How to avoid garbage values at the time of output

[Ayan Dasgupta]

I am trying to code a C program which will print the reverse of an entered string.
My code is as follows:

#include <stdio.h>
#include <conio.h>
 
void reverse(char[]);
int mystrlen(char[]);
 
int mystrlen(char s[])
{
    int i;
    for(i=0;s[i]!='\0';i++);
    return i;
}
 
void reverse(char s[])
{
     int i;
     char rev[100];
     for(i=mystrlen(s);i>1;i--)
     rev[mystrlen(s)-i-1]=s[i];
     rev[mystrlen(s)]='\0';
     printf("%s", rev);
     }
 
int main()
{
    char str[100];
    printf("Enter a string:");
    fflush(stdin);
    gets(str);
 
    printf("The reverse of %s is:", str);
    reverse(str);
 
    getch();
    return 0;
}

However when I am entering Hello World, the output I am getting is dlroW ollA.

Please provide me a solution to overcome this.

[donbock]

Consider input string "Hello World\0". mystrlen ought to return 11 for this string.

Look at the loop in reverse:

for(i=mystrlen(s);i>1;i--)
   rev[mystrlen(s)-i-1]=s[i];
rev[mystrlen(s)]='\0';

Unroll the loop and we see that you have:

rev[-1] = s[11];   // '\0'
rev[0] = s[10];    // 'd'
rev[1] = s[9];     // 'l'
rev[2] = s[8];     // 'r'
rev[3] = s[7];     // 'o'
rev[4] = s[6];     // 'W'
rev[5] = s[5];     // ' '
rev[6] = s[4];     // 'o'
rev[7] = s[3];     // 'l'
rev[8] = s[2];     // 'l'
// Exit the loop.
rev[11] = '\0';

Notice that rev[9] and rev[10] are uninitialized. Writing to rev[-1] is a bad thing.

[Ayan Dasgupta]

i have modified the code to:

void reverse(char s[])
{
     int i;
     char rev[100];
     for(i=0;i<mystrlen(s);i++)
     rev[i]=s[mystrlen(s)-1-i];
     rev[i]='\0';
     printf("%s", rev);
     }

This has solved the problem. :)