feat: implement read/write graphs from/to string

R/foreign.R

@@ -174,7 +174,7 @@ write.graph.fromraw <- function(buffer, file) {
 #' @aliases LGL Pajek GraphML GML DL UCINET
 #' @param file The connection to read from. This can be a local file, or a
 #'   `http` or `ftp` connection. It can also be a character string with
-#'   the file name or URI.
+#'   the file name or URI or contain the graph in a single string.
 #' @param format Character constant giving the file format. Right now
 #'   `edgelist`, `pajek`, `ncol`, `lgl`, `graphml`,
 #'   `dimacs`, `graphdb`, `gml` and `dl` are supported,
@@ -319,7 +319,17 @@ read_graph <- function(
   ),
   ...
 ) {
-  if (
+  from_string <- is.character(file) && length(file) == 1 && !file.exists(file)
+
+  if (from_string || inherits(file, "connection")) {
+    con <- if (inherits(file, "connection")) file else textConnection(file)
+    tmp <- tempfile()
+    writeLines(readLines(con), tmp)
+    if (!inherits(file, "connection")) {
+      close(con)
+    }
+    file <- tmp
+  } else if (
     !is.character(file) ||
       length(grep("://", file, fixed = TRUE)) > 0 ||
       length(grep("~", file, fixed = TRUE)) > 0
@@ -345,15 +355,14 @@ read_graph <- function(
   res
 }
 
-
 #' Writing the graph to a file in some format
 #'
 #' `write_graph()` is a general function for exporting graphs to foreign
 #' file formats. The recommended formats for data exchange are GraphML and GML.
 #'
 #' @param graph The graph to export.
 #' @param file A connection or a string giving the file name to write the graph
-#'   to.
+#'   to. If file is `""`, `stdout()` or `"-"`, then the output is written directly to the terminal.
 #' @param format Character string giving the file format. Right now
 #'   `pajek`, `graphml`, `dot`, `gml`, `edgelist`,
 #'   `lgl`, `ncol`, `leda` and `dimacs` are implemented. As of igraph 0.4
@@ -481,10 +490,16 @@ write_graph <- function(
   ...
 ) {
   ensure_igraph(graph)
+
+  to_stdout <- identical(file, "") ||
+    identical(file, stdout()) ||
+    identical(file, "-")
+
   if (
     !is.character(file) ||
       length(grep("://", file, fixed = TRUE)) > 0 ||
-      length(grep("~", file, fixed = TRUE)) > 0
+      length(grep("~", file, fixed = TRUE)) > 0 ||
+      to_stdout
   ) {
     tmpfile <- TRUE
     origfile <- file
@@ -509,12 +524,18 @@ write_graph <- function(
 
   if (tmpfile) {
     buffer <- read.graph.toraw(file)
-    write.graph.fromraw(buffer, origfile)
+
+    if (to_stdout) {
+      raw_connection <- rawConnection(buffer)
+      on.exit(close(raw_connection))
+      cat(readLines(raw_connection), sep = "\n")
+    } else {
+      write.graph.fromraw(buffer, origfile)
+    }
   }
 
   invisible(res)
 }
-
 ################################################################
 # Plain edge list format, not sorted
 ################################################################

tests/testthat/_snaps/foreign.md

@@ -73,3 +73,64 @@
       ! not_existing is not a valid graph type.
       i Must be one of r001, r005, r01, r02, m2D, m2Dr2, m2Dr4, m2Dr6, m3D, m3Dr2, m3Dr4, m3Dr6, m4D, m4Dr2, m4Dr4, m4Dr6, b03, b03m, ..., b09, and b09m.
 
+# read/write from/to string works
+
+    Code
+      write_graph(g, file = "", format = "graphml")
+    Output
+      <?xml version="1.0" encoding="UTF-8"?>
+      <graphml xmlns="http://graphml.graphdrawing.org/xmlns"
+               xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+               xsi:schemaLocation="http://graphml.graphdrawing.org/xmlns
+               http://graphml.graphdrawing.org/xmlns/1.0/graphml.xsd">
+      <!-- Created by igraph -->
+        <key id="g_name" for="graph" attr.name="name" attr.type="string"/>
+        <key id="g_mutual" for="graph" attr.name="mutual" attr.type="boolean"/>
+        <key id="g_circular" for="graph" attr.name="circular" attr.type="boolean"/>
+        <graph id="G" edgedefault="undirected">
+          <data key="g_name">Ring graph</data>
+          <data key="g_mutual">false</data>
+          <data key="g_circular">true</data>
+          <node id="n0">
+          </node>
+          <node id="n1">
+          </node>
+          <node id="n2">
+          </node>
+          <node id="n3">
+          </node>
+          <node id="n4">
+          </node>
+          <node id="n5">
+          </node>
+          <node id="n6">
+          </node>
+          <node id="n7">
+          </node>
+          <node id="n8">
+          </node>
+          <node id="n9">
+          </node>
+          <edge source="n0" target="n1">
+          </edge>
+          <edge source="n1" target="n2">
+          </edge>
+          <edge source="n2" target="n3">
+          </edge>
+          <edge source="n3" target="n4">
+          </edge>
+          <edge source="n4" target="n5">
+          </edge>
+          <edge source="n5" target="n6">
+          </edge>
+          <edge source="n6" target="n7">
+          </edge>
+          <edge source="n7" target="n8">
+          </edge>
+          <edge source="n8" target="n9">
+          </edge>
+          <edge source="n0" target="n9">
+          </edge>
+        </graph>
+      </graphml>
+

tests/testthat/test-foreign.R

@@ -76,3 +76,12 @@ test_that("graph_from_graphdb works", {
     error = TRUE
   )
 })
+
+test_that("read/write from/to string works", {
+  g <- make_ring(10)
+
+  expect_snapshot(write_graph(g, file = "", format = "graphml"))
+  str <- r"---(0 1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 0 9)---"
+  g2 <- read_graph(str, format = "edgelist", directed = FALSE)
+  expect_isomorphic(g, g2)
+})