MATLAB : function call returns different value for same x

Question

I am currently in the debugging process, and I found that this is happening, ruining everything. For reference, f = @(x)ax^3+bx^2+c*x+d (where in this specific case, a = 2, b = -7, c = -8, d = 16).

Also, I notice that temp == -1.6861 returns false..

I'm guessing it is related to this question; i.e., temp has a lot more significant digits than five that are suppressed by Matlab's default display format. — TroyHaskin
– TroyHaskin, Commented Oct 21, 2015 at 22:12

Andras Deak -- Слава Україні · Accepted Answer · 2015-10-21 22:13:43Z

2

This is a typical floating-point problem. Your number is NOT -1.6861, but something close to it. Try format long and then looking at temp. Or even better, try sprintf('%25.20f',temp). temp is the root of your function f as it should be, just not exactly -1.6861.

You actually have to be really careful with this:

(1/3-7/3/7)==0

is false, since

>> (1/3-7/3/7)

ans =

  -5.5511e-17

answered Oct 21, 2015 at 22:13

Andras Deak -- Слава Україні

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