C++ nested for loops representation in Python

Question

I have been coding in C++ and Java for some time. I have started to code in Python recently as well. However, I got stuck in the case of nested for loops in Python. I have written following code in C++. What will be the equivalent code of it in Python?

for(int i = 0; i<a.size();i++){
   for(int j = i; j<a.size();j++)
   {
       if a[i] != a[j]
          /*some code */
   }

}

I tried to use enumerate as follows, but it fails:

for i, val in enumerate(lst):
    for j=i,val2 in enumerate(lst):
        if val != val2
            #some code

So how can I represent the C++ nested loops in python?

Nir Friedman · Accepted Answer · 2017-03-15 00:38:01Z

4

import itertools

for val1, val2 in itertools.combinations(lst, 2):
    if val1 != val2:

combinations will loop over all cases of choosing two distinct elements from your list which is what your nested for loops are doing anyhow. Note that your second nested for loop is initialized at j=i, but that first case will always fail to process anyhow because it will never pass the conditional.

If you really, really need the indices, and usually you shouldn't, you can do:

for (i, val1), (j, val2) in itertools.combinations(enumerate(lst), 2):
    if val1 != val2:

answered Mar 15, 2017 at 0:38

Nir Friedman

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2 Comments

hshantanu Over a year ago

Thanks for the comment. What if my condition in the second for loop is j = i + 1. How will itertools work in that case?

Nir Friedman Over a year ago

That was my point, whether you do j = i or j = i+1 is the same because of the subsequent conditional. Without the subsequent conditional, the python loop code is like j = i + 1 because combinations will not take the same element twice. If you wanted to do j =i without the conditional in python it's a different situation and my answer would be different.

Cireo · Accepted Answer · 2017-03-15 00:40:42Z

1

As mentioned elsewhere, enumerate does one thing (provide indexes). It will not truncate or skip around in your list. If you wanted to use enumerate, one option is to pass a list slice instead of the full list, and use the start= keyword argument to start the index numbering correctly:

for i, val in enumerate(lst):
    for j, val2 in enumerate(lst[i:], start=i):
        if val != val2:
            #some code

answered Mar 15, 2017 at 0:40

Cireo

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Comments

vallentin · Accepted Answer · 2017-03-15 00:44:56Z

1

The enumerate function doesn't have a start index, however range does in that sense. It should be noted that while enumerate(sequence, start=0) has a start parameter, it only specifies the start of the returned index, and not the first index of which the list is accessed.

So you could do it like this:

n = len(lst)
for i in range(n):
    val = lst[i]
    for j in range(i, n):
        val2 = lst[j]
        if val != val2:
            #some code

edited Mar 15, 2017 at 0:44

answered Mar 15, 2017 at 0:36

vallentin

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