Reversing a String in C using loops

Question

This code is for converting a decimal number in its equivalent in any base numeric system.

The reversed result is stored in temp.

#include <stdio.h>
#include <math.h>
int main(){
    long decimal;
    int b = 0,i = 0,n,j = 0;
    printf("Enter Decimal Number :");
    scanf("%ld",&decimal);
    char temp[decimal],base[decimal];
    printf("Enter Base :");
    scanf("%d",&b);
    for(;decimal != 0;decimal /= b){    
      n = decimal % b;
      if(n >= 0 && n <= 9)
      temp[i++] = n + 48;
      else if(n >= 10)
      temp[i++] = n + 55;
    }
    for(j = 0;i >= 0;) //Reversing temp and storing it in base
       base[j++] = temp[i--];
       printf("Base Equivalent : %s",temp);
       printf("Base Equivalent : %s",base);
    return 0;
}

The temp part need to be reversed to get the correct result, yet, temp does not get reversed and stored in base. What's wrong with my code?

You need a NULL terminator: `base[j]=0` – DYZ Nov 23 '21 at 18:09 — DYZ, Nov 23 '21 at 18:09

0___________ · Answer 1 · 2021-11-23T18:16:05.490

char temp[decimal],base[decimal];

wow. What if decimal is negative? Do you really need to allocate 4GB memory to convert 2147483647? Thank god you do not read long long int Then it would require 2*9 223372036854775807 bytes (many hundreds of petabytes) of memory

Use functions for similar tasks:

char *reverse(char *str)
{
    char *wrk = str, *end = str;

    if(str && *str)
    {
        while(*(end + 1)) end++;
        while(end > wrk)
        {
            char temp = *end;
            *end-- = *wrk;
            *wrk++ = temp;
        }
    }
    return str;
}

Adrian Mole · Answer 2 · 2021-11-23T18:26:03.307

The main problem is in the line base[j++] = temp[i--];. Note that the i-- expression returns the value of i before the decrement (and then decrements i). You need the predecrement operation, instead:

for (j = 0; i > 0;) // Note the change to i > 0
    base[j++] = temp[--i];

There is also a potential issue with the lengths of your character arrays. Why do you assume the value of the input number (decimal) will be appropriate for the length of those string?¹ Give them fixed lengths, instead – one that will be large enough to handle all cases. You also don't add the required nul terminators to your strings; this can be achieved by initialising them with 'empty' strings (will which set all elements to zero):

char temp[64] = "", base[64] = "";

¹ For example, for an input of 2 and a base of 2, the string required for the binary representation (10) will need to be 3 characters long (one for the terminator).

score 0 · Answer 3 · answered Nov 23 '21 at 18:13

strings in c must be null-terminated.
You just need to add '\0' to the end of both of your array to make it work.

e.g.:

#include <stdio.h>
#include <math.h>
int main(){
    long decimal;
    int b = 0,i = 0,n,j = 0;
    printf("Enter Decimal Number :");
    scanf("%ld",&decimal);
    char temp[64],base[64];
    printf("Enter Base :");
    scanf("%d",&b);
    printf("MM: %ld, %d", decimal, b);
    for(;decimal != 0;decimal /= b){    
      n = decimal % b;
      if(n >= 0 && n <= 9)
      temp[i++] = n + 48;
      else if(n >= 10)
      temp[i++] = n + 55;
    }

    // add null terminators
    temp[i] = '\0';
    base[i] = '\0';
    i--;

    for(j = 0;i >= 0;) //Reversing temp and storing it in base
       base[j++] = temp[i--];
       printf("Base Equivalent : %s",temp);
       printf("Base Equivalent : %s",base);
    return 0;
}

godbolt example

they should both be properly terminated, at least asan wasn't screaming about it :D — Turtlefight, Nov 23 '21 at 18:20

Reversing a String in C using loops

3 Answers3