Why do these same codes, output differently?

Question

This is my sum calculator for numbers 1 to N.
But for example why when I input "5";
The A code outputs "21"
The B code outputs "15"
I tried manually their output should be same!
Just the line of "while" statements replaced:
https://code.sololearn.com/c0ROTx5QW7Hp/?ref=app
https://code.sololearn.com/c0yCN5y3091U/?ref=app

Answer 1

Code A : incrementing i then adding to sum. Next going to check I<=N

Code B : adding I to sum then incrementing. (Current incremented value will be added in next iteration). Next going to check I<=N

So code B dont adds 6
Code A add 6 to sum as well..
Hope it helps..

Answer 2

Simple

i = 0
sum = 0

i = i + 2 = 2
sum = sum + i = sum + 2 = 0 + 2 = 2

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sum = sum + i = 0 + 0 = 0
i = i + 1 = 2

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BTW code is not exactly same

Answer 3

In code A : i is increased before it gets added to sum where as in code B : i is added to sum first then i is increased by 1 , so there is a difference in there outputs

Answer 4

Jayakrishna🇮🇳 A͢J Swaraj Soubhagya Khandai Thank you all 🙏🌹
I thought the program checks i <= N first of all and stops when i = 5, so no matter i will increase before or after sum cause it has stopped before. But it seems it will check i <= N after doing so.