How to get rid of garbage characters in this program? - c

Question

This program is supposed to print an input string backwards. Every single time it happens, though, I get garbage characters such as \340 or of the like. Why is it doing that? Here's my code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main()
{
char mattsentence[51];
mattsentence[50] = '\0';
gets(mattsentence);
char mask[sizeof(mattsentence)];
int i, j;
j = sizeof(mattsentence);
for (i = 0; i < sizeof(mask); i++)
{
j = j - 1;
mask[i] = mattsentence[j];
printf("%c", mask[i]);
}
printf("\n");
}

Answer 1

Your approach is wrong because you reverse the entire character array while it can be filled only partially. You should use standard C function strlen declared in header <string.h> that to determine the size of the entered string. Also to use gets is unsafe because you can overwrite memory beyond the character array. It now is excluded from the C Standard
Here is shown how the program can be written.
#include <stdio.h>
#include <string.h>
#define N 51
int main(void)
{
char mattsentence[N] = { '\0' };
char mask[N] = { '\0' };
fgets( mattsentence, sizeof( mattsentence ), stdin );
size_t n = strlen( mattsentence );
if ( n != 0 && mattsentence[n-1] == '\n' ) mattsentence[--n] = '\0';
for ( size_t i = 0; n != 0; i++ )
{
mask[i] = mattsentence[--n];
printf( "%c", mask[i] );
}
printf( "\n" );
return 0;
}
If to enter
Hello, Christiana S. F. Chamon
then the program output will be
nomahC .F .S anaitsirhC ,olleH
Take into account that to output a string in the reverse order there is no need to define a second character array.
If you want only to output the source string in the reverse order then the program can look like
#include <stdio.h>
#include <string.h>
#define N 51
int main(void)
{
char mattsentence[N] = { '\0' };
fgets( mattsentence, sizeof( mattsentence ), stdin );
size_t n = strlen( mattsentence );
if ( n != 0 && mattsentence[n-1] == '\n' ) mattsentence[n-1] = '\0';
while ( n != 0 )
{
printf( "%c", mattsentence[--n] );
}
printf( "\n" );
return 0;
}

Answer 2

sizeof() operator gives the size of the datatype. So, sizeof(mattsentence) will give you a value of 51. Then, sizeof(mask) will give you 51 again.
When you use that sizeof(mask) as for loop condition, you're basically going past the actual input values, thus pritning out garbage values.
What you want here is to use strlen() to find out the actual valid length of the entered string.
So, basically you need to take care of
Point 1: replace sizeof with strlen().
Point 2: Use of gets() is dangerous. Please use fgets() instead of gets().
Point 3: int main() should be int main(void). Put an expilicit return statement at the end of main(). Good Practice.
The modified code should look like
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(void)
{
char mattsentence[51] = {0}; //always initalize local variables, here it's covering null termination , too.
fgets(mattsentence, 51, stdin); //fgets()
char mask[strlen(mattsentence) + 1]; // one more to store terminating '\0'
int i = 0, j = 0, k = 0;
j = strlen(mattsentence);
k = j;
for (i = 0; i < k; i++) // make use of k, don't call `strlen()` repeatedly
{
j = j - 1;
mask[i] = mattsentence[j];
printf("%c", mask[i]);
}
mask[i] = '\0'; // for proper string termination
printf("\n");
printf("%s\n", mask);
return 0; //added return statement
}

Answer 3

See changed code:
int main()
{
char mattsentence[51];
mattsentence[0] = '\0'; // initialization
gets(mattsentence);
char mask[strlen(mattsentence) + 1]; // +1 for string terminator '\0'
int i, j;
j = strlen(mattsentence);
for (i = 0; i < strlen(mattsentence); i++) // strlen of original string
{
j = j - 1;
mask[i] = mattsentence[j];
printf("%c", mask[i]);
}
mask[i] = '\0'; // for proper string termination
printf("\n");
printf("%s\n", mask);
}
There are several errors:
strlen() should be used to get length of string
for loop should be controlled according to input string, not output string
it is better to use fgets() instead of gets(): that way you can control how many character will be read from the input