Getting garbage after reversing string in c

Question

I am trying to reverse a string. scanf is working well but when I use fixed string then it gives garbage value. So where is the fault ?

#include<stdio.h>
#include<string.h>
int main()
{
    char s[50]="Hi I Love Programming";
    char rev[strlen(s)];
    int i,k;



    k=strlen(s);
    for(i=0; i<strlen(s); i++)
    {
        rev[k]=s[i];
    k--;
    }

    printf("The reverse string is: %s\n", rev);
}

Answer 1

Your program has two issues:

1.

char rev[strlen(s)];

You forgot to add an element for the string-terminating null character '\0'.

Use:

char rev[strlen(s) + 1];

---

Furthermore you also forgot to append this character at the end of the reversed string.

Use:

size_t len = strlen(s);
rev[len] = '\0';

Note, my len is the k in your provided code. I use the identifier len because it is more obvious what the intention of that object is. You can use strlen(s) because the string has the same length, doesn´t matter if it is in proper or reversed direction.

---

2.

k=strlen(s);

for(i=0; i<strlen(s); i++)
{
    rev[k]=s[i];
    k--;
}

With rev[k] you accessing memory beyond the array rev, since index counting starts at 0, not 1. Thus, the behavior is undefined.

k needs to be strlen(s) - 1.

---

Three things to note:

1. The return value of strlen() is of type size_t, so an object of type size_t is appropriate to store the string length, not int.

2. It is more efficient to rather calculate the string length once, not at each condition test. Use a second object to store the string length and use this object in the condition of the for loop, like i < len2.

3. char s[50]="Hi I Love Programming"; can be simplified to char s[]="Hi I Love Programming"; - The compiler automatically detects the amount of elements needed to store the string + the terminating null character. This safes unnecessary memory space, but also ensures that the allocated space is sufficient to hold the string with the null character.

---

The code can also be simplified (Online example):

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

int main(void)
{
    char s[] = "Hi I Love Programming";
    size_t len = strlen(s);

    char rev[len + 1];
    size_t i,j;

    for(i = 0, j = (len - 1); i < len; i++, j--)
    {
        rev[j] = s[i];
    }

    rev[len] = '\0';

    printf("The reverse string is: %s\n", rev);
}

Output:

The reverse string is: pgnimmargorP evoL I iH

Answer 2

your program is hard to understand. Here you have something much simpler (if you want to reverse the string of course)

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

char *revstr(char *str)
{
    char *start = str;
    char *end;

    if(str && *str)
    {
        end = str + strlen(str) - 1;
        while(start < end)
        {
            char tmp = *end;
            *end-- = *start;
            *start++ = tmp;
        }
    }
    return str;
}


int main()
{
    char s[50]="Hi I Love Programming";

    printf("%s", revstr(s));
}

https://godbolt.org/z/5KX3kP