# the difference between Read\_Ncol and Read\_edgelist **URL:** \ **Category:** Usage\ **Tags:** Python\ **Created:** [1 July 2022 17:17 UTC](https://igraph.discourse.group/t/the-difference-between-read-ncol-and-read-edgelist/1283 "2022-07-01T17:17:38Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![salaheddine\_taibi](https://avatars.discourse-cdn.com/v4/letter/s/e495f1/32.png) [@salaheddine\_taibi](https://igraph.discourse.group/u/salaheddine_taibi)\ **Post date:** [1 July 2022 17:17 UTC](https://igraph.discourse.group/t/the-difference-between-read-ncol-and-read-edgelist/1283/1 "2022-07-01T17:17:38Z")
I use **Read\_Ncol and Read\_edgelist** for read edgelist texte file so i get difference results exmple this is text file (tst): 1 10 1 9 1 8 1 2 2 4 2 3 2 1 2 5 2 6 3 9 3 4 3 6 4 3 4 5 4 6 5 4 5 7 6 2 6 3 6 7 6 4 7 5 7 6 7 8 8 9 8 1 8 10 9 10 9 1 10 1 10 9 when i use graph=Graph.Read\_Edgelist(‘/home/yac/Desktop/tst/tst’) print(graph.vcount()) Output : 11 when i use graph=Graph.Read\_Ncol(‘/home/yac/Desktop/tst/tst’) print(graph.vcount()) Output:10 this difference effects on the result of the algorithm, i do not get the same result ---
**Author:** ![szhorvat](https://yyz2.discourse-cdn.com/free1/user_avatar/igraph.discourse.group/szhorvat/32/3_2.png) [@szhorvat](https://igraph.discourse.group/u/szhorvat)\ **Post date:** [1 July 2022 17:37 UTC](https://igraph.discourse.group/t/the-difference-between-read-ncol-and-read-edgelist/1283/2 "2022-07-01T17:37:22Z")
`Read_Edgelist` assumes that the file has pairs of 0-based vertex indices. The number of vertices is the largest index + 1. `Read_Ncol` uses vertex names, not indices. You can use any string to identify vertices. There is no guarantee about the ordering of vertices in the final graph. ---
**Author:** ![salaheddine\_taibi](https://avatars.discourse-cdn.com/v4/letter/s/e495f1/32.png) [@salaheddine\_taibi](https://igraph.discourse.group/u/salaheddine_taibi)\ **Post date:** [1 July 2022 21:55 UTC](https://igraph.discourse.group/t/the-difference-between-read-ncol-and-read-edgelist/1283/3 "2022-07-01T21:55:43Z")
I generate LFR benchmark with the software package from this site [https://www.santofortunato.net/resources](https://www.santofortunato.net/resources) witch is the implementation of algorithm that describe in the paper ( [_ **Benchmark graphs for testing community detection algorithms** _]) The edgelist starts from index 1 I run the program and i get the benchmarks but when i use read\_Ncol or read\_edgelist for read the file . i get irrational results when i use read\_Ncol . So i use networkx for generate the LFR benchmark , the edgelist starts from index 0 , I use read\_edgelist then i get rational results and when i use Read\_Ncol i get irrational results !! what is the issue with read\_Ncol ? ---
**Author:** ![szhorvat](https://yyz2.discourse-cdn.com/free1/user_avatar/igraph.discourse.group/szhorvat/32/3_2.png) [@szhorvat](https://igraph.discourse.group/u/szhorvat)\ **Post date:** [2 July 2022 16:31 UTC](https://igraph.discourse.group/t/the-difference-between-read-ncol-and-read-edgelist/1283/4 "2022-07-02T16:31:01Z")
It’s unclear what you mean by “irrational results”. As I said above, using the NCOL format does not guarantee any vertex ordering, but you can of course reorder vertices based on their names. The edgelist format assumed 0-based indexing. You can simply delete vertex 0 from the result and then it’s the same as if it used 1-based indexing. ---
**Author:** ![salaheddine\_taibi](https://avatars.discourse-cdn.com/v4/letter/s/e495f1/32.png) [@salaheddine\_taibi](https://igraph.discourse.group/u/salaheddine_taibi)\ **Post date:** [2 July 2022 17:19 UTC](https://igraph.discourse.group/t/the-difference-between-read-ncol-and-read-edgelist/1283/5 "2022-07-02T17:19:27Z")
![Screenshot from 2022-07-02 18-10-04](https://global.discourse-cdn.com/free1/uploads/igraph/original/1X/7c2cff821b217415f131474054ea705241002ebf.png) Hi thank you for your respond In this code I use ledein algorthim to identify community structure and I use NMI for evaluate the quality of solution but when i use Read\_Ncol for read graph the NMI is 0.08 this is irrational result , it should be 1 when i use Read\_Edglist , NMI is 1 and this the right result…it is same results as in the paper … ---
**Author:** ![szhorvat](https://yyz2.discourse-cdn.com/free1/user_avatar/igraph.discourse.group/szhorvat/32/3_2.png) [@szhorvat](https://igraph.discourse.group/u/szhorvat)\ **Post date:** [5 July 2022 09:43 UTC](https://igraph.discourse.group/t/the-difference-between-read-ncol-and-read-edgelist/1283/6 "2022-07-05T09:43:05Z")
To understand what is happening, you need to first understand the difference between `Read_Edgelist` and `Read_Ncol`. In igraph, vertices have a consistent ordering. The ID of a vertex is its position in this order, i.e. just an index. To be specific, it’s a 0-based index, as usual in Python. The first vertex has index 0, the second 1, and so on. `Read_Edgelist` assumes that the file contains vertex IDs. This means that the ordering of vertices is defined by the file. However, the file you are dealing with uses 1-based indices, not 0-based. The first vertex is referred to as `1`, the second as `2`, and so on. `Read_Ncol` assumes that the file contains arbitrary vertex _names_, which may be any string, not necessarily numbers. You can have vertices called `Alice` and `Bob`. The vertex name `1` will not in general be the first one in your graph. I assume that the ordering is important for your application as the ground-truth communities are made up of vertices with adjacent indices. Thus, you should use `Read_Edgelist`. This will produce an additional vertex using index `0` that you don’t want. Simply delete this and you have your graph. Note that after deleting vertex `0`, all indices shift down by one, so the old vertex 1 becomes 0, old vertex 2 becomes 1, etc. Take this into account when working with the ground-truth communities. I hope this is clear. ---
**Author:** ![salaheddine\_taibi](https://avatars.discourse-cdn.com/v4/letter/s/e495f1/32.png) [@salaheddine\_taibi](https://igraph.discourse.group/u/salaheddine_taibi)\ **Post date:** [7 July 2022 00:14 UTC](https://igraph.discourse.group/t/the-difference-between-read-ncol-and-read-edgelist/1283/7 "2022-07-07T00:14:46Z")
yes thank you so much , it is clear now 😍