scanf with %s and no ampersand

Why & is not used

In scanf("%s", str), str is the name of a character array, and the name of an array is already the address of its first element. scanf needs the address of the place where the value is to be stored, so the array name alone is enough. Writing &str is not needed (it has the same address but a different type). For ordinary variables like int n, the address must be given explicitly as &n.

Can a string with spaces be entered?

No. %s reads characters until the first white-space (space, tab or new line) and stores only that word. The rest stays in the input buffer.

#include <stdio.h>
int main(void)
{
    char str[40];
    printf("Enter a string: ");
    scanf("%s", str);          /* no & : str is already an address */
    printf("Stored: %s\n", str);
    return 0;
}

Input Kathmandu Valley gives:

Enter a string: Stored: Kathmandu

To read a string with spaces use a scan set %[^\n], or fgets:

#include <stdio.h>
int main(void)
{
    char str[40];
    scanf("%39[^\n]", str);    /* reads a line including spaces */
    printf("Stored: %s\n", str);
    return 0;
}

Output for the same input:

Stored: Kathmandu Valley