Pointer Reversal Array

Answer 1

The following is an example of how to reverse an array using a pointer:

1. ** Basic train of thought **
- For an array, we can define two pointers, one pointing to the starting position of the array (the address of the first element), and the other pointing to the end position of the array (the address of the last element). Then, by swapping the elements pointed to by the two pointers and gradually moving the pointers closer to the middle, the array was reversed.

2. ** Code sample **
- Suppose we have an int array:
```c
#include
//Function used to reverse the array
void reverse(int* a, int length) {
int* i, * j, temp;
i = a;
j = a + length - 1;
//Loop the elements until the pointers meet or cross
while (i < j) {
temp = *i;
*i = *j;
*j = temp;
i++;
j--;
}
}
//function is used to print arrays
void printArr(int arr[], int length) {
for (int i = 0; i < length; i++) {
printf("%d ", arr[i]);
}
putchar('\n');
}
int main() {
int a[] = {1, 2, 3, 4, 5};
int length = sizeof(a)/sizeof(a[0]);
print ("Reverse the previous array: ");
printArr(a, length);
reverse(a, length);
print ("Array after reversal: ");
printArr(a, length);
return 0;
}
```
- In the above code, the `reverse` function accepts a pointer to an array `a` and the length of the array `length`. Inside the function, the `i` pointer is initialised to the first address of the array (namely `a`), and the `j` pointer is initialised to the last element address of the array (`a+length - 1`). Then, through the `while` loop, as long as the `i` pointer is smaller than the `j` pointer, the elements pointed to by `i` and `j` will be swapped. The `i` pointer will move backward by one bit, and the `j` pointer will move forward by one bit. Finally, in the `main` function, we first print out the original array, then call the `reverse` function to reverse the array, and then print out the inverted array.

- For arrays of other data types (such as `double`), the principle is the same, but the data type needs to be changed when the pointer and array are defined. For example:
```c
//Reverse the function of the double array
void Reverse_array(double* arrbegin, double* arrend) {
double temp;
while (arrbegin < arrend) {
temp = *arrbegin;
*arrbegin = *arrend;
*arrend = temp;
arrbegin++;
arrend--;
}
}
```
- Here, the `Reverse_array` function accepts the start pointer `arrbegin` and the end pointer `arrend` of the `double` array, and the array is reversed by exchanging the elements pointed to by the pointers.
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