Input and Output

Q: Why do scanf variables need & in front?

scanf needs to know where to store the value, and & takes the variable's address. Only array names don't need it, because an array name is already the address of its first element.

Q: Can printf and scanf format specifiers be uppercase?

Some can. %d lowercase corresponds to int; uppercase %D is non-standard. But %x prints lowercase hex (abcdef) while %X prints uppercase (ABCDEF) — this is valid.

Q: How do I safely read a string with spaces?

scanf("%s") stops at whitespace. Use fgets(name, sizeof(name), stdin) to read an entire line — it supports spaces and is safer.

Input and output are like a conversation between you and the computer — printf is the computer talking to you, scanf is you talking to the computer. Get the format right, or you'll be talking past each other.

printf Formatted Output

printf stands for "print formatted." Its first argument is a format string, followed by the values to output.

Basic Format Specifiers

Specifier	Purpose	Example	Output
`%d`	Decimal integer	`printf("%d", 42)`	`42`
`%f`	Floating-point	`printf("%f", 3.14)`	`3.140000`
`%c`	Single character	`printf("%c", 'A')`	`A`
`%s`	String	`printf("%s", "hi")`	`hi`
`%x`	Hexadecimal	`printf("%x", 255)`	`ff`
`%o`	Octal	`printf("%o", 8)`	`10`
`%p`	Pointer address	`printf("%p", &a)`	Address value
`%%`	Literal percent sign	`printf("100%%")`	`100%`

#include <stdio.h>

int main(void) {
    int age = 25;
    float height = 175.5f;
    char grade = 'A';
    printf("Age: %d, Height: %.1f, Grade: %c\n", age, height, grade);
    return 0;
}

TEXT

Age: 25, Height: 175.5, Grade: A

Width and Alignment

You can insert numbers between % and the format specifier to control output width:

%5d: Minimum 5 characters wide, right-aligned
%-5d: Minimum 5 characters wide, left-aligned
%05d: Pad with zeros up to 5 digits

#include <stdio.h>

int main(void) {
    int num = 42;
    printf("[%5d]\n", num);
    printf("[%-5d]\n", num);
    printf("[%05d]\n", num);
    printf("[%+5d]\n", num);
    return 0;
}

TEXT

[   42]
[42   ]
[00042]
[  +42]

Floating-Point Precision

%.2f: 2 digits after the decimal point
%8.2f: Total width at least 8 characters, 2 digits after the decimal point
%-8.2f: Left-aligned version

#include <stdio.h>

int main(void) {
    double pi = 3.14159265358979;
    printf("Default:  %f\n", pi);
    printf("2 places: %.2f\n", pi);
    printf("6 places: %.6f\n", pi);
    printf("8.2:      %8.2f\n", pi);
    return 0;
}

TEXT

Default:  3.141593
2 places: 3.14
6 places: 3.141593
8.2:          3.14

💡 Tip: %f defaults to 6 decimal places. Use %.0f to hide the decimal portion entirely.

scanf Reading Input

scanf stands for "scan formatted." It reads data from standard input according to the format string and stores it in variables.

Basic Usage

#include <stdio.h>

int main(void) {
    int age;
    float score;
    printf("Enter your age: ");
    scanf("%d", &age);
    printf("Enter your score: ");
    scanf("%f", &score);
    printf("Age: %d, Score: %.1f\n", age, score);
    return 0;
}

TEXT

Enter your age: 20
Enter your score: 92.5
Age: 20, Score: 92.5

⚠️ Note: scanf's variable arguments must be preceded by & (the address-of operator), telling scanf "store the value at this address." Forgetting & is the most common beginner mistake — the program won't error out but will crash.

Reading Multiple Values

#include <stdio.h>

int main(void) {
    int a, b, c;
    printf("Enter three integers (separated by spaces): ");
    scanf("%d %d %d", &a, &b, &c);
    printf("You entered: %d, %d, %d\n", a, b, c);
    return 0;
}

TEXT

Enter three integers (separated by spaces): 10 20 30
You entered: 10, 20, 30

By default, scanf uses whitespace (spaces, newlines, tabs) to separate different values.

Reading Characters and Strings

#include <stdio.h>

int main(void) {
    char ch;
    char name[20];
    printf("Enter a character: ");
    scanf(" %c", &ch);
    printf("Enter your name (no spaces): ");
    scanf("%s", name);
    printf("Character: %c, Name: %s\n", ch, name);
    return 0;
}

⚠️ Note: Adding a space before %c (" %c") skips whitespace characters, otherwise it will read the leftover newline from previous input. %s reads a string but stops at whitespace. name is an array name, which is already an address, so no & is needed.

scanf's Return Value

scanf returns the number of successfully read variables. This feature can be used to validate input:

#include <stdio.h>

int main(void) {
    int num;
    printf("Enter an integer: ");
    int result = scanf("%d", &num);
    if (result == 1) {
        printf("Successfully read: %d\n", num);
    } else {
        printf("Invalid input!\n");
    }
    return 0;
}

TEXT

Enter an integer: abc
Invalid input!

getchar and putchar

getchar() reads a single character from the keyboard, and putchar() outputs a single character to the screen. They're simpler and more efficient than scanf("%c") and printf("%c").

#include <stdio.h>

int main(void) {
    char ch;
    printf("Enter a character: ");
    ch = getchar();
    printf("You entered: ");
    putchar(ch);
    putchar('\n');
    return 0;
}

TEXT

Enter a character: X
You entered: X

Reading an Entire Line

getchar() combined with a loop can read an entire line until a newline is encountered:

#include <stdio.h>

int main(void) {
    char ch;
    printf("Enter a line of text: ");
    while ((ch = getchar()) != '\n') {
        putchar(ch);
    }
    putchar('\n');
    return 0;
}

TEXT

Enter a line of text: Hello World!
Hello World!

💡 Tip: while ((ch = getchar()) != '\n') is a classic idiom: first read a character into ch, then check if it's a newline. Note that the outer parentheses around the assignment are mandatory.

Buffer Issues

C's standard input uses line buffering: the entire line of data is only delivered to the buffer for the program to read after the user presses Enter. This can cause some confusing problems.

Problem: scanf %c Reads a Leftover Newline

#include <stdio.h>

int main(void) {
    int num;
    char ch;
    printf("Enter an integer: ");
    scanf("%d", &num);
    printf("Enter a character: ");
    scanf("%c", &ch);
    printf("Integer=%d, Character=%d\n", num, ch);
    return 0;
}

TEXT

Enter an integer: 42
Enter a character: Integer=42, Character=10

After typing 42 and pressing Enter, the buffer contains 42\n. scanf("%d") reads 42, and scanf("%c") immediately reads that leftover \n (ASCII code 10), without waiting for you to type a character.

Solutions

Solution 1: Add a space before %c (recommended)

scanf(" %c", &ch);

The space skips all whitespace characters (including newlines, spaces, and tabs).

Solution 2: Use getchar() to absorb the leftover

scanf("%d", &num);
getchar();
scanf("%c", &ch);

Solution 3: Clear the input buffer

int c;
while ((c = getchar()) != '\n' && c != EOF) {}

⚠️ Note: Don't use fflush(stdin) to clear the input buffer — it's undefined behavior in standard C. Some compilers support it, but it's not portable.

Example

Write a program that reads both an integer and a character, properly handling buffer leftovers:

#include <stdio.h>

int main(void) {
    int age;
    char gender;
    printf("Enter your age: ");
    scanf("%d", &age);
    printf("Enter your gender (M/F): ");
    scanf(" %c", &gender);
    printf("Age: %d, Gender: %c\n", age, gender);
    return 0;
}

▶ Try it Yourself

TEXT

Enter your age: 25
Enter your gender (M/F): M
Age: 25, Gender: M

Example

Use getchar() to read character by character and count the number of letters and digits in a line:

#include <stdio.h>

int main(void) {
    char ch;
    int letters = 0;
    int digits = 0;
    printf("Enter a line of text: ");
    while ((ch = getchar()) != '\n') {
        if (ch >= 'A' && ch <= 'Z' || ch >= 'a' && ch <= 'z') {
            letters++;
        } else if (ch >= '0' && ch <= '9') {
            digits++;
        }
    }
    printf("Letters: %d, Digits: %d\n", letters, digits);
    return 0;
}

▶ Try it Yourself

TEXT

Enter a line of text: Hello 2024!
Letters: 5, Digits: 4

puts and fputs

puts() outputs a string and automatically appends a newline. It's simpler than printf and ideal for plain text output:

#include <stdio.h>

int main(void) {
    puts("Hello, World!");
    puts("Auto newline, no need for \\n");
    return 0;
}

TEXT

Hello, World!
Auto newline, no need for \n

💡 Tip: puts() is more efficient than printf("%s\n", ...) because it doesn't need to parse a format string. When you don't need formatting, prefer puts.

❓ FAQ

Q Why do scanf variables need & in front?

A scanf needs to know where to store the value, and & takes the variable's address. Only array names don't need it, because an array name is already the address of its first element.

Q scanf("%c") skipped my input — what happened?

A There's a leftover newline in the buffer from a previous input. Add a space before %c: scanf(" %c", &ch) to skip whitespace.

Q Can printf and scanf format specifiers be uppercase?

A Some can. %d lowercase corresponds to int; uppercase %D is non-standard. But %x prints lowercase hex (abcdef) while %X prints uppercase (ABCDEF) — this is valid.

Q How do I safely read a string with spaces?

A scanf("%s") stops at whitespace. Use fgets(name, sizeof(name), stdin) to read an entire line — it supports spaces and is safer.

📖 Summary

printf uses format specifiers to control output: width, precision, alignment, padding
scanf requires & before variables; add a space before %c to avoid reading leftover newlines
getchar/putchar are efficient choices for single-character I/O
Line buffering causes leftover data issues between scanf and getchar calls
scanf's return value can be used to validate whether input was successful

📝 Exercises

Write a program that reads a circle's radius and outputs its area and circumference using printf's %.2f format.
Write a program that uses getchar() to read a line of input and counts how many uppercase and lowercase letters it contains.
Write a program that reads an integer and outputs it in decimal (%d), hexadecimal (%x), and octal (%o) formats.

Input and Output

printf Formatted Output

Basic Format Specifiers

Width and Alignment

Floating-Point Precision

scanf Reading Input

Basic Usage

Reading Multiple Values

Reading Characters and Strings

scanf's Return Value

getchar and putchar

Reading an Entire Line

Buffer Issues

Problem: scanf %c Reads a Leftover Newline

Solutions

Example

Example

puts and fputs

❓ FAQ

📖 Summary

📝 Exercises

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