【速通】C语言基础教程

#include <stdio.h>  // 预处理指令，导入头文件

int main() {        // 主函数入口
    printf("Hello, World!\n");  // 输出语句
    return 0;       // 返回0，表示程序正常结束
}

int a;       // 声明一个整数变量
float b;     // 声明一个浮点型变量
char c;      // 声明一个字符型变量
int d = 1;   // 也可以在声明变量时进行初始化（推荐，防止访问未初始化的变量造成程序崩溃）

const int maxAge = 100;  // 定义常量maxAge
maxAge = 2; // 错误，const定义的常量不能更改

#define PI 3.14159  // 定义常量PI
#define SQUARE(x) ((x) * (x))  // 定义宏函数SQUARE

#include <stdio.h>
#define MAX_VALUE 100
int main() {
    const int minValue = 0;   // 常量
    printf("Max value: %d, Min value: %d\n", MAX_VALUE, minValue);
    return 0;
}

printf("文本内容 %d\n", 变量名);  // %d表示输出整数

int age = 25;
printf("My age is %d\n", age);  // 输出：My age is 25

scanf("格式字符串", &变量名);  // scanf是向变量地址写入，所以要加&取地址符

char str[10];
scanf("%s",str); // 数组可以不用取地址，因为数组名可以退化为数组首地址，等价于&str[0]

int num[5];
scanf("%d",&num[5]); // 这里就要取地址，因为num是首地址，但是nun后面有[]代表取到数组元素，这时就要取地址

int a = 10, b = 3;
int sum = a + b;      // sum = 13
int diff = a - b;     // diff = 7
int product = a * b;  // product = 30
int quotient = a / b; // quotient = 3
int remainder = a % b; // remainder = 1

int a = 10, b = 3;
if (a > b) {
    printf("a is greater than b\n");  // 输出：a is greater than b
}

int a = 10, b = 3;
if (a > b && b > 0) {
    printf("Both conditions are true\n");  // 输出：Both conditions are true
}

int a = 5, b = 3; // 二进制：a = 0101, b = 0011
int and_result = a & b;  // and_result = 1 (0101 & 0011 = 0001)
int or_result = a | b;   // or_result = 7 (0101 | 0011 = 0111)

int a = 5;   // 二进制：0101
int result = a << 1;  // 结果：1010，即10

int a = 10, b = 3;
int max = (a > b) ? a : b;  // max = 10，因为a > b
// 翻译为如果a>b，则取a，否则取b

int max(int a,int b,int c){
    return (a>b?a:(b>c?b:c)); // 这里通过嵌套实现了三个数取最大值
}

int a = 10, b = 5, c = 3;
int result = a + b * c;  // 先计算b * c，再加上a，即result = 10 + 15 = 25

if (condition) {
    // 条件为真时执行的代码
}

int a = 10;
if (a > 5) {
    printf("a is greater than 5\n");  // 输出：a is greater than 5
}

if (condition) {
    // 条件为真时执行的代码
} else {
    // 条件为假时执行的代码
}

int a = 3;
if (a > 5) {
    printf("a is greater than 5\n");
} else {
    printf("a is not greater than 5\n");  // 输出：a is not greater than 5
}

if (condition1) {
    // 条件1为真时执行的代码
} else if (condition2) {
    // 条件2为真时执行的代码
} else {
    // 条件1和条件2都为假时执行的代码
}

int a = 7;
if (a > 10) {
    printf("a is greater than 10\n");
} else if (a > 5) {
    printf("a is greater than 5 but less than or equal to 10\n");  // 输出：a is greater than 5 but less than or equal to 10
} else {
    printf("a is less than or equal to 5\n");
}

switch (expression) {
    case value1:
        // expression == value1时执行的代码
        break;
    case value2:
        // expression == value2时执行的代码
        break;
    default:
        // 如果没有匹配的值，执行这段代码
}

int day = 3;
switch (day) {
    case 1:
        printf("Monday\n");
        break;
    case 2:
        printf("Tuesday\n");
        break;
    case 3:
        printf("Wednesday\n");  // 输出：Wednesday
        break;
    default:
        printf("Invalid day\n");
}

for (initialization; condition; increment) {
    // 循环体
}

for (int i = 1; i <= 5; i++) {
    printf("i = %d\n", i);  // 输出：i = 1, i = 2, i = 3, i = 4, i = 5
}

while (condition) {
    // 循环体
}

int i = 1;
while (i <= 5) {
    printf("i = %d\n", i);  // 输出：i = 1, i = 2, i = 3, i = 4, i = 5
    i++;
}

do {
    // 循环体
} while (condition);

int i = 1;
do {
    printf("i = %d\n", i);  // 输出：i = 1, i = 2, i = 3, i = 4, i = 5
    i++;
} while (i <= 5);

for (int i = 1; i <= 10; i++) {
    if (i == 5) {
        break;  // 当i == 5时跳出循环
    }
    printf("i = %d\n", i);  // 输出：i = 1, i = 2, i = 3, i = 4
}

for (int i = 1; i <= 5; i++) {
    if (i == 3) {
        continue;  // 当i == 3时跳过当前循环
    }
    printf("i = %d\n", i);  // 输出：i = 1, i = 2, i = 4, i = 5
}

int add(int a, int b) {
    return a + b;  // 返回a和b的和
}

int main() {
    int result = add(5, 3);
    printf("Result: %d\n", result);  // 输出：Result: 8
    return 0;
}

return_type function_name(parameter1, parameter2, ...) {
    // 函数体：包含具体的代码逻辑
}

#include <stdio.h>

// 函数声明
int add(int a, int b);

int main() {
    int result = add(3, 5);  // 调用函数
    printf("Sum: %d\n", result);  // 输出：Sum: 8
    return 0;
}

// 函数定义
int add(int a, int b) {
    return a + b;
}

#include <stdio.h>

float multiply(float x, float y) {
    return x * y;  // 返回两个数的乘积
}

int main() {
    float result = multiply(3.5, 2.0);
    printf("Multiplication result: %.2f\n", result);  // 输出：Multiplication result: 7.00
    return 0;
}

#include <stdio.h>

// 递归函数：计算n的阶乘
int factorial(int n) {
    if (n == 0) {
        return 1;  // 基准条件：0的阶乘是1
    } else {
        return n * factorial(n - 1);  // 递归调用
    }
}

int main() {
    int result = factorial(5);
    printf("Factorial of 5 is: %d\n", result);  // 输出：Factorial of 5 is: 120
    return 0;
}

return_type (*function_ptr)(parameter_types);

#include <stdio.h>

int add(int a, int b) {
    return a + b;
}

int subtract(int a, int b) {
    return a - b;
}

int main() {
    int (*operation)(int, int);  // 声明函数指针
    operation = add;  // 将函数指针指向add函数
    printf("Add: %d\n", operation(3, 4));  // 输出：Add: 7
    
    operation = subtract;  // 将函数指针指向subtract函数
    printf("Subtract: %d\n", operation(10, 3));  // 输出：Subtract: 7
    
    return 0;
}

// 声明与初始化
int arr[5] = {1, 2, 3, 4, 5}; 

// 访问元素
printf("%d", arr[2]); // 输出：3

int matrix[2][3] = {
    {1, 2, 3},
    {4, 5, 6}
};
printf("%d", matrix[1][2]); // 输出：6

char str1[6] = {'H', 'e', 'l', 'l', 'o', '\0'};
char str2[] = "World"; // 自动添加\0

char s1[20] = "Hello";
char s2[] = " World";
strcat(s1, s2); // s1变为"Hello World"

int arr[3] = {10, 20, 30};
int *ptr = arr; // 等价于 &arr[0]
printf("%d", *(ptr+1)); // 输出：20

int var = 10;
int *p = &var; // p存储var的地址

printf("%d", *p); // 解引用，输出：10
*p = 20; // 通过指针修改变量值

int arr[3] = {1, 2, 3};
int *p = arr;

// 以下三种写法等价
arr[1] = 5;
*(arr + 1) = 5;
*(p + 1) = 5;

void swap(int *a, int *b) {
    int temp = *a;
    *a = *b;
    *b = temp;
}

int x = 1, y = 2;
swap(&x, &y); // x=2, y=1

#include <stdlib.h>

// 分配内存
int *arr = (int*)malloc(5 * sizeof(int));

// 重新分配
arr = (int*)realloc(arr, 10 * sizeof(int));

// 释放内存
free(arr);

struct Student {
    char name[20];
    int age;
    float score;
};

// 声明变量
struct Student stu1 = {"Alice", 20, 90.5};

// 访问成员
printf("%s", stu1.name);

struct Student *pStu = (struct Student*)malloc(sizeof(struct Student));
strcpy(pStu->name, "Bob");
pStu->age = 21;

free(pStu);

union Data {
    int i;
    float f;
    char str[20];
};

union Data data;
data.i = 10; // 此时其他成员的值无意义

FILE *fp;

// 打开文件（读写模式）
fp = fopen("test.txt", "r+");
if (fp == NULL) {
    perror("Error opening file");
    return -1;
}

// 关闭文件
fclose(fp);

// 写入
fprintf(fp, "%s %d", "Hello", 123);

// 读取
char str[20];
int num;
fscanf(fp, "%s %d", str, &num);

struct Student stu;
fwrite(&stu, sizeof(struct Student), 1, fp);
fread(&stu, sizeof(struct Student), 1, fp);

// 移动文件指针
fseek(fp, 0, SEEK_SET); // 回到文件开头

// 错误检测
if (ferror(fp)) {
    printf("File error occurred\n");
}

// 分配后立即初始化
int *p = malloc(sizeof(int));
if (p != NULL) {
    *p = 0;
}

// 释放后置空
free(p);
p = NULL;

#include <stdio.h>
#include <stdlib.h>

void safe_malloc_example() {
    int *ptr = NULL;
    
    // 带错误检查的分配
    ptr = (int*)malloc(5 * sizeof(int));
    if (ptr == NULL) {
        fprintf(stderr, "Memory allocation failed\n");
        exit(EXIT_FAILURE);
    }
    
    // 使用内存
    for (int i = 0; i < 5; i++) {
        ptr[i] = i * 10;
    }
    
    // 释放内存
    free(ptr);
    ptr = NULL; // 防止悬空指针
}

int main() {
    safe_malloc_example();
    return 0;
}