## "Warning: Imaginary parts of complex X and/or Y arguments ignored" in fminserach and optimtool

### Victor (view profile)

on 5 Mar 2012
Accepted Answer by Walter Roberson

### Walter Roberson (view profile)

Dear all,

I am trying to fit a curve to experimental data by use of "optimtool" and "fminsearch".

But I receive a warning which says " Imaginary parts of complex X and/or Y arguments ignored". I have this warning from the first iteration.

Any idea to solve this warning is appreciated. Thanks in advance Best regards Victor

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### Walter Roberson (view profile)

Answer by Walter Roberson

### Walter Roberson (view profile)

on 5 Mar 2012

Turn on FunValCheck to produce an error when imaginary values are encountered.

If your fitting inherently involves imaginary quantities, then please note:

```fminsearch only minimizes over the real numbers, that is, x must only consist of real numbers and f(x) must only return real numbers. When x has complex variables, they must be split into real and imaginary parts.
```

Victor

### Victor (view profile)

on 5 Mar 2012

Dear all,
Now it is ok but I found just if I let the Matlab to calculate the numbers, it will give me the errors!
For example instead of writing the final value of log(a/b) with a=0.001 and b=1 I wrote -3 and now also instead of "1-((103-298)/(1723-298)" I wrote 1.136842105
It is so strange for me.
do you have any idea?

Walter Roberson

### Walter Roberson (view profile)

on 5 Mar 2012

Beware of log vs log10

http://matlab.wikia.com/wiki/FAQ#Why_is_0.3_-_0.2_-_0.1_.28or_similar.29_not_equal_to_zero.3F

### Sean de Wolski (view profile)

Answer by Sean de Wolski

### Sean de Wolski (view profile)

on 5 Mar 2012

Evaluate your function at the initial values you provide. Is the result complex? If so, that's the reason for the warning. We can't be of much more assistance without seeing reproduction steps.