read_graph() error on pajek file: Parse error in Pajek file, line 1 (syntax error, unexpected ALNUM, expecting end of file), Parse error

[reuning]

Describe the bug
I am having trouble reading in a ucinet network using read_graph(). The file is in the Hollywood Film Music network. For what it is worth, I can read the file in using network::read.paj() so I don't think there is anything wrong with the pajek file itself.

To reproduce

library(igraph)
net <- read_graph("Movies.paj", format="pajek")

The error:

Error in read.graph.pajek(file, ...) : 
  At core/io/pajek.c:173 : Parse error in Pajek file, line 1 (syntax error, unexpected ALNUM, expecting end of file), Parse error

Version information

• R/igraph version: 1.4.1
• R version: 4.2.2
• Operating system: MacOS

> sessionInfo()
R version 4.2.2 (2022-10-31)
Platform: x86_64-apple-darwin17.0 (64-bit)
Running under: macOS Ventura 13.2.1

Matrix products: default
LAPACK: /Library/Frameworks/R.framework/Versions/4.2/Resources/lib/libRlapack.dylib

locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] igraph_1.4.1  usethis_2.1.6

loaded via a namespace (and not attached):
 [1] pillar_1.8.1         compiler_4.2.2       tools_4.2.2          digest_0.6.31       
 [5] evaluate_0.18        lifecycle_1.0.3      tibble_3.1.8         lattice_0.20-45     
 [9] pkgconfig_2.0.3      rlang_1.1.0          DBI_1.1.3            cli_3.6.0           
[13] rstudioapi_0.14      yaml_2.3.6           xfun_0.35            fastmap_1.1.0       
[17] coda_0.19-4          dplyr_1.0.10         stringr_1.5.0        knitr_1.41          
[21] generics_0.1.3       fs_1.5.2             vctrs_0.5.2          grid_4.2.2          
[25] tidyselect_1.2.0     glue_1.6.2           R6_2.5.1             fansi_1.0.4         
[29] rmarkdown_2.18       purrr_0.3.5          magrittr_2.0.3       htmltools_0.5.4     
[33] assertthat_0.2.1     utf8_1.2.2           intergraph_2.0-2     stringi_1.7.12      
[37] network_1.18.0       statnet.common_4.7.0

[szhorvat]

This is a Pajek "project file" (.paj extension), containing not only a network, but also other kinds of data. igraph only supports Pajek network files (.net extension), not Project files. See https://igraph.org/c/html/latest/igraph-Foreign.html#igraph_read_graph_pajek

This should be better documented for the R interface as well. Above I linked to the documentation of the igraph C library.

You can effectively convert this file to a network file by removing all other kinds of data. Open it with a text editor, and remove everything starting with line *Partition Movies_top_composers.clu. Then igraph will be able to read it. Here's a fixed version:

Movies.net.gz

Once igraph's R interface is upgraded to use version 0.10 of the igraph C library, the error message will be slightly more informative, as it will include the line number. It will look something like this:

Error at src/io/pajek.c:213: Parse error in Pajek file, line 300 (syntax error, unexpected ALNUM, expecting end of file). -- Parse error

[reuning]

Oh that makes a lot of sense. Thank you.

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