Fixed-Point Blockset

Parameters for fixpt_look1_func_approx

To use the function fixpt_look1_func_approx, you must first define its parameters. The required parameters for the function are

• funcstr - The ideal function
• xmin - The minimum input of interest
• xmax - The maximum input of interest
• xdt - The x data type
• xscale - The x data scaling
• ydt - The y data type
• yscale - The y data scaling
• rndmeth - The rounding method

In addition there are three optional parameters:

• errmax - The maximum allowed error of the lookup table
• nptsmax - The maximum number of points of the lookup table
• spacing - The allowed spacing between breakpoints

You must use at least one of the parameters errmax and nptsmax. The next section Setting Function Parameters for the Lookup Table gives typical settings for these parameters.

Using Only errmax

If you use only the errmax parameter, without nptsmax, the function creates a lookup table with the fewest points, for which the worst case error is at most errmax. See Example 1: Using errmax with Unrestricted Spacing.

Using Only nptsmax

If you use only the nptsmax parameter without errmax, the function creates a lookup table with at most nptsmax points, which has the smallest worse case error. See Example 2: Using nptsmax with Unrestricted Spacing.

The section Specifying Both errmax and nptsmax describes how the function behaves when you specify both errmax and nptsmax.

Spacing

You can use the optional spacing parameter to restrict the spacing between breakpoints of the lookup table. The options are

• 'unrestricted' - The default.
• 'even' - The distance between any two adjacent breakpoints is the same.
• 'pow2' - The distance between any two adjacent breakpoints is the same and the distance is a power of two.

The section Restricting the Spacing and the examples that follow it explain how to use the spacing parameter.

Creating Lookup Tables for a Sine Function

Setting Function Parameters for the Lookup Table