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Handling Whitespace in scanf ​

Consider a program that asks for a quantity and then a yes/no confirmation:

c
#include <stdio.h>

int main() {
    int quantity;
    char confirm;
    printf("Enter quantity: ");
    scanf("%d", &quantity);
    printf("Order %d items? (y/n): ", quantity);
    scanf("%c", &confirm);
    printf("You entered: '%c'\n", confirm);
}

When you run this program and answer the first prompt by typing a number like 5 then pressing Enter, the program skips right past the confirmation prompt:

Enter quantity: 5
Order 5 items? (y/n): You entered: '
'

The program never waited for you to type y or n. What's going on?

Why This Happens ​

When you type 5 and press Enter, the input buffer contains 5\n. The scanf("%d", &quantity) reads 5 but leaves \n in the buffer. Then scanf("%c", &confirm) immediately reads that leftover newline instead of waiting for new input.

This happens because %c reads exactly one character, including whitespace. Most other format specifiers like %d and %s automatically skip leading whitespace, but %c does not. This is intentional: if %c skipped whitespace, there would be no way to read a space or newline character.

The Solution ​

Add a space before %c in the format string:

c
scanf(" %c", &confirm); // Note the space before %c

A space in a scanf format string tells it to skip any amount of whitespace before reading the next value. This consumes leftover newlines or spaces before reading the character you actually want.

c
#include <stdio.h>

int main() {
    int quantity;
    char confirm;
    printf("Enter quantity: ");
    scanf("%d", &quantity);
    printf("Order %d items? (y/n): ", quantity);
    scanf(" %c", &confirm);  // Space before %c skips the leftover newline
    printf("You entered: '%c'\n", confirm);
}

Now the program works as expected:

Enter quantity: 5
Order 5 items? (y/n): y
You entered: 'y'

Quick Reference ​

Specifiers that skip leading whitespace: %d, %i, %f, %lf, %s, %u, %x, %o, %p

Specifiers that do NOT skip whitespace: %c, %[...] (scansets)

For the specifiers that don't skip whitespace, add a space before them when needed: " %c", " %[a-z]".

Alternative: Line-Based Input ​

For more complex input parsing, consider reading entire lines with fgets and parsing with sscanf. This avoids whitespace issues entirely and gives you more control over error handling:

c
char line[256];

fgets(line, sizeof(line), stdin);
sscanf(line, "%d", &quantity);

fgets(line, sizeof(line), stdin);
sscanf(line, "%c", &confirm);

See Also ​