Why does my python program ignore/skip some cases

Question

I'm trying to write a code which does the following:

In the first line, it inputs two space-separated integers, the first presents the length of the list which will be input later, and the second presents an int, called k, which will be needed later on.
In the second line, the list I talked about will be input

The expected output is what meets the following criteria:

number of pairs of ( i , j ), where i<j and ar[ i ]+ar[ j ] is divisible by k.

Problem: I don't know why my code ignores some pairs. I have tried it with many test cases, and it fails 90% of them, by outputting less pairs than expected. I know I am pretty close to the right result, I just don't know what I'm not doing right.

For more precise information about what I want my code to do, kindly check this link (No signup or login needed): https://www.hackerrank.com/challenges/divisible-sum-pairs/problem

Here is my code:

#https://www.hackerrank.com/challenges/divisible-sum-pairs/problem

samples_and_k = [int(x) for x in input().split()]
num_list = [int(y) for y in input().split()]
num_list.sort()
counter =0

for i in range(0, samples_and_k[0]-1):
    for j in range(i+1, samples_and_k[0]-1):
        if (num_list[i]+num_list[i+1]) % samples_and_k[1] == 0:
            counter += 1

print(counter)

Here's an example:

Input:

6 3
1 3 2 6 1 2

Expected output: 5

The output of MY code: 3

In your own words, when you write `for i in range(0, samples_and_k[0]-1):`, what is the reason for doing `-1`? In your own words, if you wrote for example `for i in range(0, 5):`, what values would you expect `i` to take on? Now, did you *check the documentation*? Did you *try it*? Does the way that `range` works agree with your expectation? — Karl Knechtel, Oct 12 '20 at 17:25

Jesper · Accepted Answer · 2020-10-12T17:26:48.453

The problem is that you subtract 1 when using range():

for i in range(0, samples_and_k[0]-1):
    for j in range(i+1, samples_and_k[0]-1):
        if (num_list[i]+num_list[i+1]) % samples_and_k[1] == 0:
            counter += 1

It should be:

for i in range(0, samples_and_k[0]):
    for j in range(i+1, samples_and_k[0]):
        if (num_list[i]+num_list[i+1]) % samples_and_k[1] == 0:
            counter += 1

It's a common misunderstanding when using range(), as range() is implemented somewhat unintuitively (in my opinion). The last number is not included in the range, so list(range(0, 6)) is equal to [0, 1, 2, 3, 4, 5].

Oh I see! You solved my issue, thanks a million :)) – Liana Oct 12 '20 at 17:24 — Liana, Oct 12 '20 at 17:24

Why does my python program ignore/skip some cases

1 Answers1