Strange console/pointer behaviour

[Muglian]

I am working on a bit of code that includes arrays in and out of functions, but my programs insist on some very strange behavior that persists regardless of my many efforts to fix the problem. Pseudocode:

main
function randomArray //generates array of random numbers
function printArray //prints the array to console
function sortArray //sorts the array lowest -> highest
function printArray
end

The problem is that the second time the printArray function is run,after sorting the array, the numbers printed to console are all very strange.

Output example:

41 85 72 38 80 69 65 68 96 22 49 67 51 61 63 87 66 24 80 83

22 0 1 0 0 0 20987800 20987808 20987808 512 0 0 19921228 19921520 20921280 -1 19921360 20779373 20987800 20922800

The number 22 at the start suggests that it somehow got the first number right, but after that I see no connection at all.

Code:

#include <iostream>

int randomWithLimits(int lower, int upper){
	int diff = upper - lower + 1;
	return std::rand() % diff + lower;
}
void printArray(int *array, int array_length){
	for(int i = 0; i < array_length; i++){
		std::cout << array[i] << " ";
	}
	std::cout << std::endl;
}
int* randomizeArray(int *array, int array_length){
	for(int i = 0; i < array_length; i++){
		int rnum = randomWithLimits(0, 100);
		array[i] = rnum;
	}
	return array;
}
int* sortArray(int *array, int array_length){
	
	int sorted[20] = {};
	for(int i = 0; i < array_length; i++){
		int lowest = i;
		for(int j = 0; j < array_length; j++){
			if(array[j] < array[lowest]){
				lowest = j;
			}
		}
		sorted[i] = array[lowest];
		array[lowest] = 1000;
	}
	array = sorted;
	return array;
}

int main() {
	int empty[20] { };
	int *array = randomizeArray(empty, 20);
	
	printArray(array, 20);
	
	array = sortArray(array, 20);

	printArray(array, 20);
	return 0;
}

Can anyone explain this very strange behaviour, or what can be done to fix it? Any help is greatly appreciated. Apologies for the vague/indirect question.

[Ganado]

You're invoking undefined behavior. As a consequence of this, you're printing junk.
In your sortArray, you are returning a pointer to data allocated locally on the stack. The data in int sort[20] only exists for the duration of your sortArray function. Once the function ends, all the data that sort is pointing to is invalid.

When you do array = sorted (and then return the array pointer), that is not doing an element-wise copy. That is only copying the pointer of the array. I would suggest not returning anything, and to just modify the source array instead.

[jlb]

You can't return a local array from your function.

Both your sortArray() function and your randomize() array functions should both probably be returning nothing (void).

[Muglian]

Thanks jlb and Ganado, i see now the fault in my ways. It works now, by not having the functions return anything. I still think its strange though, that the randomizing function could return just fine and have the array printed, but the sort function could not (with seemingly the same syntax?).

It works now though, so thanks again.

[Ganado]

It's important to understand the difference. Your randomizeArray function returns the pointer to the same array that you passed in, and not a local array's pointer, so it's still valid. But it would be better to have a uniform pattern so you don't get confused (making both functions' return type be void).