CODEWITHSUNDEEP
c++visual-c++
Matt
Matt

Reputation: 69

C++ Primer exercise 1.12 results puzzleing

EXERCISE 1.12: What does the following "for" loop do? what is the final value of sum?

The code that applies to the above exercise is:

#include < iostream >

int main()    
{    
    int sum = 0;
    for (int i = -100; i <= 100; ++i)
        sum += i;

    std::cout << "sum of i is : " << sum << std::endl;
}

The result of sum or i is 0.

My question is with the condition of i <= 100. how is the answer 0?

I would think the program would count from -100 all the way up to 100.

Upvotes: 0

Views: 221

Answers (3)

RichardPlunkett
RichardPlunkett

Reputation: 2988

for(<init> ; <test>; <update>) { X;Y;Z; }

loosely translates to 

 <init>;  while (<test>) { X;Y;Z; <update>; }

We use for loops instead of while loops because its often nice to have all the looping logic in one chunk, then all the per-loop doing logic within the braces, where as a while loop can make them a little tougher to separate cognitively.

Upvotes: 0

Floris
Floris

Reputation: 46415

Indeed it counts all the way from -100 to 100 inclusive. Let's make a shorter example (-2 to <=2):

sum = -2 + -1 + 0 + 1 + 2 = 0

do you see it now?

Upvotes: 4

Dan
Dan

Reputation: 12675

It adds all of the numbers from -100 to 100. Since 100-100+99-99+...1-1+0=0, the total is 0.

Upvotes: 1

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