CODEWITHSUNDEEP
c++loops
Ely
Ely

Reputation: 11152

While loop is more efficient than for loop. What could be the reason?

I was doing a LeetCode challenge (here) for fun and was surprised that the while loop was more efficient than the for loop. I would have expected the compiler to generate identical code (also as per these question and answers), but the run times are different.

The while loop was around 3 ms, whilst the for loop took around 6ms. I repeated a couple of times and it seemd to be often like that.

I don't have the test cases unfortunately, and I don't have any information about compiler used, the architecture or optimizations set. I think it is not important because the programs are almost identical and use the same compiler, architecture and options surely.

Any ideas or experiences in that matter ?

For loop:

vector<int> twoSum(vector<int>& numbers, int target) {
    int upper = numbers.size() - 1;
    int lower = 0;
    int sum;

    for (;lower<upper;) {
        sum = numbers[lower] + numbers[upper];
        if (sum == target) {
            return vector<int> { lower+1, upper+1 };
        } else if (sum > target) {
            upper--;
        } else {
            lower++;
        }
    }
}

While loop:

vector<int> twoSum(vector<int>& numbers, int target) {
    int upper = numbers.size() - 1;
    int lower = 0;
    int sum;

    while (lower<upper) {
        sum = numbers[lower] + numbers[upper];
        if (sum == target) {
            return vector<int> { lower+1, upper+1 };
        } else if (sum > target) {
            upper--;
        } else {
            lower++;
        }
    }
}

Upvotes: 1

Views: 587

Answers (2)

Jonas
Jonas

Reputation: 7017

You did not run enough or long enough tests, benchmarks in milliseconds is hard to verify.

A better way is to compare the generated assembly: for-loop and while-loop. The snippets are compiled with maximum optimization (-O3), using g++ 6.3. From this it is clear that there is no performance difference at all, since the assembly for the two is exactly the same.

Upvotes: 3

Sterling Beason
Sterling Beason

Reputation: 630

All loops follow the same template:

{
// Initialize
LOOP: 
if(!(/* Condition */) ) {
    goto END
}

// Loop body

// Loop increment/decrement
goto LOOP
}
END:

Your test must have differed by the available processing power on your CPU.

Upvotes: 0

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