第1章 导言

1.1 入门

#include <stdio.h>
int main() {
    printf("Hello, World!\n");
}

1.2 变量与算术表达式

#include <stdio.h>

int main() {
    int fahr, celsius;
    int lower, upper, step;

    lower = 0;
    upper = 300;
    step = 20;

    fahr = lower;
    while (fahr <= upper) {
        celsius = 5 * (fahr-32) / 9;
        printf("%3d %6d\n", fahr, celsius);
        fahr = fahr + step;
    }
}

各数据类型的大小取决于具体机器.

• int
• float
• char
• short
• long
• double
• 数组
• 结构
• 联合
• 指针
• 函数

使用 float:

• 如果表达式中有 float 和 int, 那么会将 int 转为 float

#include <stdio.h>

int main() {
    float fahr, celsius;
    int lower, upper, step;

    lower = 0;
    upper = 300;
    step = 20;

    fahr = lower;
    while (fahr <= upper) {
        celsius = (5.0/9.0) * (fahr-32.0);
        printf("%3.0f %6.1f\n", fahr, celsius);
        fahr = fahr + step;
    }
}

1.3 for 语句

#include <stdio.h>

int main() {
    int fahr;
    for (fahr = 0; fahr <= 300; fahr = fahr + 20) {
        printf("%3d %6.1f\n", fahr, (5.0/9.0)*(fahr-32));
    }
}

1.4 符号常量

#include <stdio.h>
#define LOWER 0
#define UPPER 300
#define STEP  20

int main() {
    int fahr;
    for (fahr = LOWER; fahr <= UPPER; fahr = fahr + STEP) {
        printf("%3d %6.1f\n", fahr, (5.0/9.0)*(fahr-32));
    }
}

1.5 字符输入/输出

c = getchar()
putchar(c)

1.5.1 文件复制

#include <stdio.h>

int main() {
    int c;
    c = getchar();
    while (c != EOF) {
        putchar(c);
        c = getchar();
    }
}

赋值是个表达式, 可以作为更大的表达式的一部分:

#include <stdio.h>

int main() {
    int c;
    while ((c = getchar()) != EOF) {
        putchar(c);
    }
}

!= 的优先级比 = 高.

1.5.2 字符计数

#include <stdio.h>

int main() {
    long nc;

    nc = 0;
    while (getchar() != 'c') {
        ++nc;
    }
    printf("%ld\n", nc);
}

1.5.3

#include <stdio.h>

int main() {
    int c, nl;

    nl = 0;
    while ((c = getchar()) != EOF) {
        if (c == '\n') {
            ++nl;
        }
    }
    printf("%d\n", nl);
}

1.5.4 单词计数

#include <stdio.h>

#define IN  1
#define OUT 0

int main() {
    int c, nl, nw, nc, state;

    state = OUT;
    nl = nw = nc = 0;
    while ((c = getchar()) != '0') {
        ++nc;

        if (c == '\n') {
            ++nl;
        }

        if (c == ' ' || c == '\n' || c == '\t') {
            state = OUT;
        } else if (state == OUT) {
            state = IN;
            ++nw;
        }
    }
    printf("%d %d %d\n", nl, nw, nc);
}

1.6 数组

#include <stdio.h>

int main() {
    int c, i, nWhite, nOther;
    int nDigit[10];

    nWhite = nOther = 0;
    for (i = 0; i < 10; ++i) {
        nDigit[i] = 0;
    }

    while ((c = getchar()) != '0') {
        if (c >= '0' && c <= '9') {
            ++nDigit[c-'0'];
        } else if (c == ' ' || c == '\n' || c == '\t') {
            ++nWhite;
        } else {
            ++nOther;
        }
    }

    printf("digits = ");
    for (i = 0; i < 10; ++i) {
        printf(" %d", nDigit[i]);
    }
    printf(", white space = %d, other = %d\n",
           nWhite, nOther);
}

1.7 函数

#include <stdio.h>

// 函数原型
int power(int, int);

int main() {
    int i;
    for (i = 0; i < 10; ++i) {
        printf("%d %d %d\n", i, power(2, i), power(-3, i));
    }
    return 0;
}

int power(int base, int n) {
    int i, p;

    p = 1;
    for (i = 1; i <= n; ++i) {
        p = p * base;
    }
    return p;
}

1.8 参数 - 传值调用

好处:

• 减少函数内的变量声明, 直接使用形式参数

传递数组传的是数组第一个元素的地址, 所以函数对其修改可在外部可见.

1.9 字符数组

字符串常量自动使用 \0 结束.

• "hello\n"

#include <stdio.h>
#define MAXLINE 1000

int getLine(char line[], int maxLine);
void copy (char to[], char from[]);

int main() {
    int len;
    int max;
    char line[MAXLINE];
    char longest[MAXLINE];

    printf("len: %d\n", len);
    max = 0;
    while ((len = getLine(line, MAXLINE)) > 0) {
        if (len > max) {
            max = len;
            copy(longest, line);
        }
    }

    if (max > 0) {
        printf("%s", longest);
    }
    return 0;
}

int getLine(char s[], int lim) {
    int c, i;

    for (i=0; i<lim-1 && (c=getchar())!='0' && c!='\n'; ++i) {
        s[i] = c;
    }
    if (c == '\n') {
        s[i] = c;
        ++i;
    }
    s[i] = '\0';
    return i;
}

void copy(char to[], char from[]) {
    int i;

    i = 0;
    while ((to[i] = from[i]) != '\0') {
        ++i;
    }
}

1.10 外部变量与作用域

函数中的局部变量被称为自动变量.

extern:

• 如果外部变量在使用它的函数之前, 那么可以省略 extern 关键字

#include <stdio.h>
#define MAXLINE 1000

int max;
char line[MAXLINE];
char longest[MAXLINE];

int getLine(void);
void copy(void);

int main() {
    int len;
    extern int max;
    extern char longest[];

    max = 0;
    while ((len = getLine()) > 0) {
        if (len > max) {
            max = len;
            copy();
        }
    }
    if (max > 0) {
        printf("%s", longest);
    }
    return 0;
}

int getLine() {
    int c, i;
    extern char line[];

    for (i=0; i<MAXLINE-1 && (c=getchar())!='0' && c!='\n'; ++i) {
        line[i] = c;
    }
    if (c == '\n') {
        line[i] = c;
        ++i;
    }
    line[i] = '\0';
    return i;
}

void copy() {
    int i;
    extern char line[], longest[];

    i = 0;
    while ((longest[i] = line[i]) != '\0') {
        ++i;
    }
}