“From today on, study hard and make progress every day”

Repetition is the best method for memory; spend one minute every day to remember the basics of C language.

“Essential Knowledge Points for C Language Beginners: Series of 100 Notes“

15. printf() Function: Introduction to Formatting Output

1. Basic Usage of printf Function

printf is the most commonly used output function in the C standard library, declared in the stdio.h header file:

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

Basic Example

#include <stdio.h>

int main() {
    printf("Hello, World!\n");  // Output string
    int age = 25;
    printf("Age: %d\n", age);   // Format output variable
    return 0;
}

2. Detailed Explanation of Format Specifiers

printf controls the output format through format specifiers, with the basic syntax:

%[flags][width][.precision][length]type

Common Type Specifiers

Specifier	Usage	Example
%d	Decimal integer	printf(“%d”, 123)
%f	Floating-point number	printf(“%f”, 3.14)
%c	Single character	printf(“%c”, ‘A’)
%s	String	printf(“%s”, “hello”)
%p	Pointer address	printf(“%p”, &var)
%%	Percent character	printf(“%%”)

3. Formatting Control Parameters

1. Width Control

printf("%5d\n", 123);    // "  123" (right-aligned, width 5)
printf("%-5d\n", 123);   // "123  " (left-aligned)

2. Precision Control

printf("%.2f\n", 3.14159);  // "3.14" (keep 2 decimal places)
printf("%.5s\n", "hello world"); // "hello" (truncate to 5 characters)

3. Flag Characters

printf("%+d\n", 123);     // "+123" (show sign)
printf("%#x\n", 15);      // "0xf" (show base prefix)
printf("%08d\n", 123);    // "00000123" (leading zeros)

4. Special Format Output

1. Escape Characters

printf("Line1\nLine2");   // New line
printf("Tab\tColumn");    // Tab character
printf("Backslash: \\ ");  // Output backslash

2. Multiple Parameter Output

int a = 10, b = 20;
printf("Values: %d, %d\n", a, b);

3. Dynamic Width/Precision

int width = 8, precision = 3;
printf("%*.*f\n", width, precision, 3.14159); // "   3.142"

5. Type Modifiers

Modifier	Description	Example
h	short type	%hd (short int)
l	long type	%ld (long int)
ll	long long type	%lld (long long)
L	long double type	%Lf

long bigNum = 1234567890L;
printf("%ld\n", bigNum);

6. Common Error Examples

1. Type Mismatch

float f = 3.14;
printf("%d\n", f);  // Error! Should use %f

2. Insufficient Parameters

printf("%d %d\n", 1);  // Missing second parameter

3. Format String Vulnerability

char user_input[100];
scanf("%s", user_input);
printf(user_input);    // Dangerous! Should use printf("%s", user_input);

7. Advanced Usage Examples

1. Table Output

printf("%-10s %-10s %-10s\n", "Name", "Age", "Score");
printf("%-10s %-10d %-10.2f\n", "Alice", 20, 89.5);
printf("%-10s %-10d %-10.2f\n", "Bob", 22, 92.3);

2. Memory Address Output

int var = 10;
printf("Address: %p\n", (void*)&var);

3. Format Reuse

const char *fmt = "Result: %.*f\n";
printf(fmt, 2, 3.14159);  // "Result: 3.14"
printf(fmt, 4, 2.71828);  // "Result: 2.7183"

8. Return Value Explanation

printf returns the number of characters successfully output:

int count = printf("Hello");
// count = 5 (not including '\0')

9. Best Practice Recommendations

1. 1. Safety Check:

   if (printf("Test") < 0) {
       perror("printf failed");
   }

2. 2. Avoid Buffer Overflow:

   char buf[100];
   snprintf(buf, sizeof(buf), "Value: %d", large_num);

———- End ———-

[Special Statement: All articles in this public account are original or authorized by the author, some content and images are sourced from the internet and AI, please feel free to consume, the views are for learning reference only~~]

“If you like C, please like it” Click the bottom right corner to view“