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Replace the exp Function with a Lookup Table

This example shows how to replace the exp function with a lookup table approximation in the generated fixed-point code using the codegen function.

Prerequisites

To complete this example, you must install the following products:

Create Algorithm and Test Files

  1. Create a MATLAB function, my_fcn.m, that calls the exp function.

    function y = my_fcn(x)
  y = exp(x);
end

  2. Create a test file, my_fcn_test.m, that uses my_fcn.m.

    close all

x = linspace(-10,10,1e3);
for itr = 1e3:-1:1
   y(itr) = my_fcn( x(itr) );
end
plot( x, y );

Configure Approximation

Create a function replacement configuration object to approximate the exp function, using the default settings of linear interpolation and 1000 points in the lookup table.

q = coder.approximation('exp');

Set Up Configuration Object

Create a coder.FixptConfig object, fixptcfg. Specify the test file name and enable numerics testing. Associate the function replacement configuration object with the fixed-point configuration object.

fixptcfg = coder.config('fixpt');
fixptcfg.TestBenchName = 'my_fcn_test';
fixptcfg.TestNumerics = true;
fixptcfg.DefaultWordLength = 16;
fixptcfg.addApproximation(q);

Convert to Fixed Point

Generate fixed-point MATLAB code.

codegen -float2fixed fixptcfg my_fcn

View Generated Fixed-Point Code

To view the generated fixed-point code, click the link to my_fcn_fixpt.

The generated code contains a lookup table approximation, replacement_exp, for the exp function. The fixed-point conversion process infers the ranges for the function and then uses an interpolated lookup table to replace the function. By default, the lookup table uses linear interpolation, 1000 points, and the minimum and maximum values detected by running the test file.

The generated fixed-point function, my_fcn_fixpt, calls this approximation instead of calling exp.

function y = my_fcn_fixpt(x)
  fm = get_fimath();

  y = fi(replacement_exp(x), 0, 16, 1, fm);
end

You can now test the generated fixed-point code and compare the results against the original MATLAB function. If the behavior of the generated fixed-point code does not match the behavior of the original code closely enough, modify the interpolation method or number of points used in the lookup table and then regenerate code.