# On the design of igraph\_eulerian\_path
**URL:** https://igraph.discourse.group/t/on-the-design-of-igraph-eulerian-path/438
**Category:** Development
**Tags:** C
**Created:** [13 September 2020 07:49 UTC](https://igraph.discourse.group/t/on-the-design-of-igraph-eulerian-path/438 "2020-09-13T07:49:46Z")
**Posts on this page:** 2
**Page:** 1
### Author:  [@szhorvat](https://igraph.discourse.group/u/szhorvat)
#### Post date: [13 September 2020 07:49 UTC](https://igraph.discourse.group/t/on-the-design-of-igraph-eulerian-path/438/1 "2020-09-13T07:49:47Z")
While working on integrating the Eulerian functions into the Mathematica interface, I came across a small issue. The same issue will apply to other similar functions too, so I wanted to discuss it.
Currently, `igraph_eulerian_path` and `igraph_eulerian_cycle` will issue an error if the input is not Eulerian. When exposing these functions in a high-level interface, the simplest choice is to keep the same behaviour. This is reasonable in languages that have exceptions (like Python), but less so in languages that might indicate the lack of an Eulerian path with a certain return value (Mathematica, and, I presume, R).
In a typical usage situation, it may not be known in advance if the graph is Eulerian. In a language with exceptions, one may do (pseudocode):
```auto
try:
eulerian_path(g)
except:
# do something if g was not Eulerian
```
In a language where errors are not catchable exceptions, this may need to look like
```auto
if is_eulerian(g):
eulerian_path(g)
else:
# do something if g was not Eulerian
```
This duplicates the `is_eulerian` check, which is also done by `eulerian_path` internally.
In such languages, an _expected failure_ such as an Eulerian path not existing is often better indicated by returning a special value, and not displaying an error message. I _can_ implement this with the current design of the `igraph_eulerian_path` C function, but in order to do so I need an extra `igraph_is_eulerian` check, which is a waste.
I cautiously propose changing `igraph_eulerian_path` (and `_cycle`) so that instead of erroring out, it indicates that the graph is not Eulerian, with this prototype:
```auto
int igraph_eulerian_path(
const igraph_t *graph,
igraph_bool_t *eulerian,
igraph_vector_t *edge_res, igraph_vector_t *vertex_res);
```
If the graph was not Eulerian, then `eulerian` gets set to `false` and `edge_res` and `vertex_res` are not set.
Any thoughts? Alternative solutions?
@Gabor, how would you expose this function in R?
---
### Author:  [@Gabor](https://igraph.discourse.group/u/Gabor)
#### Post date: [13 September 2020 11:42 UTC](https://igraph.discourse.group/t/on-the-design-of-igraph-eulerian-path/438/2 "2020-09-13T11:42:59Z")
I don’t mind if you change it. The ideal for R would be to throw an “igraph\_not\_eulerian\_error” error I guess.