關鍵字：if、else、switch、continue、break、case、default、goto
運算符：&&、||、？
函數：getchar()、putchar()、cype.h系列
如何使用if和if else語句，如何嵌套它們
在更複雜的測試表達式中用邏輯運算符組合關系表達式
C的條件運算符
switch語句
break、continue和goto語句
使用C的字元I/O函數：getchar()和putchar()
ctype.h頭檔案提供的字元分析函數系列
目錄
7.1 if語句
7.2 if else語句
7.2.1 介紹getchar()和putchar()
7.2.2 ctype.h系列的字元函數
7.2.3 多重選擇else if
7.2.4 多層嵌套的if語句
7.3 邏輯運算符
7.3.1 備選拼寫：iso646.h頭檔案
7.3.2 優先級
7.3.3 求值順序
7.3.4 範圍
7.4 一個統計單詞的程式
7.5 條件運算符：？
7.6 循環輔助：continue和break
7.6.1 continue語句
7.6.2 break語句
7.7 多重選擇：swithc和break
7.7.1 switch語句
7.7.2 隻讀每行的首字元
7.7.3 多重标簽
7.7.4 switch和if else
7.8 goto語句
7.9 關鍵概念
7.10 本章小結
7.1 if語句
// colddays.c -- finds percentage of days below freezing
#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("%f", &temperature) == 1)
{
all_days++;
if (temperature < FREEZING)
cold_days++;
}
if (all_days != 0)
printf("%d days total: %.1f%% were below freezing.\n",
all_days, 100.0 * (float) cold_days / all_days);
if (all_days == 0)
printf("No data entered!\n");
return 0;
}
[[email protected]] ch07$ clang colddays.c
[[email protected]] ch07$ ./a.out
Enter the list of daily low temperatures.
Use Celsius, and enter q to quit.
12 5 -2.5 0 6 8 -3 -10 5 10 q
10 days total: 30.0% were below freezing.
- while循環的測試條件利用scanf()的傳回值來結束循環，因為scanf()讀到非數字字元時會傳回0.
- 未避免整數除法，強制類型轉換。其實不使用也可以，C的自動轉換類型規則，乘積會被強制轉換成浮點數。為什麼選擇用強制類型轉換，是因為明确表達轉換類型的意圖，保護程式免受不同版本的編譯器的影響。
7.2 if else語句
if ( expression )
statement1
else
statement2
7.2.1 介紹getchar()和putchar()
ch = getchar(); //隻處理字元，不需要轉換說明
scanf("%c",&ch);
putchar(ch);
printf("%c",ch)
// cypher1.c -- alters input, preserving spaces
#include <stdio.h>
#define SPACE ' ' // that's quote-space-quote
int main(void)
{
char ch;
ch = getchar(); // read a character
while (ch != '\n') // while not end of line
{
if (ch == SPACE) // leave the space
putchar(ch); // character unchanged
else
putchar(ch + 1); // change other characters
//字元實際上作為整數存儲
ch = getchar(); // get next character
}
putchar(ch); // print the newline
return 0;
}
[[email protected]] ch07$ clang cypher1.c
[[email protected]] ch07$ ./a.out
CALL ME HAL.
DBMM NF IBM/
//ch = getchar(); // read a character
while ((ch = getchar()) != '\n') // while not end of line
{
if (ch == SPACE) // leave the space
putchar(ch); // character unchanged
else
putchar(ch + 1); // change other characters
// ch = getchar(); // get next character
}
C常見程式設計風格--把兩個行為合并成一個表達式.
另外“/”對應的ASCII碼比“.”對應的ASCII碼多1
7.2.2 ctype.h系列的字元函數
// cypher2.c -- alters input, preserving non-letters
#include <stdio.h>
#include <ctype.h> // for isalpha()
int main(void)
{
char ch;
while ((ch = getchar()) != '\n')
{
if (isalpha(ch)) // if a letter,
putchar(ch + 1); // display next letter
else // otherwise,
putchar(ch); // display as is
}
putchar(ch); // display the newline
return 0;
}
[[email protected]] ch07$ clang cypher2.c
[[email protected]] ch07$ ./a.out
Look! It's a programmer!
Mppl! Ju't b qsphsbnnfs!
7.2.3 多重選擇else if
// electric.c -- calculates electric bill
#include <stdio.h>
#define RATE1 0.13230 // rate for first 360 kwh
#define RATE2 0.15040 // rate for next 108 kwh
#define RATE3 0.30025 // rate for next 252 kwh
#define RATE4 0.34025 // rate for over 720 kwh
#define BREAK1 360.0 // first breakpoint for rates
#define BREAK2 468.0 // second breakpoint for rates
#define BREAK3 720.0 // third breakpoint for rates
#define BASE1 (RATE1 * BREAK1)
// cost for 360 kwh
#define BASE2 (BASE1 + (RATE2 * (BREAK2 - BREAK1)))
// cost for 468 kwh
#define BASE3 (BASE1 + BASE2 + (RATE3 *(BREAK3 - BREAK2)))
//cost for 720 kwh
int main(void)
{
double kwh; // kilowatt-hours used
double bill; // charges
printf("Please enter the kwh used.\n");
scanf("%lf", &kwh); // %lf for type double
if (kwh <= BREAK1)
bill = RATE1 * kwh;
else if (kwh <= BREAK2) // kwh between 360 and 468
bill = BASE1 + (RATE2 * (kwh - BREAK1));
else if (kwh <= BREAK3) // kwh betweent 468 and 720
bill = BASE2 + (RATE3 * (kwh - BREAK2));
else // kwh above 680
bill = BASE3 + (RATE4 * (kwh - BREAK3));
printf("The charge for %.1f kwh is $%1.2f.\n", kwh, bill);
return 0;
}
[[email protected]] ch07$ clang electric.c
[[email protected]] ch07$ ./a.out
Please enter the kwh used.
580
The charge for 580.0 kwh is $97.50.
7.2.4 多層嵌套的if語句
// divisors.c -- nested ifs display divisors of a number
#include <stdio.h>
#include <stdbool.h>
int main(void)
{
unsigned long num; // number to be checked
unsigned long div; // potential divisors
bool isPrime; // prime flag
printf("Please enter an integer for analysis; ");
printf("Enter q to quit.\n");
while (scanf("%lu", &num) == 1)
{
/*
for (div = 2; div < num; div++)
if(num % div == 0)
printf("%lu is divisible by %lu and %lu.\n",num, div, num / div);
*/
for (div = 2, isPrime = true; (div * div) <= num; div++)
//num是144，當div = 12時停止循環 。 num是145，當div = 13時停止循環
{
if (num % div == 0)
{
if ((div * div) != num)
printf("%lu is divisible by %lu and %lu.\n",
num, div, num / div);
else
printf("%lu is divisible by %lu.\n",
num, div);
isPrime= false; // number is not prime
//如何知道一個數是素數？如果沒有進入if語句，就是素數，标記（flag）
}
}
if (isPrime)
printf("%lu is prime.\n", num);
printf("Please enter another integer for analysis; ");
printf("Enter q to quit.\n");
}
printf("Bye.\n");
return 0;
}
[[email protected]] ch07$ clang divisors.c
[[email protected]] ch07$ ./a.out
Please enter an integer for analysis; Enter q to quit.
123456789
123456789 is divisible by 3 and 41152263.
123456789 is divisible by 9 and 13717421.
123456789 is divisible by 3607 and 34227.
123456789 is divisible by 3803 and 32463.
123456789 is divisible by 10821 and 11409.
Please enter another integer for analysis; Enter q to quit.
149
149 is prime.
Please enter another integer for analysis; Enter q to quit.
2013
2013 is divisible by 3 and 671.
2013 is divisible by 11 and 183.
2013 is divisible by 33 and 61.
Please enter another integer for analysis; Enter q to quit.
q
Bye.
7.3 邏輯運算符
// chcount.c -- use the logical AND operator
#include <stdio.h>
#define PERIOD '.'
int main(void)
{
char ch;
int charcount = 0;
while ((ch = getchar()) != PERIOD)
{
if (ch != '"' && ch != '\'') //如果測試字元不是雙引号，并且也不是單引号
charcount++;
}
printf("There are %d non-quote characters.\n", charcount);
return 0;
}
[[email protected]] ch07$ clang chcount.c
[[email protected]] ch07$ ./a.out
I didn't read the "I'm a Programming Fool" best seller.
There are 50 non-quote characters.
7.3.1 備選拼寫：iso646.h頭檔案
7.3.2 優先級
a > b && b > c || b > d
((a > b) && (b > c)) || (b > d)
7.3.3 求值順序
7.3.4 範圍
if(range >= 90 && range <= 100)
if(ch >= 'a' && ch <= 'z')
if(islower(ch))
7.4 一個統計單詞的程式
// wordcnt.c -- counts characters, words, lines
#include <stdio.h>
#include <ctype.h> // for isspace()
#include <stdbool.h> // for bool, true, false
#define STOP '|'
int main(void)
{
char c; // read in character
char prev; // previous character read
long n_chars = 0L; // number of characters
int n_lines = 0; // number of lines
int n_words = 0; // number of words
int p_lines = 0; // number of partial lines
bool inword = false; // == true if c is in a word
//一個單詞裡是否有某個字元，在程式讀入單詞的首字元時把标記inword設定為1
//隻要為1，後續的非空白字元都不記為單詞的開始
printf("Enter text to be analyzed (| to terminate):\n");
prev = '\n'; // used to identify complete lines
while ((c = getchar()) != STOP)
{
n_chars++; // count characters
if (c == '\n')
n_lines++; // count lines
if (!isspace(c) && !inword) //查找單詞，不含空格
{
inword = true; // starting a new word
n_words++; // count word
}
if (isspace(c) && inword)
inword = false; // reached end of word
prev = c; // save character value
}
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);
return 0;
}
[[email protected]] ch07$ clang wordcnt.c
[[email protected]] ch07$ ./a.out
Enter text to be analyzed (| to terminate):
Reason is a
powerful servant but
an inadequate master.
!
|
characters = 58, words = 10, lines = 4, partial lines = 0
如果c不是空白字元，且inword為假
設定inword為真，并給單詞計數
如果c是空白字元，且inword為真
設定inword為假
即讀到每個單詞的開頭時，把inword設定為1（真），在讀到每個單詞的末尾時把inword設定為0（假），隻有标記從0設定為1時，遞增單詞計數。
if(inword) if(inword == true)
if(!inword) if(inword == false)
7.5 條件運算符：？
條件運算符是C語言中唯一的三元運算符
expression1 ? expression2 : expression3
/* paint.c -- uses conditional operator */
//計算給定平方英尺的面積需要多少罐油漆，用平方英尺數除以每罐油漆能刷的面積，但是，商店隻賣整罐油漆，不會拆分來賣，是以1.7罐，就需要兩罐。得到小樹結果應該進1.
#include <stdio.h>
#define COVERAGE 350 // square feet per paint can
int main(void)
{
int sq_feet;
int cans;
printf("Enter number of square feet to be painted:\n");
while (scanf("%d", &sq_feet) == 1)
{
cans = sq_feet / COVERAGE; //截斷
cans += ((sq_feet % COVERAGE == 0)) ? 0 : 1; //整除 ； 餘數
printf("You need %d %s of paint.\n", cans,
cans == 1 ? "can" : "cans");
printf("Enter next value (q to quit):\n");
}
return 0;
}
[[email protected]] ch07$ clang paint.c
[[email protected]] ch07$ ./a.out
Enter number of square feet to be painted:
349
You need 1 can of paint.
Enter next value (q to quit):
351
You need 2 cans of paint.
Enter next value (q to quit):
q
7.6 循環輔助：continue和break
7.6.1 continue語句
/* skippart.c -- uses continue to skip part of loop */
#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 quit): ");
while (scanf("%f", &score) == 1)
{
if (score < MIN || score > MAX)
{
printf("%0.1f is an invalid value. Try again: ",
score);
continue; // jumps to while loop test condition
//跳過本次疊代的剩餘部分，并開始下一輪疊代。
//continue語句在嵌套循環内，則隻影響包含該語句的記憶體循環
}
/*
//減少主語句組的一級縮進，提高代碼可讀性
if(score < MIN || score > MAX) {}
else{
}
if(score >= 0 && score <=100)
*/
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 quit): ");
}
if (n > 0)
{
printf("Average of %d scores is %0.1f.\n", n, total / n);
printf("Low = %0.1f, high = %0.1f\n", min, max);
}
else
printf("No valid scores were entered.\n");
return 0;
}
[[email protected]] ch07$ clang skippart.c
[[email protected]] ch07$ ./a.out
Enter the first score (q to quit): 50
Accepting 50.0:
Enter next score (q to quit): 200
200.0 is an invalid value. Try again: q
Average of 1 scores is 50.0.
Low = 50.0, high = 50.0
count = 0;
while (count < 10)
{
ch = getchars();
if(ch == '\n')
continue;
putchar(ch);
count++;
}
//除換行符，因為ch是換行符，程式會跳過count++語句，并重新顯示它們
for(count = 0; count < 10; count++)
{
ch = getchars();
if(ch == '\n')
continue;
putchar(ch)
}
//換行符也計算在内
7.6.2 break語句
/* break.c -- uses break to exit a loop */
#include <stdio.h>
int main(void)
{
float length, width;
printf("Enter the length of the rectangle:\n");
//while(scanf("%f %f",&length,&width)==2)
while (scanf("%f", &length) == 1)
{
printf("Length = %0.2f:\n", length);
printf("Enter its width:\n");
if (scanf("%f", &width) != 1)
break;
printf("Width = %0.2f:\n", width);
printf("Area = %0.2f:\n", length * width);
printf("Enter the length of the rectangle:\n");
}
printf("Done.\n");
return 0;
}
[[email protected]] ch07$ clang break.c
[[email protected]] ch07$ ./a.out
Enter the length of the rectangle:
10
Length = 10.00:
Enter its width:
20
Width = 20.00:
Area = 200.00:
Enter the length of the rectangle:
ab
Done.
7.7 多重選擇：swithc和break
switch(整型表達式)
{
case 常量1:
語句
case 常量2
語句
default:
語句
}
/* animals.c -- uses a switch statement */
#include <stdio.h>
#include <ctype.h>
int main(void)
{
char ch;
printf("Give me a letter of the alphabet, and I will give ");
printf("an animal name\nbeginning with that letter.\n");
printf("Please type in a letter; type # to end my act.\n");
while ((ch = getchar()) != '#')
{
if('\n' == ch)
continue;
if (islower(ch)) /* lowercase only */
switch (ch)
{
case 'a' :
printf("argali, a wild sheep of Asia\n");
break;
case 'b' :
printf("babirusa, a wild pig of Malay\n");
break;
case 'c' :
printf("coati, racoonlike mammal\n");
break;
case 'd' :
printf("desman, aquatic, molelike critter\n");
break;
case 'e' :
printf("echidna, the spiny anteater\n");
break;
case 'f' :
printf("fisher, brownish marten\n");
break;
default :
printf("That's a stumper!\n");
} /* end of switch */
else
printf("I recognize only lowercase letters.\n");
while (getchar() != '\n')
continue; /* skip rest of input line */
printf("Please type another letter or a #.\n");
} /* while loop end */
printf("Bye!\n");
return 0;
}
[[email protected]] ch07$ clang animals.c
[[email protected]] ch07$ ./a.out
Give me a letter of the alphabet, and I will give an animal name
beginning with that letter.
Please type in a letter; type # to end my act.
a
argali, a wild sheep of Asia
Please type another letter or a #.
dab
desman, aquatic, molelike critter
Please type another letter or a #.
r
That's a stumper!
Please type another letter or a #.
Q
I recognize only lowercase letters.
Please type another letter or a #.
#
Bye!
7.7.1 switch語句
7.7.2 隻讀每行的首字元
7.7.3 多重标簽
// vowels.c -- uses multiple labels
#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;
} // end of switch
} // while loop end
printf("number of vowels: A E I O U\n");
printf(" %4d %4d %4d %4d %4d\n",
a_ct, e_ct, i_ct, o_ct, u_ct);
return 0;
}
[[email protected]] ch07$ clang vowels.c
[[email protected]] ch07$ ./a.out
Enter some text; enter # to quit.
I see under the overserr.#
number of vowels: A E I O U
0 6 1 1 1
7.7.4 switch和if else
7.8 goto語句
7.9 關鍵概念
7.10 本章小結