MATLAB engine for python - How to build graph with weights

Hey,

When trying to build the simplest graph on matlab engine with python -

s1 = ['a', 'b', 'c', 'd'];
t1 = ['b', 'c', 'd', 'e'];
w1 = [10, 20, 30, 40];
g = eng.graph(s1,t1,w1);

I'm getting this error-

MatlabExecutionError                      Traceback (most recent call last)
~\AppData\Local\Temp/ipykernel_15984/3466945527.py in 
      8 t1 = ['b', 'c', 'd', 'e'];
      9 w1 = [10, 20, 30, 40];
---> 10 g = eng.graph(s1,t1,w1);
     11 
     12 

c:\Users\amitz\anaconda3\lib\site-packages\matlab\engine\matlabengine.py in __call__(self, *args, **kwargs)
     68             return FutureResult(self._engine(), future, nargs, _stdout, _stderr, feval=True)
     69         else:
---> 70             return FutureResult(self._engine(), future, nargs, _stdout,
     71                                 _stderr, feval=True).result()
     72 

c:\Users\amitz\anaconda3\lib\site-packages\matlab\engine\futureresult.py in result(self, timeout)
     65                 raise TypeError(pythonengine.getMessage('TimeoutCannotBeNegative'))
     66 
---> 67         return self.__future.result(timeout)
     68 
     69     def cancel(self):

c:\Users\amitz\anaconda3\lib\site-packages\matlab\engine\fevalfuture.py in result(self, timeout)
     80                 raise TimeoutError(pythonengine.getMessage('MatlabFunctionTimeout'))
...
  File C:\Program Files\MATLAB\R2022b\toolbox\matlab\graphfun\+matlab\+internal\+graph\constructFromEdgeList.m, line 256, in constructFromEdgeList

  File C:\Program Files\MATLAB\R2022b\toolbox\matlab\graphfun\@graph\graph.m, line 325, in graph.graph
Graph edge weights must be double or single, real, and not sparse

I tried looking everywhere but didn't find the cause.

Really appreciate your assistance.

Answer 1

[Vinayak Choyyan]

Hi Amit,

As per my understanding, you are trying to use the ‘graph()’ function when you are facing an issue with assignment of weight to the graph edges.

‘graph(s,t)’ accepts 2 types of input, namely ‘Node index’ and ‘Node name’. Further, ‘Node name’ type input accepts one of the following types of input for multiple nodes:

• Cell array of character vectors. Example: {'A' 'B' 'C'}
• String array. Example: ["A" "B" "C"]
• Categorical array. categorical(["A" "B" "C"])

Also, in MATLAB, as shown below, the following code results to ‘abcd’ rather than creating an array of characters as you were expecting.

s1 = ['a', 'b', 'c', 'd']

s1 = 'abcd'

Hence you had only created 2 nodes with names ‘abcd’ and ‘bcde’ and were passing the function 4 edge weights instead of 1 edge weight.

s1 = ['a', 'b', 'c', 'd'];
t1 = ['b', 'c', 'd', 'e'];
g = graph(s1,t1);
plot(g)

What you might want to try out is the following,

s2 = {'a', 'b', 'c', 'd'};
t2 = {'b', 'c', 'd', 'e'};
g2 = graph(s2,t2);
plot(g2)

Now you can also assign the 4 weights to the edges respectively.

w1 = [10, 20, 30, 40];
g3 = graph(s2,t2,w1);
plot(g3)

I hope this resolves the issue you were facing.