Format Specifiers — Controlling Output in C

C in 100 Seconds, Episode 3

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printf doesn't just print values — it formats them. Format specifiers give you precise control over how numbers, strings, and addresses appear on screen.

The Basics

int n = 42;
float pi = 3.14159;
char name[] = "Alice";

Each type has a matching specifier:

printf("%d\n", n);       // 42 — decimal integer
printf("%x\n", n);       // 2a — hexadecimal
printf("%f\n", pi);      // 3.141590 — float (6 decimals default)
printf("%.2f\n", pi);    // 3.14 — float (2 decimals)
printf("%s\n", name);    // Alice — string
printf("%p\n", &n);      // 0x7ff... — memory address

Width and Alignment

You can control column width with a number between % and the specifier:

printf("[%10d]\n", n);   // [        42] — right-aligned, 10 wide
printf("[%-10d]\n", n);  // [42        ] — left-aligned, 10 wide

The - flag flips the alignment. This is how you build aligned tables and formatted reports in C.

Why Does This Matter?

Format specifiers are not optional in C — they're the only way to convert data to text. Higher-level languages handle this automatically. In C, you're telling printf exactly how to interpret each argument's bits.

Get the specifier wrong (like using %d for a float) and you won't get an error — you'll get silent, incorrect output.

Full Code

#include <stdio.h>

int main() {
  int n = 42;
  float pi = 3.14159;
  char name[] = "Alice";

  printf("%d\n", n);
  printf("%x\n", n);
  printf("%f\n", pi);
  printf("%.2f\n", pi);
  printf("%s\n", name);
  printf("%p\n", &n);
  printf("[%10d]\n", n);
  printf("[%-10d]\n", n);

  return 0;
}

Compile and Run

gcc format.c -o format
./format

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Next episode: Arithmetic Operators — doing math in C (and why integer division might surprise you).

Student code: github.com/GoCelesteAI/c-in-100-seconds/episode03