Program to reverse a string using pointers in C

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hello guys i am trying to make c program for reversing the string by returning the pointer to char from the function

What I have tried:

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

char* reversestring(char* input);
int main()
{
	int i;
	char* output;
	char inputstring[100];
	printf("enter the string\n");
	gets(inputstring);
	output = reversestring(inputstring);

i want to print the same string in both way that is by using printf and by running the loop please suggest me both way to print in same program

here i have taken i=0 to 4 for fixed string here i m assuming that user will enter a fixed string of length 5 char because i dont know the proper way

	printf("%s\n",output);
	for(i=0;i<4;i++) {
		printf("%c",(output+i));
	}
}

reverse function start from here

char* reversestring(char* input)
{
    int count=0,begin,end;

	char* output=(char*) malloc(100);
	while(*input!=0) {
		count++;
	}

	end=count-1;

	for(begin=0;begin<count;begin++) {
		*(output+begin)=*(input+end);
		 end--;
	}

	output[begin]='\0';
	return output;
}

there is no error and warning it is taking input only not giving the output(cursor is blinking only program is not ending)

Posted 11-Jun-18 2:34am

thakurcoder

Updated 11-Jun-18 3:05am

User 7429338

v2

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User 7429338 · Accepted Answer · 2018-06-11T03:03:00

Solution 1

This loop won't finish:

while(*input!=0) {
    count++;
}

You could replace it with:

while (*(input + count) != 0) {
    count++;
}

To print your reversed string character by character, use strlen[^]:

for (int i = 0; i<strlen(output); i++) {
    printf("%c", *(output + i));
}

Posted 11-Jun-18 3:03am

User 7429338

Comments

CPallini 11-Jun-18 9:12am

for (int i = 0; i<strlen(output); i++)
is quadratic on string length

[no name] 11-Jun-18 9:15am

That is a fair point. Since we're dealing with short strings here I went for the short/simple form, but for performance reasons it makes sense to put it in a separate variable first.

CPallini 11-Jun-18 12:07pm

My 5.

CPallini · Accepted Answer · 2018-06-11T03:07:00

In my opinion the memory allocation should be caller responsibility. Moreover, never ever use gets function.
Try

C

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

char * reverse( char * rev, const char *  str, size_t length )
{
  size_t n;
  for (n=0; n<length; ++n)
  {
    rev[n] = str[length-1-n];
  }
  rev[n] ='\0';
  return rev;
}

int main()
{
  char s[100];
  printf("please enter the string:\n");

  if ( ! fgets( s, sizeof(s), stdin) ) return -1;

  size_t length = strlen(s);

  char * r = (char *) malloc( length+1 );
  if ( ! r ) return -2;

  reverse( r, s, length);

  printf("reversed: %s\n", r);

  free(r);
  return 0;
}