Question

I am trying to print the distinct nodes in a graph when given a list of edges with the following:

def find_nodes(graph):
    # get the distinct nodes from the edges
    nodes = []
    l = len(graph)
    for i in range(l):
        edge = graph[i]
        n1 = edge[0]
        n2 = edge[1]
        if n1 not in nodes:
            nodes.append(n1)
        if n2 not in nodes:
            nodes.append(n2)
    return nodes

graph = ((1,2),(2,3), (3,1))
print find_nodes(graph)

But I only get (1,2) how am i missing the 3?

Answer 1

When I look at the text you’ve inserted, it looks like you’re mixing tabs and spaces as left-hand whitespace:

This can be confirmed by looking at the repr of each line:

'    def find_nodes(graph):'
'        # get the distinct nodes from the edges'
'        nodes = []'
'        l = len(graph)'
'        for i in range(l):'
'        \tedge = graph[i]'
'        \tn1 = edge[0]'
'        \tn2 = edge[1]'
'        \tif n1 not in nodes:'
'        \t\tnodes.append(n1)'
'        \tif n2 not in nodes:'
'        \t\tnodes.append(n2)'
'    \treturn nodes'

This can result in lines not being indented to the level you think they are. Here’s what I get as a result from copying and pasting your input into a console:

>>> s = """
...     def find_nodes(graph):
...         # get the distinct nodes from the edges
...         nodes = []
...         l = len(graph)
...         for i in range(l):
...             edge = graph[i]
...             n1 = edge[0]
...             n2 = edge[1]
...             if n1 not in nodes:
...                     nodes.append(n1)
...             if n2 not in nodes:
...                     nodes.append(n2)
...             return nodes
...     
...     graph = ((1,2),(2,3), (3,1))
...     print find_nodes(graph)
... 
... """

It looks to me like the return nodes line will be executed too early. Write the code into a file and use the python -tt option to check for whitespace problems.