Why this for loop doesn't do the same that the while loop?

Question

I'm trying to implement the cocktail sort, and in the main loop I'm trying to use a for loop instead of a while, but it doesn't give me the same results. Here is the code of the while loop:

template<typename T>
void cocktail(std::vector<T>& vec)
{
    std::size_t beg = 1;
    std::size_t end = vec.size()-1;
    bool change = true;

    while((beg < end) && (change))
    //for(std::size_t beg = 1, end = vec.size() - 1; (beg < end) && (change); change = false)
    {
        change = false;
        for(std::size_t j = beg; j <= end; j++){
            if(vec[j] < vec[j-1]){
                std::swap(vec[j-1], vec[j]);
                change = true;
            }
        }
        end--;
        for(std::size_t j = end; j >= beg; j--){
            if(vec[j] < vec[j-1]){
                std::swap(vec[j-1], vec[j]);
                change = true;
            }
        }
        beg++;
    }
}

Output:

Before: 3 2 0 9 5 7 8 1 4 6
 After: 0 1 2 3 4 5 6 7 8 9

And here is the for loop:

template<typename T>
void cocktail(std::vector<T>& vec)
{
    //std::size_t beg = 1;
    //std::size_t end = vec.size()-1;
    bool change = true;

    //while((beg < end) && (change))
    for(std::size_t beg = 1, end = vec.size() - 1; (beg < end) && (change); change = false)
    {
        //change = false;
        for(std::size_t j = beg; j <= end; j++){
            if(vec[j] < vec[j-1]){
                std::swap(vec[j-1], vec[j]);
                change = true;
            }
        }
        end--;
        for(std::size_t j = end; j >= beg; j--){
            if(vec[j] < vec[j-1]){
                std::swap(vec[j-1], vec[j]);
                change = true;
            }
        }
        beg++;
    }
}

Output:

Before: 3 2 0 9 5 7 8 1 4 6
 After: 0 2 1 3 5 7 8 4 6 9

Do you know why I'm getting different output? My code is located here if you want to see the main file or something.

Answer 1

In your For Loop remove (change = false) from inside the for() statement. Let that for statement be

for(std::size_t beg = 1, end = vec.size() - 1; (beg < end) && (change);)
{
    // Write here
    change= false;
    ......
}

It would work.

Explanation: In your while loop code, your change is set to false at the first step. But in your for loop the change = false statement is set to false at the end of the loop. As the third argument of for loop executed as the last statement of the loop.

Consider an example:

for(i=0;i< n;i++)

i is incremented at last not at first. This is the problem with your solution.

Answer 2

In a for loop, the "increment" (3rd part) is executed before the condition (2nd part) is evaluated. In your case

for(std::size_t beg = 1, end = vec.size() - 1; (beg < end) && (change); change = false)

you are not incrementing a variable, but rather setting change = false;. However, this is still executed before the condition is evaluated. Therefore, the condition is always false. In your while loop, instead, you are setting change = false; inside the loop, after the condition has been evaluated. This means your 2 loops are not equivalent.

Answer 3

for (size_t i = 0; i < 10; i++)
    body();

is equivalent to

size_t i = 0;
while (i < 10) {
    body();
}

Your code uses

for (bool change = true; change; change = false)
    body();

which is equivalent to

bool change = true;
while (change) {
    body();
    change = false;
}

After the first pass through this loop, change will always be false during the conditional test at the start of the loop.

for ( initial ; condition ; iterate )
               ^-------------------v