between ’=’ and ’==’.
AUTHOR
Emden R. Gansner <erg@research.att.com>
SEE ALSO