reverse string in c incorrect output

Question

I was writing a program to reverse an entered string in C and my code is :

#include<stdio.h>
#include<string.h>
void main()
{
    int sz;
    printf("Enter the size of the string : ");
    scanf("%d",&sz);
    char str[sz];
    gets(str);
    printf("Enter the string : \n");
    gets(str);
    char str1[sz];
    int i =0;
    --sz;
    for(i=0;i<=sz;i++)    
    {
          str1[sz-i]=str[i];

    }
    printf("%s",str1);
}

well this program is giving an weird output for string sizes 8,9 and 10 for size 8 the reversed string is being printed followed by a space and 2 garbage characters,for size 9 the reversed string is being printed followed by 2 garbage characters and for size 10 the reversed string is being printed by a garbage character and for other string sizes the program is running properly. why is this happening?

@Antti Haapala Yeah I didn't null terminate them but why are these garbage characters being displayed only for string sizes 8,9and 10 why not for other string sizes also — Rittam Das, Dec 21 '17 at 08:19
because you didn't null-terminate. The behaviour is undefined then. — Antti Haapala -- Слава Україні, Dec 21 '17 at 08:20
Yeah I haven't null terminated it and that is true for string sizes less than 8 or more than 10 but why is the error occurring only for 8,9 and 10 and not for any other string size? — Rittam Das, Dec 21 '17 at 08:31
Do you need to store the reverse string in memory ? And although you can use variable sized array, you should consider how they would behave if `sz` is quite large, check where the string is terminating as you should not trust what the user is putting as input. I would recommend to overallocate the first string then use strlen to get en end of the string. — dvhh, Dec 21 '17 at 08:32
I don't understand one thing - you wish to reverse **one** string by your code. Then why do you have two `gets()` statements in your code? — Aditi Rawat, Dec 21 '17 at 08:56
[___DO NOT___ use `gets()`, it is dangerous. use `fgets()` instead.](https://stackoverflow.com/a/41383540/2173917) — Sourav Ghosh, Dec 21 '17 at 09:05

score 0 · Answer 1 · 2018-01-07T16:15:31.660

Note:

It is not smart to try to limit the input string by asking the user, the user can still write string with length greater/ less
Reading string from console does by default not allow spaces

First option:
Read a string of a static size:

 char original[5];
 scanf("%5s", original); // String containing 5 chars

Second option:
Read a string of variable size:

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

// Length of String
unsigned expected;

printf("Enter the length of the string: ");
scanf("%u", &expected);

char *temp; // Warning: Uninitialized

// Get string
printf("Enter the string: ");
scanf("%s", temp);

unsigned actual = strlen(temp);
unsigned length = actual > expected ? expected : actual;

char *string = (char *) malloc(length * sizeof(char));
char *reverse = (char *) malloc(length * sizeof(char));

 // Trim string to proper size
for (int i = 0; i < length; i++) {
  string[i] = temp[i];
}

// Reverse string
for (int i = 0, j = length - 1; i <= j; i++) {
  reverse[i] = string[j - i];
}

// Print Strings
printf("%s", string);
printf("\n%s", reverse);

reverse string in c incorrect output

1 Answers1