3-4

运行结果1

结果2

结果3

#include
     
      int main(void){    double x,y;    printf("Enter x:");    scanf("%lf",&x);    if(x<0){        y=0;    }    else if(x<=15){        y=4*x/3;    }    else{        y=2.5*x-10.5;    }    printf("f(%.2f)=%.2f\n",x,y);    return 0;}
     

 

3-5

 

#include
     
      int main(void){    double value1,value2;    char op;    printf("Type in an expression:");    scanf("%lf%c%lf",&value1,&op,&value2);    if(op=='+')        printf("=%.2f\n",value1+value2);    else if(op=='-')        printf("=%.2f\n",value1-value2);    else if(op=='*')        printf("=%.2f\n",value1*value2);    else if(op=='/')        printf("=%.2f\n",value1/value2);    else        printf("Unknown operator\n");    return 0;}
     

 

 3-7

#include
     
      int main(void){    int digit,letter,other;    char ch;    int i;    digit=letter=other=0;    printf("Enter 10 characters:");    for(i=1;i<=10;i++){        ch=getchar();        if((ch>='a'&&ch<='z')||(ch>='A'&&ch<='Z'))            letter++;        else if(ch>='0'&&ch<='9')            digit++;        else            other++;    }    printf("letter=%d,digit=%d,other=%d\n",letter,digit,other);    return 0;}