reversing a string in c without library function - c

Question

In this program I am trying to take a string as input from the user guessing that the maximum length of the string here is 40 (obviously can exceed).
I'm finding out the length and using the length to create another character array dynamically( only to prevent assigning this array with any random value) and finally, adding the characters from the last to get the reversed string.
It compiles just fine, but upon running, provides no output.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main()
{
char word[40];
char *rev;
int i=0;
int l=0;
printf("Enter any statement \n");
scanf("%[^\n]", word);
while(word[i]!='\0')
{
i++;
}
// i contains the length of the string
rev=(char *)malloc(i*sizeof(char));
while(i>0)
{
rev[l]=word[i];
i--;
l++;
}
printf("\n %s", rev);
return 0;
}

Answer 1

You have two (at least) problems: The first one is that you swap characters twice. The second problem is that you also include the terminator character in your swapping.

Answer 2

I believe you have a 'one-off' type error and are not seeing any output because the teminating \0 character is being copied to the first position of rev.

Answer 3

rev[l]=word[i]; should rev[l]=word[i-1];

Answer 4

Point 1:-
In C a string is always ended with '\0'(NULL Termination),you should always manually insert a '\0' at the end,which i think you forgot.But no worries even i did not remember it as well when i started...:) ,Experiences like these will make you from next time.
Point 2:-
Assume you have a string char str[]="ABCD" ;,In memory this would look like this
------------IN Memory----------------
| 'A' || 'B' || 'C' | | 'D' | | '\0' |
-------------------------------------
0 1 2 3 4
Just by looking we can say that its length would be 4 (we know that we don't include '\0' when calculating length). For copying in reverse we need to start with character 'D' whose index is 3,that means you have to start copying from 2nd last character whose index can be can be found from length of string -1.The mistake you did in your above program is you have used length of string (which is 4), to start copying.which copies '\0' as first character in rev[] ,Hence it provides no output.
Other points:-
Based on the above two points i have corrected the mistakes, see comments in the below program to understand it well.And i have used int j;instead of int l; because i felt so confusing.
and please use width specifier when reading strings using scanf(); In your while loop,you should change the condition from i>0 to i>=0 because when i reaches 0,i>0 condition will break the loop before copying the first character from word[] to rev[].
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main()
{
char word[40];
char *rev;
int i=0;
int j=0;
printf("Enter any statement \n");
scanf("%39[^\n]", word);//using width specifier,(POINT 1)here 39 because sizeof(word)-1, one left to accommodate '\0' at end.
while(word[i]!='\0')//calculating length of string
{
i++;
}
rev=(char *)malloc(i*sizeof(char)+1);// (POINT 1),here making +1 to accommodate '\0' at end.
i=i-1;//(POINT 2)- Last index of the array is always length of string - 1
while(i>=0)//This should be i>=0 because you also want to copy first character of input string.
{
rev[j]=word[i];
i--;
j++;
}
rev[j]='\0';//(POINT 1) You should end with '\0'.
printf("\n %s", rev);
return 0;
}

Answer 5

Your scanf format string looks weird. Perhaps you are using some strange feature of it that I'm unaware of, so I'll pass on that. But you need to make sure that word is a null terminated string after the user input is done.
while(word[i]!='\0')
This loops counts until it finds the null term. So i will contain the string length: you just invented the strlen() function. It will not contain the string length plus the null terminator length (1).
Therefore
rev=(char *)malloc(i*sizeof(char));
is incorrect, you need to allocate i+1 bytes.
Also, you shouldn't cast the result of malloc, because doing so is completely pointless.
And when you copy, you have to ensure that you also copy the null terminator.

Answer 6

Below code with just one modification should return just correct answer. I changed in 2nd while loop from:
rev[l]=word[i];
to
rev[l]=word[i-1];
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main()
{
char word[40];
char *rev;
int i=0;
int l=0;
printf("Enter any statement \n");
scanf("%[^\n]", word);
while(word[i]!='\0')
{
i++;
}
// i contains the length of the string
rev=(char *)malloc(i*sizeof(char));
while(i>0)
{
rev[l]=word[i-1];
i--;
l++;
}
printf("\n %s", rev);
return 0;
}

Answer 7

The code has several memory mistakes and using static memory is also an issue. But for if you are using for practice purpose below are the changes and its explanation.
i--; // you can't place a '\0' to your rev[0] else it would always be NULL string when you print
while(i>=0)
{
rev[l]=word[i];
i--;
l++;
}
rev[l]='\0'; // string should be terminated by '\0'
printf("\n%s\n", rev);

Answer 8

//Reverse of a string without using string functions
#include<stdio.h>
#include<conio.h>
void main()
{
char a[40];
int j=0,len=0;
clrscr();
printf("please enter your name\n");
gets(a); // takes white spaces as characters
printf("the reverse of your name is\n");
while (a[j]!=NULL) // finding the length of the string array
{
len++;
j++;
}
for(int k=len;k>=0;k--)
{
printf("%c",a[k]);
}
getch();
}
Output - please enter your name
dwayne rock johnson
the reverse of your name is
nosnhoj kcor enyawd

Answer 9

As noted, array indices begin at zero and end at length-1. Moreover, if you like, you could rewrite your code in a more cryptic fashion:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main()
{
char word[40];
char *rev;
int i=0;
int l=0;
printf("Enter any statement \n");
scanf("%[^\n]", word);
while(word[++i]);
// i contains the length of the string
rev=(char *)malloc(i*sizeof(char));
while(rev[l++]=word[i---1], i>0);
printf("%s\n", rev);
return 0;
}