Delete space error in C programming - c

Question

#include <stdio.h>
#include <stdlib.h>
#include <windows.h>
#pragma warning(disable:4996)
#define SIZE 100
int main(void){
char str[SIZE];
char str2[SIZE];
int i;
int len;
gets(str);
len = strlen(str);
for (i = 0; str[i] != NULL; i++) {
if (str[i] != ' '){
str2[i] = str[i];
}
}
for (i = 0; i < len; i++){
printf("%c", str2[i]);
}
return 0;
}
It returns the following error:
What is the problem?
I just want to copy some elements in str to str2 without spaces, but when I run, it has got some weird letters.

Answer 1

You need two index variables
one to go through str
one to tell where to write next to str2
code:
len = strlen(str);
int j;
for (i=0, j=0 ; str[i] != '\0' ; i++) {
if (str[i] != ' '){
str2[j++] = str[i]; // store to str2 and increment j
}
}
Then store a final \0 to str2 at index j
str2[j] = '\0';
Finally, to print the result, you can do that one shot
printf("%s\n", str2);
instead of printing one char at a time.

Answer 2

For starters these headers
#include <stdlib.h>
#include <windows.h>
can be removed because neither declaration from the headers is used in the program.
The function gets is unsafe and is not supported by the C Standard any more. Instead use standard C function fgets.
When str is copied in str2 you have to use separate index to access characters in str2 because some characters from str are not copied. Otherwise the array str2 will contain gaps. As result you can not use the previous value of the variable len with the array str2.
Also it is desirable not to copy any other white space characters.
The program can look the following way
#include <stdio.h>
#include <ctype.h>
#define SIZE 100
int main(void)
{
char str[SIZE];
char str2[SIZE];
fgets( str, sizeof( str ), stdin );
const char *p = str;
char *p2 = str2;
do
{
if ( !isspace( ( unsigned char )*p ) )
{
*p2++ = *p;
}
} while ( *p++ );
for ( p2 = str2; *p2; ++p2 )
{
printf( "%c", *p2 );
}
return 0;
}
Its output might be
Hello World
HelloWorld
If you do not study yet pointers then the program can look like
#include <stdio.h>
#include <ctype.h>
#define SIZE 100
int main(void)
{
char str[SIZE];
char str2[SIZE];
fgets( str, sizeof( str ), stdin );
size_t i = 0, j = 0;
do
{
if ( !isspace( ( unsigned char )str[i] ) )
{
str2[j++] = str[i];
}
} while ( str[i++] );
for ( j = 0; str2[j]; ++j )
{
printf( "%c", str2[j] );
}
return 0;
}