Reversing a string in c, problem using strlen function

Question

I wrote a code to reverse a string, but the strlen function is giving me the wrong length of string, that's why the reversing of the string is not done properly. Here is the code I wrote:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

void reversestring(char string[], int start, int end);

int main() {
    char str[500];
    int n;
    n = strlen(str);
    reversestring(str, 0, n - 1);
    printf("%d\n", n);
    printf("The reverse string is %s", str);
    return 0;
}

void reversestring(char string[], int start, int end) {
    printf("enter the string:\n");
    scanf("%s", string);
    int temp;
    while (start < end) {
        //printf("insidewhile\n");
        temp = string[start];
        string[start] = string[end];
        string[end] = temp;
        start++;
        end --;
    }
}

Answer 1

strlen() can´t give you the length of a string, when its argument does not point to a valid string, which is the case on your example:

char str[500];
int n;
n = strlen(str);

str isn´t initialized with a string.

Providing a pointer to a char array which doesn´t contain a string as argument to strlen() causes undefined behavior.

Also strlen() doesn´t return an int . It´s return value is of type size_t .

Also use fgets() instead of scanf() when input a string. It is a little bit more safe.

---

Solution:

Let the string get entered in main() into str , then use strlen() and thereafter call the reversestring() function:

char str[500];         
size_t n;

printf("enter the string:\n");
fgets(str,500,stdin);

n = strlen(str);   
reversestring(str, 0, n-1);

I also edited the function declaration and the printf() accordingly to take care of the size_t type.

---

Here is the full code ( Online Example ):

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

void reversestring(char string[], size_t start, size_t end);

int main()
{
    char str[500];
    size_t n;

    printf("enter the string:\n");
    fgets(str,500,stdin);
    str[strcspn(str, "\n")] = 0;    // removing trailing newline from fgets

    n = strlen(str);   
    reversestring(str, 0, n-1);

    printf("%zu\n", n);
    printf("The reverse string is %s", str);
    return 0;
}


void reversestring(char string[], size_t start, size_t end)
{
    int temp;
    while(start < end)
    {   //printf("insidewhile\n");
        temp = string[start];
        string[start] = string[end];
        string[end] = temp;
        start++;
        end --;
    }
}

Output:

enter the string:             
helloworld     
10                     
The reverse string is dlrowolleh

Answer 2

You should use strlen after the initialization of str and set a correct size of third argument of reversestring :

size_t max_length = 128;

reversestring(str, 0, max_length);
n = strlen(str);

Answer 3

You have no string in the declared character array

{   char str[500];
    int n;

The character array is not initailzied. So this call

n = strlen(str);

results in undefined bahavior.

This code snippet from the function reversestring

printf("enter the string:\n");
scanf("%s", string);

shall be outside the function and used before the function is called.

The function should be declared at least like

char * reversestring( char string[], size_t n );

There is no need to declare the function with three parameters like you did

void reversestring(char string[], int start, int end);

because a call to this function can be substituted for the call of the previous shown function like

reversestring( string + start, end - start + 1 );

Also pay attention to that using the format %s does not allow to enter a sentence. Moreover using this format specifier with the function scanf is unsafe, And the function strlen has the return type size_t .

Here is the function definition

char * reversestring( char s[], size_t n )
{
    for ( size_t i = 0; i < n / 2; i++ )
    {
        char c = s[i];
        s[i] = s[n-i -1];
        s[n-i-1] = c;
    }

    return s;
}

Here is a demonstrative program.

#include <stdio.h>
#include <string.h>

char * reversestring( char s[], size_t n )
{
    for ( size_t i = 0; i < n / 2; i++ )
    {
        char c = s[i];
        s[i] = s[n-i -1];
        s[n-i-1] = c;
    }

    return s;
}

int main(void) 
{
    enum  { N = 500 };
    char s[N];

    printf( "Enter a string: " );

    fgets( s, N, stdin );

    s[ strcspn( s, "\n" ) ] = '\0';

    size_t n = strlen( s );

    puts( reversestring( s, n ) );

    char *p = strchr( s, ' ' );

    if ( p != NULL && ( p = strchr( p + 1, ' ' ) ) != NULL )
    {
        reversestring( s, p - s );
        puts( s );
    }

    return 0;
}

Its output might look for example like

Enter a string: Hello Shivam Gupta
atpuG mavihS olleH
Shivam Gupta olleH