How to fix the result equals to zero everytime i run the program

Question

I have a homework that is all about computing the grade of a student. The formula is multiplying it by 30% then dividing by 100. I'm sure that I typed properly the code but when I run the program the answer is always zero.

int grade, s1,s2,s3,s4;
float average;

cout << "Enter grade in 1st quiz: ";
cin >> s1;
cout << "Enter grade in 2nd quiz: ";
cin >> s2;
cout << "Enter grade in 3rd quiz: ";
cin >> s3;
cout << "Enter grade in 4th quiz: ";
cin >> s4;

s1=(s1*=.30)/100;
s2=(s2*=.30)/100;
s3=(s3*=.20)/100;
s4=(s4*=.20)/100;

average = s1+s2+s3+s4;

cout << "Your average is " << average*100.0 << endl ; //average should not be zero.

I expect the ouput to be 80 to 90 but the actual output is always 0.

Instead of the type int use the type float for the variables s1, s2, s3, and s4. — Vlad from Moscow, Aug 31 '19 at 12:27
When you have integer division the result must be an integer so something like 47/100 is 0. — drescherjm, Aug 31 '19 at 12:29
Do you know the difference between `s1=(s1*=.30)/100;` and the more usual `s1=(s1*.30)/100;`? What is your reason for doing it your way? — Yunnosch, Aug 31 '19 at 12:33
Also `s1=(s1*=.30)/100` is undefined behavior in C++ **because you modify the same variable more than once in a single expression**. — Phil1970, Aug 31 '19 at 12:35
@Phil1970 I am not sure that "modify the same variable more than once in a single **expression**" is applicable here. I do not doubt twice within a **statement**, but expression? — Yunnosch, Aug 31 '19 at 12:37
If variables are of type `int`, results stored to them will have integral values. So, if `s1 = 100` is entered, `s1 *= .3` will produce a result of about `30` (subject to rounding error since the calculation is done as floating point). If that result is divided by `100`, the result will always be zero (rounding is toward zero for positive values). Also, it is a really bad idea to modify a variable more than once in an expression. `s1 = (s1 *= .3)/100` modifies `s1` twice -> undefined behaviour (older standards) or unspecified behaviour (in some cases, in recent standards). — Peter, Aug 31 '19 at 12:47
Well, even if the behavior might be defined in newer standard, it is still a bad idea to write it that way (less readable). And since `s1` is an integer, then you have truncation (doing the `*=`) and not the intended result anyway. — Phil1970, Aug 31 '19 at 12:50
@Yunnosch - the behaviour is mostly undefined if modifying a variable twice in an expression. In some cases, in more recent standards, there is some sequencing or the behaviour is unspecified. — Peter, Aug 31 '19 at 12:50
Also assuming that `s1` to `s4` and `average` are number between 0 and 100, then you are essentially dividing twice by 100. So even if you were using floating point, the result would be 1% of the expected value. Thus in your example, you might get 1 if all notes are 100 (or above if some bonus are possible). You really want something like `int average = (s1 * 30 + s2 * 30 + s3 * 20 + s4 * 20) / 100;` with a possible `+50` if you want to round the final average. **That code is easier to read, correct and more efficient**. With than change remove `* 100` in `cout` statement. — Phil1970, Aug 31 '19 at 12:57

score 0 · Answer 1 · answered Aug 31 '19 at 12:32

In C++, the result of integer division is always an integer. You want those integers to be doubles or floats:

double grade, s1, s2, s3, s4;

This fixes the problem.

Also for calculations:

s1 = (s1 *= .30) / 100;

I am almost sure you meant:

s1 = (s1 * .30) / 100;

score 0 · Answer 2 · answered Aug 31 '19 at 12:32

Either declare the variables s1, s2, s3 and s4 as having the type float or double or exclude these statements

s1=(s1*=.30)/100;
s2=(s2*=.30)/100;
s3=(s3*=.20)/100;
s4=(s4*=.20)/100;

and write instead

average = (s1 *.30)/100 + s2 *.30)/100 + (s3 * .20)/100 + ( s4 * .20)/100;

Otherwise there is a truncation in assignments like this

s1=(s1*=.30)/100;
 ^^^  ^^^

because s1 is an integer and there is a conversion from the type double to the type int.

How to fix the result equals to zero everytime i run the program

2 Answers2