c4第四章选择结构程序设计.pptVIP

第四章 选择结构程序设计 教学目的：掌握选择结构的形式，掌握典型算法。 教学要求：掌握关系表达式和逻辑表达式以及在选择结构中的使用。学会把现实问题通过表达式的形式表示。 (补充)从数学而言，当y是正时，x+yx等价于y0，它们的逻辑结果都是true。但从计算机而言，当x与y的绝对值相差很大时，这个值就有可能是false #include stdio.h main() { double x=1e20,y=10; printf(%d, x+yx); getch(); } /* 输出是0 */ 例如：已知三角形的三条边求面积 程序 #include stdio.h #include math.h main ( ){ float a,b,c,s,area; printf(Side a=); scanf(%f,a); printf(Side b=); scanf(%f,b); printf(Side c=); scanf(%f,c); if (a+bca+cbb+ca){ s=0.5*(a+b+c); area=sqrt(s*(s-a)*(s-b)*(s-c)); printf(area=%6.2f\n,area); } else printf(“Sorry,reentry please.); getch(); } A code fragment is to be used to separate integer values into five ranges. For example: 0 .. 29, 30 .. 49, 50 .. 59, 60 .. 69, 70 .. 100. What is the fewest number of if .. else selection statements needed to do this? Note, all data presented will already have been validated to ensure they are in the range 0 ..100. break语句（中断语句） 语句形式：break; 作用：中止switch结构或循环结构的执行， 转去执行相应结构的下一条语句。 说明：break语句只能用于switch结构或者循 环结构之中。 switch statements 使用if语句嵌套层次太多，容易发生错误，因此在多分支结构中，通常使用switch语句 。 Suppose we had an integer variable j, and wanted to execute different statements depending on the value of j. Lets suppose we wanted to print j is one if j had value 1, j is two if j has the value 2, otherwise we want to print j is neither 1 nor 2. We could write the following code: if (j == 1) printf(“j is one”); else if (j == 2) printf(“ j is two”); else printf(“j is neither 1 nor 2”); OR we could use a switch statement. switch statement example switch (j) { case 1: printf(“ j is one”); break; case 2: printf(“ j is two”); break; default: printf(“j is neither 1 nor 2”); } When used in a switch statement , the break statement causes control to pass to the statement following the closing brace. switch statement switch语句的一般形式 switch (expr) { case constant1: statement1; break; ca