Question

Could anyone explain to me why this keeps returning 0 when it should return a value of 42? it works on paper so i know the math is right I’m just wondering as to why it isn’t translating across?

    int a = 60;
    int b = 120;
    int c = 85;
    int progress;

    progress = ((c-a)/(b-a))*100;

    NSLog(@"Progess = %d %%",progress);

Answer 1

It’s because your math is all using integers.

In particular, your inner expression is calculating 25 / 60, which in integer math is zero.

In effect you have over-parenthesised your expression, and the resulting order of evaluation is causing integer rounding problems.

It would have worked fine if you had just written the formula so:

progress = 100 * (c - a) / (b - a);

because the 100 * (c – a) would first evaluate to 2500, and would then be divided by 60 to give 41.

Alternative, if any one (or more) of your variables a, b, or c were a float (or cast thereto) the equation would also work.

That’s because an expression in which either operand is a float will cause the other (integer) operand to be promoted to a float, too, at which point the result of the expression will also be a float.