CODEWITHSUNDEEP
c++
sircodesalot
sircodesalot

Reputation: 11439

Increment order in a while loop

Here I have a simple algorithm that computes the square of a number by adding up all of the odd numbers equal to the inputted number, such that:

1 = 1 = 1-squared
1 + 3 = 4 = 2-squared
1 + 3 + 5 = 9 = 3-squared

and so forth. The algorithm is as follows:

int triangleSquare(int number) {
    int product = 0;
    int currentOdd = 1;

    while (number--) {
        product += currentOdd;
        currentOdd += 2;
    }

    return product;
}

My question is about the segment: 

while (number--) { ... }

For some reason I get different results between while (--number) and while (number--), which I assume is because --number should decrement first before performing the while loop (and vice-versa for number--). My question is, why does it make a difference? Shouldn't the while loop always execute last, regardless of the increment order? I must be missing something obvious, but that seems very odd to me, almost as though number-- makes the conditional behave like a do {...} while() loop.

Upvotes: 1

Views: 2517

Answers (3)

TobiMcNamobi
TobiMcNamobi

Reputation: 4813

while (number--) does the following:

  1. evaluate while (number)
  2. then number--

On the other hand while (--number) does this:

  1. calculate --number
  2. then evaluate while (number) with the new value

And that's the difference.

Upvotes: 2

James Kanze
James Kanze

Reputation: 153929

This is a horrible way to write code, but...

An expression has two characteristics: a return value, and side effects. The decrementation is the side effect; it is the same for --number and number--. The condition in the while, however, depends on the return value; --number returns the value after the decrementation; number-- the value before the decrementation. When the decrementation actually takes place on the variable is not really specified (for either): some time after the preceding sequence point and before the following one.

In other words: --number is the equivalent of:

int
decr( int& number )
{
    number -= 1;
    int results = number;
    return results;
}

and number-- is the equivalent of:

int
decr( int& number )
{
    int results = number;
    number -= 1;
    return return;
}

In both cases, you're testing the return value of the function.

Upvotes: 1

Jonathan Leffler
Jonathan Leffler

Reputation: 754130

The two candidate loops could be written (are equivalent to):

while (number-- != 0)
while (--number != 0)

Consider what happens when number == 1 at loop entry:

  1. The value of number is compared to zero and then decremented; since 1 is not 0, the loop body is executed.
  2. The value of number is decremented and then compared to zero; since 0 is 0, the loop body is not executed.

That's the difference!

Upvotes: 3

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