C++ IF Statement inside a function issue

An expression like:

a < b < c

does not do what you think it does. It actually evaluates as:

(a < b) < c

where a < b is a truth value giving 0 for false and 1 for true`. Then that value is used in the rest of the expression.

By way of example, the expression 100 < 200 < 42 will be true because 100 < 200 evaluates to 1, which is definitely less than 42.

Instead you should be using something like:

(a < b) && (b < c)

In any case, you can clean up that code quite a bit by realising that:

1. The construct if (condition) return else ... is totally unnecessary - the return means that the rest of the code will execute only if condition is false, rendering the else superfluous.

2. Because your ranges are mutually exclusive, the checks can be simplified. In other words, no need to check if (m - (1.5*s)) <= score in the D case since, if that weren't true, the code would already have returned F.

3. It is currently impossible to get an X grade since there is no score that doesn't get captured one of the if statements, excepting possibly some edge cases like NaN but I'm going to assume you're not worried about that.

With those points in mind, the code can be simplified to:

char grade(double score, double m, double s) {
    if (score < m - 1.5 * s) return 'F';
    if (score < m - 0.5 * s) return 'D';
    if (score < m + 0.5 * s) return 'C';
    if (score < m + 1.5 * s) return 'B';
    return 'A';
}

We would need to know the input data values to have a complete understanding, however as others have pointed out, comparisons don't work like that in C++.

Take this comparison: (m - (1.5*s)) <= score < (m - (0.5 * s))

What this is doing is first evaluating: (m - (1.5*s)) <= score

Due to the first if statement failing, this will always evaluate to true. In C++, boolean values are integers, and true is usually represented by "1" (in practice it doesn't have to be, and can be represented by any non-zero number, but I think in this specific case it's being set to 1).

Then it's evaluating: 1 < (m - (0.5*s))

Which I'm assuming always returns true given your values of "m" and "s".

What you should be doing instead is using a compound statement:

if (m - 1.5*s <= score && score < m - 0.5*s)

(you can keep the parentheses if you like, but they're unnecessary here due to C++'s operator precedence rules)

Just drop all of the lower-bound comparisons (which, as others have said, do not do what you thought): if any of them don’t apply, you would already have returned one of the lower scores.