Command line input issues in Python

Question

I am using this Tarjan algorithm in python to find circuits in a directed graph. I have tried all types of inputs but it's not working.

Here is complete code and following is the code that takes input.

if len(sys.argv) != 2:
    print "usage: echo \"v1 v2\nv1 v3\n...\" | %s num_vertices"%(sys.argv[0])

A = [[] for a in range(int(sys.argv[1]))]

for edge in sys.stdin.readlines():
    v1,v2 = edge.split(' ', 1)
    A[int(v1)].append(int(v2));

I have given following inputs:

$ echo "3" "0 1 0 2 1 0 2 0 2 1" | python cycles.py
$ echo "3 0 1 0 2 1 0 2 0 2 1" | python cycles.py
$ echo "0 1\n0 2\n1 0\n1 3\n2 0\n3 0\n3 1\n3 2" | python cycles.py //as instructed.

but none of them is working. Can somebody help in figuring out the input format or is there some bug in the program?

Matt Davidson · Accepted Answer · 2015-04-17 05:33:33Z

1

Length of sys.argv

The length of sys.argv is always going to include the name of the script itself. So for your script:

>> print len(sys.argv)
3
>> print sys.argv
['cycles.py', '3', '0 1 0 2 1 0 2 0 2 1']

Error in your script

You make a check that sys.argv must not be any other length than 2. When you pass in 2 arguments, sys.argv will have a length of 3.

if len(sys.argv) != 2:
    print "usage: echo \"v1 v2\nv1 v3\n...\" | %s num_vertices"%(sys.argv[0])

Should be

if len(sys.argv) != 3:
    print "usage: echo \"v1 v2\nv1 v3\n...\" | %s num_vertices"%(sys.argv[0])

Invoking python scripts with arguments

A much more readable way to invoke scripts would be to use this syntax:

python -i cycles.py "3" "0 1 0 2 1 0 2 0 2 1"

answered Apr 17, 2015 at 5:33

Matt Davidson

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