Type Conversion 类型转换

Type conversion is the process of converting a variable from one data type to another.

1. Implicit Conversion (Automatic)

Automatic Type Promotion 自动类型提升

char c = 'A';
int i = c;           // char → int (自动)
float f = 3.14;
double d = f;        // float → double (自动)

// In expressions 表达式中
int a = 5;
double b = 2.5;
double result = a + b;  // a automatically converted to double

Conversion Hierarchy 转换层次结构

char → short → int → unsigned int → long → unsigned long → 
long long → float → double → long double

Lower types are automatically promoted to higher types in expressions
在表达式中，较低类型自动提升为较高类型

2. Explicit Conversion (Casting)

Cast Operator Syntax 强制转换运算符语法

(target_type) expression

Examples 示例

#include <stdio.h>

int main() {
    // Float to int 浮点数转整数
    float pi = 3.14159;
    int int_pi = (int)pi;        // Truncates to 3 截断为3
    printf("(int)%.2f = %d\n", pi, int_pi);

    // Int to float 整数转浮点数
    int count = 10;
    float float_count = (float)count;  // Becomes 10.0 变为10.0
    printf("(float)%d = %.1f\n", count, float_count);

    // Char to int 字符转整数
    char letter = 'A';
    int ascii = (int)letter;     // Becomes 65 变为65
    printf("'%c' → ASCII %d\n", letter, ascii);

    return 0;
}

3. Common Conversion Scenarios 常见转换场景

Integer Division vs Floating Division 整数除法 vs 浮点数除法

int a = 7, b = 2;

int int_result = a / b;          // 3 (integer division 整数除法)
float float_result = (float)a / b; // 3.5 (floating division 浮点数除法)

printf("Integer division: %d\n", int_result);
printf("Floating division: %.1f\n", float_result);

Pointer Type Casting 指针类型转换

int value = 65;
void *void_ptr = &value;

// Cast void pointer back to int pointer
int *int_ptr = (int*)void_ptr;  // 空指针转整型指针
printf("Value: %d\n", *int_ptr);

4. Data Loss and Precision Issues 数据丢失和精度问题

Potential Problems

#include <stdio.h>
#include <limits.h>

int main() {
    // Overflow 溢出
    int big = INT_MAX;
    short small = (short)big;    // Data loss 数据丢失
    printf("%d → %d (overflow)\n", big, small);

    // Precision loss 精度丢失
    double precise = 123.456789;
    float less_precise = (float)precise;  // Precision loss 精度丢失
    printf("double: %.6f → float: %.6f\n", precise, less_precise);

    // Truncation 截断
    float decimal = 9.999;
    int truncated = (int)decimal;  // Truncates to 9 截断为9
    printf("%.3f → %d (truncation)\n", decimal, truncated);

    return 0;
}

5. Type Conversion Rules Table 类型转换规则表

Conversion Type	Behavior	Example
int → float	Exact if possible	(float)5 → 5.0
float → int	Truncates decimal	(int)3.99 → 3
char → int	ASCII value	(int)'A' → 65
int → char	Keep lower 8 bits	(char)65 → 'A'
small → large	No data loss	int → long
large → small	Potential data loss	long → int
## Key Points to Remember 关键要点

1. Implicit conversion happens automatically in expressions
2. Explicit casting gives you control: (type)variable
3. Watch for data loss when converting to smaller types
4. Use parentheses to make casting clear
5. Integer division truncates - use casting for floating results