The variable declared in for-loop of c is not initialised-CodePudding

#include <iostream>
#include <Windows.h>

void selection_sort(int*,int);
void output_array(int*, int);

int main() {
    using namespace std;
    int numbers[] = { 4, 6, 8, 2, 7, 5, 0,1, 3, 9 };
    int length = 10;

    selection_sort(numbers, length);
    output_array(numbers, length);

    Sleep(10000);
    return 0;
}

void output_array(int* start, int length) {
    for (int i = 0; i < length; i  ) {
        std::cout << *(start   i) << " ";
    }
    std::cout << std::endl;
}

void selection_sort(int* start, int length) {
    for (int i = 0; i < length; i  ) {
        int max = *start;
        int max_i = 0;
        for (int j = 1; j < (length - i); j  ) {
            //find out the maximum
            if (max < *(start   j)) {
                max = *(start   j);
                max_i = j;
            }
        }
        //put it at the end
        for (int k = max_i; k < (length - i -1); k  ) { //The problem is HERE
            *(start   k) = *(start   k   1);
        }
        *(start   length - i) = max;
    }
}

Perhaps it is a problem simple enough, but why when it comes to the last for loop, k is undefined? Is it because max_i is not a compile-time constant? Variables Program at this step This doesn't make sense to me.

When it comes to the second largest number, k behaves as expected.

CodePudding user response：

It seems I have got it. length is 10, max_i is 9, i is 0 till 9. Your loop becomes

for (int k = 9; k < (10 - i -1); k  ) {
    *(start   k) = *(start   k   1);
}

k < 9 - i never gets true and the loop never runs. You have an error in the program logic.

CodePudding user response：

In the following line:

*(start   length - i) = max;

i has the range 0 to length - 1 this means the above line evaluates at the extreme values of i to:

*(start   1) = max;

And:

*(start   length) = max;

This means you both never write to the first element in your array and you also write after the last element in the array. The latter means your program has undefined behaviour, in this particular case as start is on the stack it will likely cause other variables on the stack to be corrupted.

The correct code is:

*(start   length - i - 1) = max;

Which results in:

*(start) = max;

And:

*(start   length - 1) = max;

On an unrelated note you can also use the simpler syntax:

start[length - i - 1] = max;

CodePudding user response：

void selection_sort(int* start, int length) {
    for (int i = 0; i < length; i  ) {
        int max = *start;
        int max_i = 0;
        for (int j = 1; j < (length - i); j  ) {
            //find out the maximum
            if (max < *(start   j)) {
                max = *(start   j);
                max_i = j;
            }
        }
        //put it at the end
        for (int k = max_i; k < (length - i -1); k  ) {
            *(start   k) = *(start   k   1);
        }
        // The problem was here, you wanted to access memory that does not belong to you.
        *(start   length - i - 1) = max;
    }