Reverse of a string using pointers without using string handling functions? [closed] - c

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I have updated the code as per the comments. As I am not near a PC, could you please check it and update me on that?
#include<stdio.h>
int main()
{
char s[100],rev[100],*ps1,*ps2,temp,*ptemp=&temp;
int i,n=0;
printf("Enter a string: ");
scanf("%s",s);
printf("The string is: %s\n",s);
for(i=0;s[i]!='\0';i++)
n++;
ps1=&s[0];
ps2=&s[n-1];
for(i=0;i<n/2;i++)
{
*ptemp=*ps1;
*ps1=*ps2;
*ps2=*ptemp;
*ps2--;
*ps1++;
}
printf("The reverse of the string is: %s\n",s);
}

Answer 1

Trying to keep code the closer to yours and making it works:
#include<stdio.h>
int main()
{
char s1[100], rev[100], *ps1=s1, *prev=rev;
int value1, value2 = 0;
int *i = &value1, *length = &value2; // Why the hell do you need pointers? But if you do want them, use them the right way
printf("Enter a string: ");
scanf("%99s", s1); // parameter must be a 'char *', not a 'char **'
printf("The string is: %s\n",s1); // Ends line with \n
for(*i=0;*(ps1+(*i))!='\0';(*i)++)
{
(*length)++;
}
printf("The length is: %d\n", *length);
for(*i=1;*(ps1+(*i)-1)!='\0';(*i)++)
{
*(prev+(*i)-1)=*(ps1+(*length)-(*i));
}
*(rev+(*i)-1)='\0';
printf("\nThe reverse string is: %s\n",rev); // Ends line with \n
}
But I must confess your last loop is really hard to understand. Also, why do you need those pointers?
Edit: Also you should check return value of scanf() is 1. Because if is is not, it means scanf() did not set a value to s1.
Edit 2: Correction of the new version of the question
#include<stdio.h>
int main()
{
char s[100],*ps1,*ps2,temp,*ptemp=&temp; // removing unused rev
int i,n=0;
printf("Enter a string: ");
scanf("%s",s);
printf("The string is: %s\n",s);
for(i=0;s[i]!='\0';i++)
n++;
ps1=&s[0]; // ps1 = s; would be fine
ps2=&s[n-1]; // ps2 = s + (n-1); would be fine
for(i=0;i<n/2;i++)
{
*ptemp=*ps1;
*ps1=*ps2;
*ps2=*ptemp;
ps2--; // You want ps2 point to previous character
ps1++; // You want ps1 point to next character
}
printf("The reverse of the string is: %s\n",s);
}

Answer 2

The first problem I see is that scanf expects a pointer, but you give it a pointer to a memory address which is effectively a pointer to pointer:
scanf("%s", &s1);
Should be changed to:
char s1[100] = {0};
scanf("%s", s1);
As scanf does not check for memory overflow, it would be a wiser choice to make use of this syntax over the above suggested one
char s1[100] = {0};
scanf("%99s", s1);
This way, you don't get into any buffer overflow issues
Next problem, you dereference a pointer before assigning it a valid location in memory:
for(*i=0;*(ps1+(*i))!='\0';(*i)++)
Change to:
int i;
for(i = 0; ps1[i] != '\0'; i++)
Do the same for your other for-loop
Finally, remove this line:
*(rev+(*i)-1)='\0';
And change the print statement to:
printf("\nThe reverse string is: %s",s1);