Reversing an array using pointers - c++

Question

I have this program that makes and populates an array. Then it is sent to a function called reverse, which reverses the order in the array. The compiler keeps giving errors. I'm not quite sure why.
CODE
void reverse(int* array, int size) {
for (int i = 0; i < size/2; i++) {
int temp = array[i];
array[i] = array[size-i];
array[size-i] = temp;
} // end of for loop
} // end of reverse
int main( int argc, char** argv ) {
int array[8];
// get and print size of the array
int size = sizeof(array) / sizeof(array[0]);
printf("Size is %d\n", size);
// populate array
for (int i = 0; i < size; i++) {
array[i] = i;
} // end of for loop
// display array before reversing
for (int i = 0; i < size; i++) {
printf("%d ", array[i]);
} // end of for loop
// new line
printf("\n");
// reverse the array
reverse(&array, size);
// display the array again after reversing
for (int i = 0;i < size; i++) {
printf("%d ", array[i]);
} // end of for loop
} // end of main
It keeps giving me this error
main.cc:17:14: error: indirection requires pointer operand ('int' invalid)
int temp = *array[i];
^~~~~~~~~
main.cc:18:3: error: indirection requires pointer operand ('int' invalid)
*array[i] = *array[size-i];
^~~~~~~~~
main.cc:18:15: error: indirection requires pointer operand ('int' invalid)
*array[i] = *array[size-i];
^~~~~~~~~~~~~~
main.cc:19:3: error: indirection requires pointer operand ('int' invalid)
*array[size-i] = temp;
^~~~~~~~~~~~~~
4 errors generated.
make: *** [main.o] Error 1

Answer 1

I did solve this problem a little differently, maybe you will use this code:
#include <iostream>
void displayArray(int table[], int size);
void rev(int table[], int size);
void fillTheArray(int table[], int size);
int main(int argc, char** argv) {
int myArray[8];
int size = sizeof(myArray) / sizeof(myArray[0]);
std::cout << "Array size is: " << size << std::endl;
fillTheArray(myArray, size);
displayArray(myArray, size);
std::cout << std::endl;
rev(myArray, size);
displayArray(myArray, size);
std::cin.get();
return 0;
}
void fillTheArray(int table[], int size) {
for (int i = 0; i < size; i++) {
table[i] = i;
}
}
void displayArray(int table[], int size) {
for (int i = 0; i < size; i++) {
std::cout << table[i] << " ";
}
std::cout << std::endl;
}
void rev(int table[], int size) {
int *start = table;
int *end = table + (size - 1);
for (int i = 0; i < size; i++) {
if (start < end) {
int temp = *end;
*end = *start;
*start = temp;
}
start++;
end--;
}
}

Answer 2

I can see two errors in this code. First is: wrong way of passing parametr to function:
// reverse the array
reverse(&array, size);
you should do this just like this(array name is pointer to first element of this array):
reverse(array, size);
Second problem is with reverser - you try to access some random memory outside arrar range:
array[i] = array[size-i];
Remember that in C++ array index's start with 0 not 1. So if your array is size of 8 - largest insext of this array is 7 (0, 1, 2, 3, 4, 5, 6, 7). Your code should look like this:
array[i] = array[size -i -1];
And it should work as you expected.
It is my solution with pointers:
void reverse(int arr[], int count)
{
int* head = arr;
int* tail = arr + count - 1;
for (int i = 0; i < count/2; ++i)
{
if (head < tail)
{
int tmp = *tail;
*tail = *head;
*head = tmp;
head++; tail--;
}
}
for (int i = 0; i < count; ++i)
{
std::cout << arr[i] << " ";
}
}
or just use functions build in C++: std::reverse in 'algorithm' library.
It's a lot of examples on stackoverflow with this kind of examples:
Reverse Contents in Array

Answer 3

You have fixed most of the compiler errors in your code except one.
The line
reverse(&array, size);
should be
reverse(array, size);
After that is fixed, you have to fix logic errors in reverse.
You are using the wrong index for accessing the upper half of the array.
void reverse(int* array, int size) {
for (int i = 0; i < size/2; i++) {
int temp = array[i];
array[i] = array[size-i]; // When i is 0, you are accessing array[size]
// That is incorrect.
array[size-i] = temp;
} // end of for loop
} // end
You need to use
void reverse(int* array, int size) {
for (int i = 0; i < size/2; i++) {
int temp = array[i];
array[i] = array[size-i-1];
array[size-i-1] = temp;
}
}
Another way to approach the algorithm would be to use two indices.
void reverse(int* array, int size) {
for (int i = 0, j = size-1; i < j; ++i, --j) {
int temp = array[i];
array[i] = array[j];
array[j] = temp;
}
}
Working program: http://ideone.com/ReVnGR.

Answer 4

You're passing **int instead of *int to the reverse method:
reverse(&array, size);
Pass it like that:
reverse(array, size);