Std::cout not printing expected float?

Question

This is really basic question, I'm testing std::cout to print float's, the following works correctly:

std::cout<<10 / 3.2<<std::endl; //Output: 3.125

But when I try:

std::cout<< 500000 / 1000000<<std::endl; //Output: 0

Why is the output of my second example not 0.5? Is this automatically rounding down?

I'm compiling with g++ -std=c++14

`500000 / 1000000` is *not* a `float` expression, it's an `int` expression. Make it a `float` with a cast, or a `double` by adding `.0` to one or both numbers. — Sergey Kalinichenko, Jan 18 '17 at 12:10
Both operands are integers, so you just do an integer division. Force one operand to float or double to have a double operation. — Serge Ballesta, Jan 18 '17 at 12:10

alexeykuzmin0 · Answer 1 · 2017-01-18T12:16:50.520

4

In your second code example 500000 and 1000000 both have integer type, so, when you divide one to another, you have integer division (the result is rounded bottom), which is 0. To fix this, try multiplying them by 1.0 or casting to any floating-point type.

edited Jan 18 '17 at 12:16

answered Jan 18 '17 at 12:10

alexeykuzmin0

6,344
2
28
51

Thanks, that makes sense. – Babra Cunningham Jan 18 '17 at 12:12
They might not be `int`s on a plaform with 16 bit `int`. That's why I'm careful in my answer not to talk about the specific type. – Bathsheba Jan 18 '17 at 12:16
@Bathsheba you right – alexeykuzmin0 Jan 18 '17 at 12:16
Much nicer. Have an upvote. – Bathsheba Jan 18 '17 at 12:17

Bathsheba · Accepted Answer · 2017-01-18T12:35:27.480

The compile time evaluable constant expression 500000 / 1000000 will be evaluated in integer arithmetic, which effectively means that any remainder is discarded. (Note that / has higher precedence than << so the division is evaluated first).

A clear rememdy is to promote one of the arguments to floating point (the other one is then promoted automatically by the compiler). I always pick the first one to signal to the reader of your code that you know what you're doing:

500000.0 / 1000000

Std::cout not printing expected float?

2 Answers2