I want to set a range of variable to three or four places of decimals

How can i set a range of variable to three or four places of decimals? I'm going to write a function of the variable and float a graph. I want to find optimal value under specific function with the variable.

Answer 1

[John D'Errico]

Are you asking to create a variable that only carries 3 or 4 digits of precision? You can't choose to set an arbitrary precision, unless you use a tool like my HPF, but that would take far more effort on your part to learn than it would help you, and it has limits.

At best, you can use single precision, giving you roughly 8 digits. And Cleve Moler has posted some tools for shorter prescision, but those tools will also be limited in which functions can use them.

Or, are you asking how to plot a function over a domain that varies over 3 or 4 orders of magnitude for a plot?

That is far easier, since you can use tools like logspace, or semilogx, etc. And of course, you can always work using logs.

[WooYong Lee]

thanks for answer!!

[WooYong Lee]

in logspace/semilogx, can I set a range of variable's order of magnititude in integer range?

[Walter Roberson]

logspace(A, B, N) is the same as 10.^(linspace(A,B,N))

For example,

format long g
logspace(-2, 5, 3)

ans = 1×3

0.01 31.6227766016838 100000

●

10.^linspace(-2,5,3)

ans = 1×3

0.01 31.6227766016838 100000

●

You can do things like:

mat2str(10.^(-2:5))

ans = '[0.01 0.1 1 10 100 1000 10000 100000]'

but that does not restrict the range of the variable for optimization routines.

Answer 2

[Walter Roberson]

I want to find optimal value under specific function with the variable.

The MATLAB minimizers such as fmincon() and ga() all accept an options parameter, and the options parameter permits you to set a function tolerance -- so that you could stop searching when the function is changing less than a certain value, instead of comtinuing on to try to find the absolute best position.