C程序設(shè)計英文課件：CHAPTE 3 Control Flow

1、CHAPTE 3statements specify the order in which computations are performedContents1. Statements and Blocks2. If-else3. Else-if4. Switch 5. Loops- While and For 6. Loops-Do-while7. Break and Continue8. Goto and Labels3.1 Statements and Blocks3.1 Statements and Blocks（語句與分程序）; (Semicolon) is a statement

2、 terminator分號是語句的結(jié)束符號 (Braces) are used to group declarations and statements together into a compound statement or block. 花括號表示了復(fù)合語句或是語句組3.1 Statements and Blocksmain( ) while(c=getchar()!=EOF ) main() c=min(a,b);int min(int a, int b)if( c!=EOF) .3.2 If-Else3.2 If-ElseSyntax(語法規(guī)則):if(expression)stat

3、ement1;elsestatement2;Statement ended with ;條件判斷NOTE: else 與最近的 if 進(jìn)行組合3.2 If-Elseif(n0)if(ab)z=a;elsez=b;if (n0)if(ab)z=a;elsez=b;if(n0) if(ab)z=a; elsez=b;用 可以界定if-else的作用范圍3.2 If-Elseif(n=0) for(i=0;i0) printf(“”);else printf(“error- n is a negativen”);Note:Its a good habit to use braces when the

4、re are nested ifs.if(n=0) for(i=0;i0) printf(“”);else /*wrong */ printf(“error- n is a negativen”);3.3 Else-If3.3 Else-Ifif(expression1)statements1;else if(expression2)statements2;else if(expression3)statements3;elsestatementsn;syntaxNone of the above3.3 Else-If3.3 Else-Ifint binsearch(int x,int v ,

5、 int n)int low,high,mid;low=0;high=n-1;while(low=high) mid=(low+high)/2;if(xvmid)low=mid+1;else /* found match*/return mid;return 1;/* no match*/例：在一個數(shù)組中找與給定的值相等的數(shù)。假定數(shù)組已經(jīng)按從小到大的順序排好。3.4 Switch3.4 Switchsyntaxswitch(expression) case const-expr1: statements 1case const-expr2: statements 2.default: stat

6、ements n執(zhí)行過程：1、當(dāng)switch后面“表達(dá)式”的值，與某個case后面的“常量表達(dá)式”的值相同時，就執(zhí)行該case后面的語句（組）；2、如果沒有任何一個case后面的“常量表達(dá)式”的值，與“表達(dá)式”的值匹配，則執(zhí)行default 后面的語句（組）。然后，再執(zhí)行switch語句的下一條。3.4 Switchswitch(grade) case A : printf(“85100n”); case B : printf(“7084n”); case C : printf(“6069n”); case D : printf(“60n”);defaut : printf(“errorn”)

7、;3.4 Switch說明（1）switch后面的“表達(dá)式”，可以是int、char和枚舉型中的一種。（2）每個case后面“常量表達(dá)式”的值，必須各不相同，否則會出現(xiàn)相互矛盾的現(xiàn)象（即對表達(dá)式的同一值，有兩種或兩種以上的執(zhí)行方案）。（3）case后面的常量表達(dá)式僅起語句標(biāo)號作用，并不進(jìn)行條件判斷。系統(tǒng)一旦找到入口標(biāo)號，就從此標(biāo)號開始執(zhí)行，不再進(jìn)行標(biāo)號判斷，所以必須加上break語句，以便結(jié)束switch語句。3.4 Switchswitch(grade) case A : printf(“85100n”); case B : printf(“7084n”); case C : printf(

8、“6069n”); case D : printf(“60n”);default : printf(“errorn”);若grade的值等于B，執(zhí)行結(jié)果會是什么？7084606960errorbreak可以中止switch的繼續(xù)執(zhí)行switch(grade) case A : printf(“85100n”); break; case B : printf(“7084n”); break; case C : printf(“6069n”); break; case D : printf(“60n”); break;default : printf(“errorn”);Example：count

9、 digit,white space, othersmain( )int c,i,nwhite,nother,ndigit10;nwhite=nother=0;for(i=0;i10;i+) ndigiti=0;while(c=getchar()!=EOF) switch(c) case 0: case 1: case 2: case 3: case 4: case 5: case 6: case 7: case 8: case 9:ndigitc-0+;break; case : case n: case t: nwhite+; break;default:nother+; break;br

10、eak statement causes an immediate exit from swicth注：各case及default子句的先后次序，不影響程序執(zhí)行結(jié)果。多個case子句，可共用同一語句（組）。3.4 SwitchExample:Write a program that inputs arithmetic operator and variables, printing the result.main() int a,b; char op; scanf(%d%c%d,&a,&op,&b); switch(op) case +: printf(%dn“,a+b);break; cas

11、e -: printf(%dn“,a-b);break; case *: printf(%dn“,a*b);break; case /: if(b=0) printf(“division by zeron”); else printf(%dn“,a/b); break; default: printf(“unknown operatorn); 3.4 SwitchExample:Write a program that inputs arithmetic operator and variables, printing the result.main() int a,b; char op; s

12、canf(%d%c%d,&a,&op,&b); if(op=+) printf(%dn“,a+b); else if(op=-) printf(%dn“,a-b); else if(op=*) printf(%dn“,a*b); else ifop=/) if(b=0) printf(“division by zeron”); else printf(%dn“,a/b); else printf(“unknown operatorn);3.5 Loops-While and For3.5 Loops-While and Forsyntaxwhile(expression) statement;

13、for(expr1;expr2;expr3)statement;expr1;while(expr2) statement; expr3;3.5 Loops-While and Forfor(expr1;expr2;expr3)statement; expr1 and expr3 are assignment or function calls, expr2 is a relational expression . all of them can be omitted, if expr2 omitted, taken as permanently true.for(變量賦初值；循環(huán)繼續(xù)條件；循環(huán)

14、變量增值) 循環(huán)體語句組；3.5 Loops-While and Forthe expr1 is omittedfor( ;i=100; i+ )sum+=i;the expr2 is omittedfor(i=1 ; ; i+ )sum+=i;the expr3 is omittedfor(i=1;i=100; ) sum+=i; i+; the expr1 and expr3 are omitted, equal to whilefor( ;i=100;) sum+=i;i+; all are omittedfor ( ; ;) while(1)the expr1 can initiali

15、ze other expressionsfor( sum=0;i=100; i+ )sum+=i;while(i=100) sum+=i; i+; 3.5 Loops-While and Forthe expr1 and expr3 may be a Comma expression;for(sum=0,i=1; i=100; i+) sum+=i; for(i=0,j=100 ; ij; i+,j-)k=i+j; the expr2 is relational expression, 0 or not 0for ( ; (c=getchar()!=n ;)3.5 Loops-While an

16、d ForExample:convert s to integer#include int atoi(char s) int i,n,sign;for(i=0;isspace(si);i+);sign=(si=-)? 1:1;if(si=+ | si=-) i+;for(n=0;isdigit(si);i+)n=10*n+(si-0);return sign*n;3.5 Loops-While and ForExample: sort an array into increasing ordervoid popsort(int v ,int n) /*起泡法*/ int i,j,t; for(

17、j=0;jn-1;j+) for(i=0;ivi+1) t=vi; vi=vi+1; vi+1=t; 相鄰兩數(shù)相比，大數(shù)后移；大數(shù)沉底，小數(shù)浮起3.5 Loops-While and ForExample:reverse string s in place#include void reverse(char s)int c,i,j;for(i=0, j=strlen(s)-1; ij; i+, j-) c=s j ;s j =si;si=c;3.5 Loops-While and For3.6LoopsDo-While syntaxdostatementwhile (expression);N

18、ote: while and for loops test the termination condition at the top, Do-while tests at the bottom. the body is always executed at least once.3.6 LoopsDo-WhileExample:convert n to charaters in s(version1)void itoa(int n, chars)int i,sign;if(sign=n)0);if (sign=0;n-)if(sn!= &sn!=t&sn!=n)break;sn+1=0;ret

19、uen n;3.7 Break and ContinueContinue causes the next iteration of the enclosing loop(while, for, do-while) to begin. only used in loop, not in switch. 終止當(dāng)前循環(huán)運算，轉(zhuǎn)入下一次循環(huán)，不能用于switch語句。3.7 Break and Continuefor(i=0,in;i+)if(ai0) /*skip negative elements*/continue;./* do positive elements*/3.7 Break and Continuewhile(expr1) . if(expr2) b