DAY4 C Language Input and Output Functions

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1. What Are Input and Output?

In C language, input and output are defined from the computer’s perspective:

• Input → Data sent to the computer.
• Output → Data displayed from the computer to the screen or a file.

To use input and output functions, include the standard header file:

#include <stdio.h>

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2. Character Input and Output Functions

1. putchar()

Used to print a single character to the terminal.

int putchar(int c);

• Parameter: The character to be printed.
• Return value: The ASCII value of the printed character.

Example:

#include <stdio.h>
int main()
{
    char c = 'c';
    putchar('a');
    putchar('a' + 1);  // prints 'b'
    putchar(c);        // prints 'c'
    return 0;
}

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2. getchar()

Reads one character from the keyboard (blocking input).

int getchar(void);

• Return value: The ASCII value of the character entered.

Example:

#include <stdio.h>
int main()
{
    char c = 0;
    c = getchar();   // waits for user input
    putchar(c);      // prints the same character
}

Blocking input means the program waits for you to type and press Enter before continuing.

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3. Formatted Output: printf()

1. Basic Syntax

int printf(const char *format, ...);

• format: Format control string.
• ...: One or more variables to display.

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2. Common Format Specifiers

Type	Specifier	Example Output
Signed integer	%d	26
Unsigned integer	%u	888899
Octal	%o	32
Hexadecimal (lowercase)	%x	1a
Hexadecimal (uppercase)	%X	1A
Character	%c	'a'
String	%s	"hello"
Float	%f	3.141500
Double	%lf	700000.000000
Scientific notation	%e	7.000000e+05
Pointer (address)	%p	0x7ffee123abcd
Percent sign	%%	%

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3. Example: 04printf.c

#include <stdio.h>
#include <unistd.h>

int main()
{
    int num = 26;
    printf("Signed int: %d\n", num);
    printf("Octal: %o\n", num);
    printf("Hexadecimal: %x\n", num);

    char c = 'a';
    printf("Character: %c, ASCII: %d\n", c, c);

    unsigned int b = 888899;
    printf("Unsigned int: %u\n", b);

    float f = 3.1415;
    printf("Float: %f\n", f);

    double d = 7e5;
    printf("Double: %lf (scientific: %e)\n", d, d);

    long l = 8899712;
    printf("Long: %ld\n", l);

    printf("String example: %s\n", "Zhang San");
    printf("Memory address: %p\n", &num);
    printf("100%% Complete!\n");
}

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4. Extended Format Options

Format	Description	Example
%5d	Width 5, right-aligned	26
%-5d	Width 5, left-aligned	26
%05d	Padded with zeros	00026
%.2f	Two decimal places	3.14
%#o	Show prefix 0 for octal	032
%#x	Show prefix 0x for hex	0x1a

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4. Formatted Input: scanf()

int scanf(const char *format, ...);

1. Important Notes

1. Use & before variables to pass their addresses.
2. Do not use & with strings.
3. Avoid adding \n or extra characters in the format string.

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2. Example

#include <stdio.h>
int main()
{
    int num = 0;
    printf("Please enter an integer: ");
    scanf("%d", &num);
    printf("You entered: %d\n", num);
    return 0;
}

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3. Practice Example (from 06scan_lianxi.c)

int num1 = 0, num2 = 0;
printf("Enter two integers: ");
scanf("%d %d", &num1, &num2);
printf("Sum: %d\n", num1 + num2);

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5. String Input and Output

1. gets() ( Not recommended)

Reads one line of text from input — unsafe because it can overflow memory.

char str[50];
gets(str);
printf("You entered: %s\n", str);

Use fgets() instead:

fgets(str, 50, stdin);

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2. puts()

Prints a string and automatically adds a newline.

#include <stdio.h>

int main()
{
    char str[50] = "Hello, Zhangsan!";
    puts("Hi!");
    puts(str);
}

Output:

Hi!
Hello, Zhangsan!

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6. Common Escape Sequences in printf

Escape	Meaning	Example
\n	New line	printf("Hello\n");
\t	Tab (4 spaces)	printf("\tHi");
\b	Backspace	printf("Y\b=");
\\	Backslash	printf("\\");
\r	Return to line start	used for dynamic refresh
%%	Percent sign	printf("100%%");

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7. Summary Table

Function	Purpose	Return Value
putchar()	Output one character	ASCII value of the printed char
getchar()	Input one character	ASCII value of the input char
printf()	Formatted output	Number of characters printed
scanf()	Formatted input	Number of successfully read values
puts()	Print string with newline	Non-negative on success
gets()	Read string (unsafe)	Pointer to string