C++ weird rounding when decimal passed into function

Question

I'm new to C++ and messing around with functions. I have two cpp files and one header. One of my cpp files is main.cpp where the following code is:

a->change(42.57);

I then have another cpp where the following code for change is:

bool Student::change(float mark)

and then the header where the following code is:

bool change(float mark);

When I pass the float in, it is 42.57. However, after it is passed in it suddendly chnages to 42.5699997. Could someone please explain why this happens?

what happens when you look at the value without passing it to any function? How are you inspecting it anyhow? — 463035818_is_not_an_ai, Dec 16 '20 at 15:37
Because 42.57 cannot be represented exactly in binary floating point. — Fred Larson, Dec 16 '20 at 15:38
@AsteroidsWithWings: Absolutely. It's no more broken than expecting something like `int n = 1.1` to set `n` to anything other than 1. — Bathsheba, Dec 16 '20 at 15:48
@Bathsheba Or calling your notepad "broken" because it failed to properly hold all the decimal places of the number ⅓ when you wrote it out. — Asteroids With Wings, Dec 16 '20 at 15:49
@AsteroidsWithWings: It was a joke of course hence the `;-)`. But you are right on one point some could really think that its is so I will try to avoid to say it now (and I have deleted the comment) — Serge Ballesta, Dec 16 '20 at 16:01
The **text** that represents the value is `42.57`. Text has to be converted to a value. Don’t skip that step in your analysis. — Pete Becker, Dec 16 '20 at 18:30

Bathsheba · Accepted Answer · 2020-12-16T15:45:56.090

3

42.57 is a double type. (The nearest double is 42.57000000000000028421709430404007434844970703125).

Use 42.57f for a float type. (The nearest float is 42.56999969482421875).

Both therefore are approximations (the float is a looser approximation than the double) but at least your debugger will display the float more appropriately.

edited Dec 16 '20 at 15:45

answered Dec 16 '20 at 15:37

Bathsheba

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C++ weird rounding when decimal passed into function

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