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Complete guide to printf() in C: format specifiers, width, precision, flags, escape sequences, and practical formatting examples. Master formatted output in C programming.
The printf() function is your primary tool for displaying output to the console in C. It stands for "print formatted" and lives in the <stdio.h> header. What makes printf() powerful is its format string — a mini-language that gives you precise control over how numbers, characters, and strings appear on screen.
Basic Syntax
c example
int printf(const char *format, ...);The function takes a format string followed by optional arguments. It returns the number of characters printed, or a negative value on error.
c example
#include <stdio.h>
int main() {
// Simple text
printf("Hello, World!\n");
// With format specifiers
printf("I am %d years old\n", 25);
printf("Pi is approximately %f\n", 3.14159);
printf("My name is %s\n", "Alice");
// Return value
int chars_printed = printf("Hello\n");
printf("Previous printf printed %d characters\n", chars_printed);
return 0;
}Output
Hello, World! I am 25 years old Pi is approximately 3.141590 My name is Alice Hello Previous printf printed 6 characters
Format Specifiers
Format specifiers begin with % and tell printf() how to interpret and display the corresponding argument:
| Specifier | Data Type | Example Output |
|---|---|---|
%d or %i | int (signed decimal) | 42, -7 |
%u | unsigned int | 42 |
%f | float/double | 3.141590 |
%e or %E | Scientific notation | 3.141590e+00 |
%g or %G | Shorter of %f or %e | 3.14159 |
%c | Character | A |
%s | String | Hello |
%p | Pointer address | 0x7ffd5e8a |
%x or %X | Hexadecimal | 1a or 1A |
%o | Octal | 52 |
%% | Literal % | % |
%ld | long int | 1234567890 |
%lld | long long int | 9876543210 |
%lf | double (in scanf) | 3.14 |
c example
#include <stdio.h>
int main() {
int num = 255;
float pi = 3.14159;
char ch = 'Z';
char name[] = "World";
printf("Decimal: %d\n", num);
printf("Unsigned: %u\n", num);
printf("Octal: %o\n", num);
printf("Hex (lower): %x\n", num);
printf("Hex (upper): %X\n", num);
printf("Float: %f\n", pi);
printf("Scientific: %e\n", pi);
printf("General: %g\n", pi);
printf("Character: %c\n", ch);
printf("String: %s\n", name);
printf("Pointer: %p\n", (void*)&num);
printf("Percent sign: 100%%\n");
return 0;
}Output
Decimal: 255 Unsigned: 255 Octal: 377 Hex (lower): ff Hex (upper): FF Float: 3.141590 Scientific: 3.141590e+00 General: 3.14159 Character: Z String: World Pointer: 0x7ffd5e8a3c4c Percent sign: 100%
Width and Precision
The complete format specifier syntax is:
Example
%[flags][width][.precision][length]specifier
Width — Minimum Field Width
Width specifies the minimum number of characters to output. If the value is shorter, it's padded with spaces (by default, right-aligned):
Example
#include <stdio.h>
int main() {
// Right-aligned (default) with width
printf("[%10d]\n", 42); // 10 chars wide
printf("[%10d]\n", 12345); // 10 chars wide
printf("[%10s]\n", "Hi"); // Right-aligned string
printf("[%10s]\n", "Hello!"); // Right-aligned string
// Left-aligned with - flag
printf("[%-10d]\n", 42); // Left-aligned
printf("[%-10s]\n", "Hi"); // Left-aligned
return 0;
}
Output
[ 42] [ 12345] [ Hi] [ Hello!] [42 ] [Hi ]
Precision
For floats, precision controls decimal places. For strings, it limits characters:
Example
#include <stdio.h>
int main() {
float pi = 3.14159265;
printf("Default: %f\n", pi); // 6 decimal places (default)
printf("2 decimals: %.2f\n", pi);
printf("4 decimals: %.4f\n", pi);
printf("0 decimals: %.0f\n", pi);
// Precision with strings (truncation)
printf("Full string: %s\n", "Hello, World!");
printf("5 chars: %.5s\n", "Hello, World!");
printf("3 chars: %.3s\n", "Hello, World!");
// Width AND precision combined
printf("[%10.2f]\n", pi); // Width 10, 2 decimals
printf("[%-10.2f]\n", pi); // Left-aligned
return 0;
}
Output
Default: 3.141593 2 decimals: 3.14 4 decimals: 3.1416 0 decimals: 3 Full string: Hello, World! 5 chars: Hello 3 chars: Hel [ 3.14] [3.14 ]
Flags
Flags modify the output format:
| Flag | Description | Example |
|---|---|---|
- | Left-align within width | %-10d |
+ | Show + for positive numbers | %+d |
(space) | Space before positive numbers | % d |
0 | Pad with zeros instead of spaces | %05d |
# | Alternate form (0x for hex, etc.) | %#x |
Example
#include <stdio.h>
int main() {
int pos = 42, neg = -42;
// + flag: show sign for positive numbers
printf("Plus flag: %+d, %+d\n", pos, neg);
// Space flag: space for positive, - for negative
printf("Space flag: % d, % d\n", pos, neg);
// 0 flag: zero-padding
printf("Zero-pad: %05d\n", 42);
printf("Zero-pad: %08.2f\n", 3.14);
// # flag: alternate form
printf("Hex: %x vs %#x\n", 255, 255);
printf("Octal: %o vs %#o\n", 8, 8);
// Combining flags
printf("[%-+10d]\n", 42); // Left-aligned with sign
printf("[%010d]\n", 42); // Zero-padded
return 0;
}
Output
Plus flag: +42, -42 Space flag: 42, -42 Zero-pad: 00042 Zero-pad: 00003.14 Hex: ff vs 0xff Octal: 10 vs 010 [+42 ] [0000000042]
Escape Sequences
Special characters in format strings:
| Sequence | Meaning |
|---|---|
\n | Newline |
\t | Tab |
\\ | Backslash |
\" | Double quote |
\0 | Null character |
\a | Alert (bell) |
\r | Carriage return |
c example
#include <stdio.h>
int main() {
printf("Line 1\nLine 2\n");
printf("Col1\tCol2\tCol3\n");
printf("He said \"Hello\"\n");
printf("Path: C:\\Users\\Admin\n");
printf("100%% complete\n");
return 0;
}Output
Line 1 Line 2 Col1 Col2 Col3 He said "Hello" Path: C:\Users\Admin 100% complete
Printing Different Data Types
c example
#include <stdio.h>
int main() {
// Integer types
short s = 32767;
int i = 2147483647;
long l = 2147483647L;
long long ll = 9223372036854775807LL;
printf("short: %hd\n", s);
printf("int: %d\n", i);
printf("long: %ld\n", l);
printf("long long: %lld\n", ll);
// Unsigned types
unsigned int ui = 4294967295U;
printf("unsigned: %u\n", ui);
// Floating-point types
float f = 3.14f;
double d = 3.141592653589793;
printf("float: %f\n", f);
printf("double: %lf\n", d);
printf("double (e): %e\n", d);
// Size
printf("sizeof(int): %zu bytes\n", sizeof(int));
return 0;
}Output
short: 32767 int: 2147483647 long: 2147483647 long long: 9223372036854775807 unsigned: 4294967295 float: 3.140000 double: 3.141593 double (e): 3.141593e+00 sizeof(int): 4 bytes
Practical Formatting Examples
Table Output
c example
#include <stdio.h>
int main() {
printf("%-15s %8s %10s\n", "Product", "Qty", "Price");
printf("%-15s %8s %10s\n", "-------", "---", "-----");
printf("%-15s %8d %10.2f\n", "Laptop", 2, 999.99);
printf("%-15s %8d %10.2f\n", "Mouse", 5, 19.99);
printf("%-15s %8d %10.2f\n", "Keyboard", 3, 49.50);
printf("%-15s %8s %10s\n", "", "", "----------");
printf("%-15s %8s %10.2f\n", "Total", "", 2249.45);
return 0;
}Output
Product Qty Price
------- --- -----
Laptop 2 999.99
Mouse 5 19.99
Keyboard 3 49.50
----------
Total 2249.45Progress Bar
Example
#include <stdio.h>
int main() {
int total = 50;
for (int i = 0; i <= total; i += 10) {
int percent = (i * 100) / total;
printf("\r[");
for (int j = 0; j < 50; j++) {
if (j < percent / 2)
printf("=");
else if (j == percent / 2)
printf(">");
else
printf(" ");
}
printf("] %3d%%", percent);
}
printf("\n");
return 0;
}
Output
[==================================================>] 100%
Common Mistakes
c example
#include <stdio.h>
int main() {
// Mismatch: using %d for float
float f = 3.14;
printf("Wrong: %d\n", f); // Undefined! Use %f
printf("Right: %f\n", f);
// Forgetting arguments
// printf("%d %d\n", 5); // Missing second argument - undefined behavior
// Using %d for long long
long long big = 9876543210LL;
printf("Wrong: %d\n", big); // Truncated!
printf("Right: %lld\n", big);
return 0;
}Interview Questions
Q1: What is the difference between %d and %i in printf()?
In printf(), %d and %i are identical — both print a signed integer in decimal. The difference only matters in scanf(): %d reads only decimal, while %i reads decimal, octal (if prefixed with 0), or hexadecimal (if prefixed with 0x).
Q2: What does printf() return?
printf() returns the total number of characters successfully printed. On error, it returns a negative value. For example, printf("Hello\n") returns 6 (5 characters + newline).
Q3: What happens if the format specifier doesn't match the argument type?
It's undefined behavior. For example, using %d with a float argument may print garbage because printf() interprets the bit pattern of the float as an integer. Always ensure format specifiers match argument types.
Q4: How do you print a literal % sign using printf?
Use %%. A single % is interpreted as the start of a format specifier, so you must escape it by doubling it: printf("100%%") prints 100%.
Q5: What is the difference between %f and %lf in printf?
In printf(), both %f and %lf print a double (since floats are promoted to double when passed to variadic functions). The l modifier is ignored in printf. However, in scanf(), %f reads a float while %lf reads a double — this distinction is critical.
Summary
printf()formats and prints data to the console using format specifiers- The format string syntax is
%[flags][width][.precision][length]specifier - Common specifiers:
%d(int),%f(float),%c(char),%s(string) - Width controls minimum output width; precision controls decimal places
- Flags like
-,+,0,#modify alignment and display - Always match format specifiers to argument types to avoid undefined behavior
printf()returns the number of characters printed
Exam Focus
Revise definitions, diagrams, examples, and short-answer points for printf() Function in C Programming.
Interview Use
Prepare one clear explanation, one practical example, and one common mistake for this C Programming topic.
Search Terms
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