Reverse string with external function - c

Question

I recently started a course in Programming in C. I got an assignment where I'm going to make an external function with this declaration:
void reverse(char *array, int numberOfChars)
My code looks like this at the moment. The problem is that the code will just reverse the first half of the word, and keep the second half as normal.
#include <stdio.h>
#include <string.h>
void reverse(char *array, int numberOfChars) {
int i = 0;
int j = 0;
for (i = numberOfChars - 1; i >= 0; i--) {
array[j] = array[i];
j = j + 1;
}
}
#define NAMESIZE 100
int main() {
char word[NAMESIZE];
int length;
printf("Enter a word: ");
scanf("%s", word);
getchar();
length = strlen(word);
reverse(word, length);
printf("%s\n", word);
}

Answer 1

Even when you use an intermediate variable, your code will reverse the string twice, leaving it as it was. A small change to the code compares i with j as well as using a temporary variable to do the swap.
int i, j = 0;
char temp;
for(i = numberOfChars-1; i > j; i--){
temp = array[j];
array[j] = array[i];
array[i] = temp;
j = j + 1;
}

Answer 2

Your function has a classic problem: you are reversing the string in place. You overwrite the characters at the start of the string with the characters from the end. The initial characters are lost by the time you reach the mid point. You should instead swap the characters from both ends and stop at the mid point.

Answer 3

You are going to need a secondary char array to reverse the string.Try like this:
#include <stdio.h>
#include <string.h>
#define NAMESIZE 100
void reverse(char *arr,int numberOfChars);
int main()
{
char word[NAMESIZE];
printf("Enter a word: ");
scanf("%s",word);
getchar();
size_t length=strlen(word);
reverse(word,length);
printf("%s\n",word);
}
void reverse(char *arr,int numberOfChars)
{
char buffer[NAMESIZE];
int j = 0,i = numberOfChars;
while( i-- )
buffer[j++] = arr[i];
buffer[j] = '\0';
strcpy(arr,buffer);
}

Answer 4

A simple way to prevent overwriting your array is to copy your array into a temporary one and write the reversed readed temporary array into the other.
If you want to have an unknown array size at compilation to reverse, I recommend you to use a heap array (see man malloc) instead of stack one to prevent segmentation fault if numberOfChars is too hight.