Simple counter won't trigger if statement

Hello, there is something strange happening when I run this code (workspace is empty before I run this)

 tplot = 0;
 for t = 1:100
    tplot = tplot+0.1;
    if tplot == 1
        tplot
        tplot = 0;
    end
 end

It never jumps inside the if block. When I increase tplot by 0.2 or 0.5 instead of 0.1 it works. I used the debugger to see what's happening when I use 0.1. When tplot reaches 1.0000 it totally ignores the if part.

Do you encounter the same problem? If so, what could be the reasen for this?

Answer 1

[Roger Stafford]

This is a classic beginner's problem. Your computer uses binary floating point numbers, and as such cannot exactly represent the fraction 0.1. For that reason after you have multiplied the approximation to 0.1 by ten, the result won't be an exact 1. It will probably be off in the least bit or bits. You should allow room for a small deviation from an exact 1 in your 'if' statement.

Answer 2

[Tilman]

Thank you Roger. I use Matlab for 3 years now and never run into this problem.

[Roger Stafford]

You should get familiar with the double and single precision binary formats of floating point numbers. It would be well worth your time. It would allow you to have a better grasp of what to expect in rounding errors for various computations.