Trying to reverse a C-string but it is not working - c

Question

I am trying to reverse this C-string and I thought I did it correct but the string remains the same when it passes through the function.
#include <stdio.h>
char* reverse(char* string);
int main(int arc, char* argv[]) {
char word[] = "Hello World!";
printf("%s\n", word);
printf("%s\n", reverse(word));
return 0;
}
char* reverse(char* string) {
int i, j, n = 0;int len = 0;char temp;
//Gets string length
for (i = 0; *(string + i) != '0'; i++) {
len++;
}
//Reverses string
for (j = len - 1; j >= 0; j--) {
temp = string[n];
string[n] = string[j];
string[j] = temp;
n++;
}
return &string[0];
}
Expected output:
Hello World!
!dlroW olleH

Answer 1

For starters there is a typo
for (i = 0; *(string + i) != '0'; i++) {
You have to write
for (i = 0; *(string + i) != '\0'; i++) {
That is instead of the character '0' you have to use '\0' or 0.
In this for loop
for (j = len - 1; j >= 0; j--) {
temp = string[n];
string[n] = string[j];
string[j] = temp;
n++;
}
the string is reversed twice.:) As a result you get the same string.
The function can look for example the following way
char * reverse(char *string)
{
//Gets string length
size_t n = 0;
while ( string[n] != '\0' ) ++n;
//Reverses string
for ( size_t i = 0, m = n / 2; i < m; i++ )
{
char temp = string[i];
string[i] = string[n - i - 1];
string[n - i - 1] = temp;
}
return string;
}
Or the function can be defined the following way using pointers
char * reverse(char *string)
{
//Gets string length
char *right = string;
while ( *right ) ++right;
//Reverses string
if ( right != string )
{
for ( char *left = string; left < --right; ++left )
{
char temp = *left;
*left = *right;
*right = temp;
}
}
return string;
}
The same approach of the function implementation without using pointers can look the following way
char * reverse(char *string)
{
//Gets string length
size_t n = 0;
while ( string[n] != '\0' ) ++n;
//Reverses string
if ( n != 0 )
{
for ( size_t i = 0; i < --n; ++i )
{
char temp = string[i];
string[i] = string[n];
string[n] = temp;
}
}
return string;
}
Here is one more solution. I like it most of all. Tough it is inefficient but it is not trivial as the early presented solutions. It is based on an attempt of one beginner to write a function that reverses a string.:)
#include <stdio.h>
#include <string.h>
char *reverse( char *string )
{
size_t n = 0;
while (string[n]) ++n;
while (!( n < 2 ))
{
char c = string[0];
memmove( string, string + 1, --n );
string[n] = c;
}
return string;
}
int main( void )
{
char string[] = "Hello World!";
puts( string );
puts( reverse( string ) );
}
The program output is
Hello World!
!dlroW olleH
Of course instead of manually calculating the length of a string in all the presented solutions there could be used standard string function strlen declared in the header <string.h>.

Answer 2

The problem is that the input word[] is an array, which decays to a pointer when passed to the reverse function.
In the for loop, instead of using n to keep track of the position, I suggest you to use i and j to keep track of the start and end of the string, and increment and decrement them respectively and use strlen to get the length of string.
Also, as it is mentionned above by #Vlad from Moscow, in your for loop you are checking for 0 but it should be \0 which is the null character.
Please find down below an update of your posted code that is generating the expected result :
#include <stdio.h>
char* reverse(char* string);
int main(int arc, char* argv[]) {
char word[] = "Hello World!";
printf("%s ", word);
printf("%s\n", reverse(word));
return 0;
}
char* reverse(char* string) {
int i, j;
char temp;
int len = strlen(string);
//Reverses string
for (i = 0, j = len - 1; i < j; i++, j--) {
temp = string[i];
string[i] = string[j];
string[j] = temp;
}
return &string[0];
}
The output is as expected: Hello World! !dlroW olleH
Aditionnally, you can include the header <string.h> or explicitly
provide a declaration for 'strlen' to avoid the warning that indicate to implicitly declaring library function 'strlen' with type 'unsigned long (const char *)' [-Wimplicit-function-declaration]