Swapping elements in Array of Pointers

Question

I have the following array in C (Used random names)

char * inputs[6] = {
  "Kangaroo my shoe", "Fly high dragonfly",
  "Philosophical Monkey", "Jumping Ape",
  "Fearful lemurs", "Tall Giraffes"
};

The goal is to swap the first element by the next. As in to do this

char * inputs[6] = {
  "Fly high dragonfly", "Kangaroo my shoe",
  "Jumping Ape", "Philosophical Monkey",
  "Tall Giraffes", "Fearful lemurs"
};

I have tried the following and some more.

ArrSwap(char *Arr[]){
  int i;
  for(i=0;i<6;i++){
    void*temp = Arr[i];
    Arr[i]=Arr[i+1];
    Arr[i+1] = temp;
  }
} 

Thanks in advance! I've spent some hours trying to figure this out, each time I do it it ends up with a messed up order or not swapping any elements expect for the first and last.

Answer 1

There are two problems in your code -

1. In function ArrSwap() you are looping over index 0 to 5. So, when i becomes 5, Arr[i+1] points to Arr[6] which leads to read from a memory past your array bounds. Which is undefined behavior.
2. Secondly, if I get your requirement correctly, you want to swap the first and second elements swapped, then third and fourth and so on. So, your code has a logic error. In that case, in ArrSwap() the loop needs to increment by 2 instead of 1.

So the code may look like this after both issues are addressed -

void ArrSwap(char *Arr[]){
    int i;
    for(i=0; i<5; i+=2) {
        char *temp = Arr[i];
        Arr[i]=Arr[i+1];
        Arr[i+1] = temp;
    }
}

Notice there are following changes done in the ArrSwap() function -

• The loop test condition changed from i < 6 to i < 5 (the reason mentioned in point 1 above). As ikegami mentioned, best practice would be to pass the array size as an argument (say n) and use n - 1 in loop condition.
• The logic is fixed (as mentioned in point 2 above)
• For swap, use char * instead of void * (The type of elements in Arr is char *)
• The return type of ArrSwap() is mentioned as void. With void it is clearly conveyed that ArrSwap() will not return anything.

After fix the inputs will be:

{"Fly high dragonfly", "Kangaroo my shoe", "Jumping Ape", "Philosophical Monkey", "Tall Giraffes", "Fearful lemurs"}