C Format Specifiers: Beginner-Friendly Guide

In C programming, format specifiers are used to tell the compiler the type of data to expect for input or output. They are mainly used with functions like printf() and scanf().

Let’s break it down step-by-step.

What Are Format Specifiers?

A format specifier starts with a percent sign % followed by a character that represents the data type.

Used in:

printf() – to display values
scanf() – to receive input from users

List of Common Format Specifiers in C

Format Specifier	Data Type	Example Value	Used With
`%d` or `%i`	`int`	10	`printf`, `scanf`
`%f`	`float`	3.14	`printf`, `scanf`
`%lf`	`double`	10.89	`scanf` only
`%c`	`char`	‘A’	`printf`, `scanf`
`%s`	`string`	“Hello”	`printf`, `scanf`
`%u`	`unsigned int`	30000	`printf`, `scanf`
`%ld`	`long int`	123456L	`printf`, `scanf`
`%lu`	`unsigned long`	123456UL	`printf`, `scanf`
`%x` / `%X`	`hex int`	0x1A	`printf`
`%o`	`octal int`	075	`printf`
`%%`	Literal `%`	%	`printf`

Example: Using Format Specifiers with printf()

#include <stdio.h>

int main() {
    int age = 25;
    float pi = 3.1415;
    char grade = 'A';
    char name[] = "Alex";

    printf("Name: %s\n", name);
    printf("Age: %d\n", age);
    printf("PI: %.2f\n", pi);   // 2 decimal places
    printf("Grade: %c\n", grade);

    return 0;
}

✅ Output:

Example: Using Format Specifiers with scanf()

#include <stdio.h>

int main() {
    int age;
    float height;
    char name[30];

    printf("Enter your name: ");
    scanf("%s", name);

    printf("Enter your age and height: ");
    scanf("%d %f", &age, &height);

    printf("Hello %s, age: %d, height: %.1f\n", name, age, height);
    return 0;
}

Tips for Accurate Usage

Always use & with scanf() variables (except for arrays/strings).
Match the specifier to the correct data type.
Use .2f or .1f for limiting decimal places in floats/doubles.
scanf("%s", name) doesn’t need & because arrays decay to pointers.

❌ Common Mistakes to Avoid

Mistake	What Happens
Using wrong format specifier	Garbage value or program crash
Forgetting `&` in `scanf()`	Runtime error or wrong input
Mismatched types	Unexpected behavior
Printing float with `%d`	Incorrect output

Formatting Tricks

Format	Purpose	Example
`%.2f`	Print 2 decimal places	`3.14`
`%5d`	Print integer in 5-character space	`" 25"`
`%-5d`	Left-align in 5-character space	`"25 "`
`%02d`	Print integer padded with zeros	`"07"`