String works but prints random symbols - c

Question

I want my program to display reversed string using pointers. It is working but it is also showing random symbols. Soo.. how can I make it show only the reversed string?
Example: input - hello , output - olleh.
#include <stdio.h>
int main(void)
{
char string[30];
printf("Enter string: ");
scanf("%s",&string);
int x =sizeof(string)/sizeof(string[0]);
char *p1 = string;
char *p2 = &string[x-1];
char temp;
while(p1<=p2)
{
temp = *p1;
*p1 = *p2;
*p2 =temp;
p1++;
p2--;
}
for(p1 =&string[0];p1<=&string[x - 1];p1++)
{
printf("%c",*p1);
}
return 0;
}

Answer 1

the problem is in calculating the string length
it does not matter the length of input this statement
int x =sizeof(string)/sizeof(string[0]);
will always return 30.
you can do two things to solve this problem
you can #include<string.h> and use strlen(string) to get the actual length.
you can write your own function that calculates the length and it will look like this
int string_length(char s[]) {
int c = 0;
while (s[c] != '\0')
c++;
return c;
}
Sample of working code using strlen
#include <stdio.h>
#include <string.h>
int main(void)
{
char string[30];
printf("Enter string: ");
scanf("%s",string);
int x =strlen(string);
printf("%d\n", x); // here the length of the input will be printed.
char *p1 = string;
char *p2 = &string[x-1];
char temp;
while(p1<=p2)
{
temp = *p1;
*p1 = *p2;
*p2 =temp;
p1++;
p2--;
}
for(p1 =&string[0];p1<=&string[x - 1];p1++){
printf("%c",*p1);
}
return 0;
}
for input "asd" the result will be "dsa".
Sample of working code using string_length function
#include <stdio.h>
int string_length(char s[]) {
int c = 0;
while (s[c] != '\0')
c++;
return c;
}
int main(void)
{
char string[30];
printf("Enter string: ");
scanf("%s",string);
int x =string_length(string);
printf("%d\n", x); // here the length of the input will be printed.
char *p1 = string;
char *p2 = &string[x-1];
char temp;
while(p1<=p2)
{
temp = *p1;
*p1 = *p2;
*p2 =temp;
p1++;
p2--;
}
for(p1 =&string[0];p1<=&string[x - 1];p1++){
printf("%c",*p1);
}
return 0;
}
for input "asd" the result will be "dsa".

Answer 2

This call of scanf
scanf("%s",&string);
is incorrect. The second argument has the type char( * )[30] instead of char *.
You should write
scanf("%29s",string);
The variable x keeps the size of the array itself instead of the stored string
int x =sizeof(string)/sizeof(string[0]);
Instead you should write
#include <string.h>
//...
size_t x = strlen( string );
The condition in the while loop should look like
while(p1 < p2)
instead of
while(p1<=p2)
because there is no great sense to swap a character with it itself.
The program can look the following way
#include <stdio.h>
#include <string.h>
int main(void)
{
char string[30];
printf("Enter string: ");
if ( scanf( "%29s", string ) == 1 )
{
char *p1 = string;
char *p2 = string + strlen( string );
if ( p1 != p2 )
{
for ( ; p1 < --p2; ++p1 )
{
char c = *p1;
*p1 = *p2;
*p2 = c;
}
}
for ( p1 = string; *p1 != '\0'; ++p1 )
{
printf( "%c", *p1 );
}
putchar( '\n' );
}
return 0;
}
You could write a separate function that reverse a string using pointers. Just move the code shown in the demonstration program in a function. For example
#include <stdio.h>
#include <string.h>
char * reverse_string( char *s )
{
char *p1 = s;
char *p2 = s + strlen( s );
if ( p1 != p2 )
{
for ( ; p1 < --p2; ++p1 )
{
char c = *p1;
*p1 = *p2;
*p2 = c;
}
}
return s;
}
int main(void)
{
char string[30];
printf("Enter string: ");
if ( scanf( "%29s", string ) == 1 )
{
puts( reverse_string( string ) );
}
return 0;
}

Answer 3

Apart from abysmal formatting of the code, the "right end" of the "string" was set to the limit of the buffer size, not the actual end of the string entered by the user.
Here's a cleaned-up, annotated version for your consideration:
#include <stdio.h>
int main() {
char buf[100]; // name variable better
printf("Enter string: ");
fgets( buf, sizeof buf, stdin ); // use fgets(), not scanf()
char *pS = buf, *pE = buf; // two pointers, initialised
// find the end of the string with-or-without LF
while( *pE && *pE != '\n' ) pE++; // newline
if( *pE == '\n' ) *pE = '\0'; // clobber possible newline
pE--;
// swap characters, meeting in the middle
while( pS < pE ) {
char tmp = *pE;
*pE-- = *pS;
*pS++ = tmp;
}
printf( "%s\n", buf ); // output
return 0;
}
You can guess what the output is like.
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