if语句
#include<stdio.h>
int main(void)
{
const int FREEZING = 0;
float temperature;
int cold_days = 0;
int all_days = 0;
printf("Enter the list of daily low temperatures.\n");
printf("Use Celsius,and enter q to quit.\n");
while (scanf_s("%f", &temperature)) {
all_days++;
if (temperature < FREEZING)
cold_days++;
}
if (all_days != 0)
printf("%d days total:%.lf%% were below freezing.\n", all_days,
100.0*(float)cold_days / all_days);
if (all_days == 0)
printf("No data entered!\n");
system("pause");
return 0;
}
/*
if (expression) 分支语句。如果expression为真，则执行statement
statement
*/if else语句
/*取自上个例子*/
if (all_days != 0)
printf("%d days total:%.lf%% were below freezing.\n", all_days,
100.0*(float)cold_days / all_days);
else
printf("No data entered!\n");
/*
if (expression) 如果条件为真，执行statement1；如果条件为假，则执行statement2
statement1
else
statement2
*/getchar()和putchar()
两者只处理字符
#include<stdio.h>
#define SPACE ' '
int main(void)
{
char ch;
ch = getchar();
/*该函数不带任何其他参数，它从输入队列中返回一个字符*/
while (ch != '\n')
{
if (ch == SPACE)
putchar(ch); //打印字符
else
putchar(ch + 1);
ch = getchar();
}
putchar(ch);
system("pause");
return 0;
}ctype.h头文件
| 函数名 | 含义判断 |
|---|---|
| isalnum() | 字母或数字 |
| isalpha() | 字母 |
| isblank() | 标准的本地化空白字符 |
| iscntrl() | 控制字符 |
| isdigit() | 数字 |
| isgraph() | 除空格之外的任意可打印字符 |
| islower() | 小写字母 |
| isprint() | 可打印字符 |
| ispunct() | 除空格或字母数字字符以外的任何可打印字符 |
| isspace() | 空白字符 |
| isupper() | 大写字母 |
| isxdigit() | 十六进数字符 |
| tolower() | 返回小写字符 |
| toupper() | 返回大写字符 |
多重选择else if
#include<stdio.h>
#define RATE1 0.13230
#define RATE2 0.15040
#define RATE3 0.30025
#define RATE4 0.34025
#define BREAK1 360.0
#define BREAK2 468.0
#define BREAK3 720.0
#define BASE1 (RATE1*BREAK1)
#define BASE2 (BASE1+(RATE2*(BREAK2-BREAK1)))
#define BASE3 (BASE1+BASE2+(RATE3*(BREAK3-BREAK2)))
int main(void)
{
double kwh;
double bill;
printf("Please enter the kwh used.\n");
scanf_s("%lf", &kwh);
if (kwh <= BREAK1)
bill = RATE1 * kwh;
else if (kwh <= BREAK2)
bill = BASE1 + (RATE2*(kwh - BREAK1));
else if (kwh <= BREAK3)
bill = BASE2 + (RATE3*(kwh - BREAK2));
else
bill = BASE3 + (RATE4*(kwh - BREAK3));
printf("The charge for %.1f kwh is $%1.2f.\n", kwh, bill);
system("pause");
return 0;
}else与if配对
如果没有花括号，else与离他最近的if匹配，除非最近的if被花括号括起来了。
逻辑运算符
| 逻辑运算符 | 含义 |
|---|---|
| && | 与 |
| || | 或 |
| ！ | 非 |
备用拼写：ios646.h头文件
| 逻辑运算符 | 替代 |
|---|---|
| && | and |
| || | or |
| ! | not |
优先级
！运算符只比圆括号优先级低，&&运算符比||高，但是二者的优先级都比关系运算符低，比赋值运算符高。
求值顺序
求值顺序是从左到右，程序在从一个运算对象执行到下一个运算对象之前，所有副作用都会生效。而且，C保障一旦发现某个元素让整个表达式无效，便立即停止求值。
范围
if (range >=90&&range<=100)统计单词的程序
#include<stdio.h>
#include<ctype.h>
#include<stdbool.h>
#define STOP '|'
int main(void)
{
char c;
char prev;
long n_chars = 0L;
int n_lines = 0;
int n_words = 0;
int p_lines = 0;
bool inword = false;
printf("Enter text to be analyzed(| to terminate):\n");
prev = "\n";
while ((c = getchar()) != STOP) {
n_chars++;
if (c == '\n') {
n_lines++;
}
if (!isspace(c) && !inword) {
inword = true;
n_words++;
}
if (isspace(c) && inword) {
inword = false;
}
prev = c;
}
if (prev != '\n') {
p_lines = 1;
}
printf("characters = %ld,words = %d,lines=%d,", n_chars, n_words, n_lines);
printf("partial lines = %d\n", p_lines);
system("pause");
return 0;
}条件运算符：？：
expression1?expression2:expression3
/*
如果expression1为真，那么整个条件表达式的值与expression2的值相同；
如果expression1为假，那么整个条件表达式的值与expression3的值相同；
*/
max = (a>b)?a:b;循环辅助：continue和break
continue语句
#include<stdio.h>
int main(void)
{
const float MIN = 0.0f;
const float MAX = 100.0f;
float score;
float total = 0.0f;
int n = 0;
float min = MAX;
float max = MIN;
printf("Enter the first score(q to qiut):");
while (scanf_s("%f",&score)==1)
{
if (score<MIN||score>MAX)
{
printf("%0.1f is an invalid value.Try again:", score);
continue; //继续执行while
}
printf("Accepting %0.1f:\n", score);
min = (score < min) ? score : min;
max = (score > max) ? score : max;
total += score;
n++;
printf("Enter next score(q to qiut):");
}
if (n > 0)
{
printf("Average of %d scores is %0.1f.\n", n, total);
printf("Low = %0.1f,high=%0.1f\n", min, max);
}
else
printf("No valid scores were entered.\n");
system("pause");
return 0;
}break
#include<stdio.h>
int main(void)
{
float length, width;
printf("Enter the length of the rectangle:\n");
while ((scanf_s("%f", &length)) == 1)
{
printf("Length = %0.2f\n", length);
printf("Enter its width:\n");
if ((scanf_s("%f", &width) != 1))
break; //使得跳出这个循环
printf("Width = %0.2f", width);
printf("The area of the rectangle is %f", length*width);
printf("Enter the length of the rectangle:\n");
}
printf("Done!\n");
system("pause");
return 0;
}多重选择：switch和break
switch(expression) //expression只能是一个值，而不能是范围
{
case expression_1: //只会读取首字母
statement1;
break;
case expression_2:
statement2;
break;
case expression_3:
statement3;
break;
......
default:expression_end;//如果没有对应的case，则跳转到default来
}多重标签：
#include<stdio.h>
int main(void)
{
char ch;
int a_ct, e_ct, i_ct, o_ct, u_ct;
a_ct = e_ct = i_ct = o_ct = u_ct = 0;
printf("Enter some text;enter # to quit.\n");
while ((ch=getchar())!='#')
{
switch (ch)
{
case 'a':
case 'A':a_ct++;
break;
case 'e':
case 'E':e_ct++;
break;
case 'i':
case 'I':i_ct++;
break;
case 'o':
case 'O':o_ct++;
break;
case 'u':
case 'U':u_ct++;
break;
default:break;
}
}
printf("number of vowels:A:%4d E:%4d I:%4d O:%4d U:%4d\n",a_ct,e_ct,i_ct,o_ct,u_ct);
system("pause");
return 0;
}goto语句
goto part;//跳转到part去
part:statement;//必须有一个标签