我不知道为什么它可以进行计算？

Question

My programming is about making a water bill.I had trouble running on my calculation part and it just show 0 when i key in any data. The calculation is amount=sqrfeet*2.05; is it because of the floating part?? And i am using if else statement.

int main()
    { char ch;
    int a;
    float amount, sqrfeet;
    start:
    printf("\n-------------Selangor Water Bill--------------\n");
    printf("\n1.Domestic\n");
    printf("\n2.Commercial\n");
    printf("\n3.Industrial");
    printf("\nChoose one of the category:",a);
    scanf("%d",&a);
    system("CLS");
    if (a!=1&&a!=2&&a!=3){
    goto start;
    }
    if (a=1){
    printf("\n1.Domestic");
    printf("\nEnter the amount of consumption water per sqrfeet in 35days:");
    scanf("%f",&amount);
    if(sqrfeet<=35){
        amount=sqrfeet*0.57;
    }
    else if(sqrfeet>35&&sqrfeet<=50){
        amount=sqrfeet*1.03;
    }
    else if (sqrfeet>50){
        amount=sqrfeet*2.00;
    }
    printf("\nTotal Amount is RM%.2lf", amount);
    }
    else if(a=2){
    printf("\n2.Commercial");
    printf("\nEnter the amount of consumption water per sqrfeet in 
    35days:");
    scanf("%f",&amount);
    if(sqrfeet<=35){
        amount=sqrfeet*2.07;
    }
    else if(sqrfeet>35&&sqrfeet<=50){
        amount=sqrfeet*2.28;
    }
    printf("\nTotal Amount is RM%.2lf", amount);
    }
    else if (a=3){
    printf("\n3.Industrial");
    printf("\nEnter the amount of consumption water per sqrfeet in 35days:");
    scanf("%f",&amount);
    if(sqrfeet<=35){
        amount=sqrfeet*2.49;
    }
    else if(sqrfeet>35&&sqrfeet<=50){
        amount=sqrfeet*2.70;
    }
    printf("\nTotal Amount is RM%.2lf", amount);
    }

    printf("\nPress'q' to quit ");
    scanf("%c",&ch);
    return 0;
    }

Answer 1

I don't know why it can run on calculation?

Either the compiler is weak or warnings are not fully enabled.

Save time and enable all warnings and receive ones like the below. This will help locate many, though not all, problems.

// gcc example
gcc -std=c11 -O3 -pedantic -Wall -Wextra -Wconversion -c ...

---

warning: suggest parentheses around assignment used as truth value [-Wparentheses]

 if (a = 1) {

The compiler sees this assignment and suggests if ((a = 1)) { to quiet the warning. Yet OP certainly does not want an assignment here, but a compare. @dbush

 if (a == 1) {

---

warning: 'sqrfeet' is used uninitialized in this function [-Wuninitialized]

Code is using sqrfeet before it is assigned. Certainly scanf("%f",&sqrfeet); was desired. @Tomek Piechocki

// scanf("%f",&amount);
scanf("%f",&sqrfeet);
if(sqrfeet<=35){

---

warning: too many arguments for format [-Wformat-extra-args]

The printf() call is mal-formed.

// printf("\nChoose one of the category:", a);
printf("\nChoose one of the category:");

---

warning: conversion to 'float' from 'double' may alter its value [-Wfloat-conversion]

0.57 is a double . No reason to use float for typical floating point math. Use double . Save float for large arrays or timing critical concerns.

// float amount, sqrfeet;
//scanf("%f", &amount);
double amount, sqrfeet;
scanf("%lf", &amount);

amount = sqrfeet * 0.57;

If you want to stay with float , use a float constant: 0.57f .

---

Also: all scanf() specifiers consume leading white-space except, c, [, n . Add a space to consume the prior input line's left-over '\\n' ( Enter ).

// scanf("%c",&ch);
//     v
scanf(" %c",&ch);

Answer 2

I think that the problem is this line

scanf("%f",&amount);

I assume that you want it like this

scanf("%f",&sqrfeet);

Answer 3

int main()
{ char ch;
int a;
float amount, sqrfeet;
start:
printf("\n-------------Selangor Water Bill--------------\n");
printf("\n1.Domestic\n");
printf("\n2.Commercial\n");
printf("\n3.Industrial");
printf("\nChoose one of the category:",a);
scanf("%d",&a);
system("CLS");
if (a!=1&&a!=2&&a!=3){
goto start;
}
if (a=1){
printf("\n1.Domestic");
printf("\nEnter the amount of consumption water per sqrfeet in 35days:");
scanf("%f",&sqrfeet); // Change made here
if(sqrfeet<=35){
    amount=sqrfeet*0.57;
}
else if(sqrfeet>35&&sqrfeet<=50){
    amount=sqrfeet*1.03;
}
else if (sqrfeet>50){
    amount=sqrfeet*2.00;
}
printf("\nTotal Amount is RM%.2lf", amount);
}
else if(a=2){
printf("\n2.Commercial");
printf("\nEnter the amount of consumption water per sqrfeet in 
35days:");
scanf("%f",&sqrfeet); // Change made here
if(sqrfeet<=35){
    amount=sqrfeet*2.07;
}
else if(sqrfeet>35&&sqrfeet<=50){
    amount=sqrfeet*2.28;
}
printf("\nTotal Amount is RM%.2lf", amount);
}
else if (a=3){
printf("\n3.Industrial");
printf("\nEnter the amount of consumption water per sqrfeet in 35days:");
scanf("%f",&sqrfeet); // Change made here
if(sqrfeet<=35){
    amount=sqrfeet*2.49;
}
else if(sqrfeet>35&&sqrfeet<=50){
    amount=sqrfeet*2.70;
}
printf("\nTotal Amount is RM%.2lf", amount);
}

printf("\nPress'q' to quit ");
scanf("%c",&ch);
return 0;
}

Answer 4

Several issues:

1. You never assign a value to sqrfeet , so all your computations with sqrfeet are going to be bad. You probably want a separate scanf statement to read that when you also read amount .
2. Your if statements are using = (assignment) instead of == (comparison), which is re-assigning a . Instead of if ( a = 1 ) use if ( a == 1 ) .
3. While it's not the cause of your problem, avoid using goto when possible. Your input loop can be better written as

    int a = -1; // initialize a to a known bad value do { // print menu options /** * Check the result of scanf - if it isn't 1, then the the * user entered non-numeric input. Clear the bad input up * to the next newline before trying again. */ if ( scanf("%d", &a ) != 1 ) while ( getchar() != '\\n' ) ; } while ( a < 1 && a > 3 );