C语言学习2：sizeof ,数据类型，scanf，算式操作，赋值与比较操作，逻辑操作，位的操作，条件运算符，特异赋值结果，强制转换，使用变量要注意的

1，sizeof用法以及各个类型占据的字节大小
#include <stdio.h>

int main(void)
{
    char ch;
    int i;
    long l;
    long long ll;
    float f;
    double d;
    long double ld;

    printf("size of char       : %u\n", sizeof(ch));
    printf("size of int        : %u\n", sizeof(int));
    printf("size of long       : %u\n", sizeof l);
    printf("size of long long  : %u\n", sizeof(long long));
    printf("size of float      : %u\n", sizeof f);
    printf("size of double     : %u\n", sizeof d);
    printf("size of long double: %u\n", sizeof ld);

    return 0;
}

结果：

size of char       : 1
size of int        : 4
size of long       : 4
size of long long  : 8
size of float      : 4
size of double     : 8
size of long double: 12

#include <stdio.h>

// 字面值常量
// 字符型字面值: 'a'
// 整型字面值:   
//            十进制: 12, +123, -126, 0, 16UL
//            十六进制: 0x0a6f
//            八进制: 0462
// 浮点型字面值:
//     3.14 -2.56
//     -3.14e-2  2.56e3
// 字符串字面值:
//     "china unix"

int main(void)
{
    char ch = 'a';
    int  a  = 32;
    float f = 3.14e-2;
    double d = 2.567e2;

    printf("ch = %c\n", ch);
    printf("a = %d\n", a);
    printf("f = %f\n", f);
    printf("d = %lf\n", d);

    printf("------------\n");

    ch = 98;
    printf("ch = %c\n", ch);

    return 0;
}

结果：

ch = a
a = 32
f = 0.031400
d = 256.700000
------------
ch = b

3，scanf用法

#include <stdio.h>

int main(void)
{
    int a = 3;

    printf("a = %d\n", a);
    printf("addr of a = %p\n", &a);

    printf("pls input a num: ");
    scanf("%d", &a);

    printf("a = %d\n", a);

    return 0;
}

结果：

a = 3
addr of a = 0xbfdca58c
pls input a num: 5648
a = 5648

4.算术操作

#include <stdio.h>

/* arithmatic operators */
/* * / % + - */
int main(void)
{
    double ret, x, y, z;

    printf("pls input x, y, z: ");
    scanf("%lf%lf%lf", &x, &y, &z);

    ret = 3 * x + 4 * y - 5 * z;

    printf("ret = %lf\n", ret);

    return 0;
}

结果：

pls input x, y, z: 23,24,67

ret = 69.000000

5，比较操作

#include <stdio.h>

/* compare operators */
/* > < >= <= == != */
int main(void)
{
    int a = 3, b = 5;
    int ret;

    ret = a > b;
    printf("ret = %d\n", ret);

    ret = a < b;
    printf("ret = %d\n", ret);

    return 0;
}

结果：

ret = 0

ret = 1

6，逻辑操作

#include <stdio.h>

/* logic operators */
/* ! && || */
/* 逻辑运算符的左右操作数具有求值顺序 */
int main(void)
{
    int a = 3, b = 5;
    int ret;

    ret = a && b;
    printf("ret = %d\n", ret);

    ret = !a;
    printf("ret = %d\n", ret);

    ret = a < b && a > b;
    printf("ret = %d\n", ret);

    ret = a < b || a > b;
    printf("ret = %d\n", ret);

    return 0;
}

结果：

ret = 1

ret = 0

ret = 0

ret = 1

7，位操作

#include <stdio.h>

/* bit operators */
/* ~ << >> & | ^ */
int main(void)
{
    int a = 3;
    int ret;

    /* 左移 低位补0 */
    ret = a << 2;
    printf("ret = %d, a = %d\n", ret, a);

    a = -3;
    ret = a << 2;
    printf("ret = %d, a = %d\n", ret, a);

    /* 右移 */
    /* 有符号数, 视最高位补 */
    /* 无符号数, 补0 */
    ret = a >> 2;
    printf("ret = %d, a = %d\n", ret, a);
    unsigned u = -3;
    printf("u = %u\n", u);
    ret = u >> 2;
    printf("ret = %u, u = %u\n", ret, u);

    printf("----------------\n");

    /* 按位取反 */
    // 2 ^ 32 ==> 0
    // 1 0000 0000  0000 0000  0000 0000  0000 0000
    // 2 ^ 32 - 1 ==> -1
    // 1111 1111  1111 1111 1111 1111 1111 1111
    // 0000 0000  0000 0000 0000 0000 0000 0101
    // ----------------------------------------
    // 1111 1111  1111 1111 1111 1111 1111 1010
    a = 5;
    ret = ~a;
    printf("ret = %d, a = %d\n", ret, a);

    printf("----------------\n");

    /* 按位与 */
    int b = 7;
    ret = a & b;
    printf("ret = %d\n", ret);

    printf("----------------\n");

    /* 按位或 */
    ret = a | b;
    printf("ret = %d\n", ret);

    printf("----------------\n");

    /* 按位异或 相异为1 */
    ret = a ^ b;
    printf("ret = %d\n", ret);

    return 0;
}

结果：

ret = 12, a = 3

ret = -12, a = -3

ret = -1, a = -3

u = 4294967293

ret = 1073741823, u = 4294967293

----------------

ret = -6, a = 5

----------------

ret = 5

----------------

ret = 7

----------------

ret = 2

8，条件运算符

#include <stdio.h>

/* ? : */
int main(void)
{
    int a = 3, b = 5;

    int min = a > b ? b : a;
    int max = a > b ? a : b;

    printf("max = %d, min = %d\n", max, min);

    return 0;
}

结果：

max = 5, min = 3

9，特异赋值结果，这些赋值都是不正确的

#include <stdio.h>

int main(void)
{
    int a, b, c;
    int ret;

    ret = a = 3, b = 5, c = 75;
    printf("ret = %d, a = %d, b = %d, c = %d\n", ret, a, b, c);

    printf("-------------------\n");

    ret = (a = 3, b = 5, c = 75);
    printf("ret = %d, a = %d, b = %d, c = %d\n", ret, a, b, c);

    return 0;
}

结果：

ret = 3, a = 3, b = 5, c = 75

-------------------

ret = 75, a = 3, b = 5, c = 75

10.强制转换类型

#include <stdio.h>

int main(void)
{
    int a = 5;
    double d = 3.14;
    double ret;

    ret = a + d;
    printf("ret = %lf\n", ret);

    ret = a + (int)d;
    printf("ret = %lf\n", ret);

    return 0;
}

结果：

ret = 8.140000
ret = 8.000000
11，使用变量要小心的地方

#include <stdio.h>

int main(void)
{
    char ch = -1;
    unsigned char uch = -1;
    int a;

    /* 符号位扩展 */
    a = ch;
    printf("a = %d, ch = %d\n", a, ch);
    ch = 8;
    a = ch;
    printf("a = %d, ch = %d\n", a, ch);

    /* 零扩展 */
    a = uch;             
    printf("a = %d, uch = %d\n", a, uch);

    printf("----------------\n");

    /* 截断 */
    a = 312;
    ch = a;
    printf("ch = %d, a = %d\n", ch, a);

    printf("----------------\n");

    /* 溢出 */
    unsigned char uch1 = 136, uch2 = 149;
    a = uch1 + uch2;
    printf("a = %d\n", a);

    return 0;
}

结果：

a = -1, ch = -1          

a = 8, ch = 8           

a = 255, uch = 255    //-1只能是超出无符号的范围只能是255了

----------------

ch = 56, a = 312   //312-256=56，这个就是超出256的截断

----------------

a = 285   //溢出就是数据太大了