CODEWITHSUNDEEP
python
dannyyy
dannyyy

Reputation: 149

Why am I getting two different values for the same arithmetic operation with the same numbers?

I'm doing a very simple hourglass sum function that adds up values in a 2d matrix in Python. While computing the sum, my printed statement gets one value, while putting it into a variable gets another value.

def hourglassSum(arr):
    max_sum = 0

    for row in range(1, len(arr) - 1):
        for col in range(1, len(arr[row]) - 1):
            print('Printed Sum', arr[row][col] + arr[row - 1][col] 
            + arr[row - 1][col - 1] + arr[row - 1][col + 1] + arr[row + 1][col] 
            + arr[row + 1][col - 1] + arr[row + 1][col + 1])

            computed_sum = arr[row][col] + arr[row - 1][col] 
            + arr[row - 1][col - 1] + arr[row - 1][col + 1] + arr[row + 1][col] 
            + arr[row + 1][col - 1] + arr[row + 1][col + 1]

            print('Computed Sum',computed_sum)

            if computed_sum > max_sum:
                max_sum = computed_sum
    return max_sum

I would expect the computed_sum to be the same that gets printed out. However my output shows two completely different numbers every loop.

I'm very new to python so I'm sure I either missed a dumb little bug, or there is something I'm not understanding. Any help is appreciated!

Upvotes: 0

Views: 71

Answers (1)

walnut
walnut

Reputation: 22152

Statements in Python usually end at a new-line. Therefore 

computed_sum = arr[row][col] + arr[row - 1][col]

is evaluated completely separately from the following to statements

+ arr[row - 1][col - 1] + arr[row - 1][col + 1] + arr[row + 1][col] 
+ arr[row + 1][col - 1] + arr[row + 1][col + 1]

These are two statements summing variables, but doing nothing with the result. The first + is an unary + operator.

With the print( at the beginning of

print('Printed Sum', arr[row][col] + arr[row - 1][col]

the statement has to continue into the next lines, because there is still an open ( at the new-line. So the following two lines are evaluated as part of the expression. Only the ) with a following new-line ends the statement.

You can avoid this problem by putting parantheses around the whole expression:

computed_sum = (arr[row][col] + arr[row - 1][col] 
             + arr[row - 1][col - 1] + arr[row - 1][col + 1] + arr[row + 1][col] 
             + arr[row + 1][col - 1] + arr[row + 1][col + 1])

Or you can put \ before the problematic line breaks to instruct Python to consider the following line a continuation of the current one (basically ignoring the new-line).

Upvotes: 3

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