# ApplyFunctionWithLookupTable function

Warning

This documentation refers to the Classic QDK, which has been replaced by the Modern QDK.

Please see https://aka.ms/qdk.api for the API documentation for the Modern QDK.

Namespace: Microsoft.Quantum.Arithmetic

Package: Microsoft.Quantum.Numerics

This function creates a lookup table operator for the function that you want to approximate, as well as the parameters required to make the two FixedPoint registers that need to be used as inputs to the operator.

function ApplyFunctionWithLookupTable (func : (Double -> Double), domain : (Double, Double), epsIn : Double, epsOut : Double) : Microsoft.Quantum.Arithmetic.FunctionWithLookupTable


## Input

### func : Double -> Double

The Q# arithmetic function that you want to implement with the lookup table

### domain : (Double,Double)

A tuple consisting of the minimum and maximum values of the input values to the function

### epsIn : Double

The maximum allowed error of the input value to the computation (i.e. |x'-x|)

### epsOut : Double

The maximum allowed error of the output without taking into account the error in input value (i.e. |f'(x')-f(x')|)

## Example

The following code creates a quantum operation based on ExpD in the (inclusive) range from -5.0 to 5.0 with an input error of 1e-3 and an output error of 1e-4.

// Create operation from lookup table
let domain = (-5.0, 5.0);
let epsIn = 1e-3;
let epsOut = 1e-4;

let lookup = ApplyFunctionWithLookupTable(ExpD, domain, epsIn, epsOut);

// Allocate qubits
use input = Qubit[lookup::IntegerBitsIn + lookup::FractionalBitsIn];
use output = Qubit[lookup::IntegerBitsOut + lookup::FractionalBitsOut];

// Represent qubit registers as fixed points
let inputFxP = FixedPoint(lookup::IntegerBitsIn, input);
let outputFxP = FixedPoint(lookup::IntegerBitsOut, output);

// Apply operation
lookup::Apply(inputFxP, outputFxP);


## Remarks

The operator guarantees that given an input value $x$ and a function $f(x)$, it will compute $\hat{f}(\hat{x})$ where $\hat{f}$ is an approximation of $f$ with a maximum error of epsOut and $\hat{x}$ is an approximation of the input value $\hat{x}$ with a maximum error of epsIn. This is useful for most reasonably behaved functions, but note that it computes $\hat{f}(\hat{x})$ and not $\hat{f}(x)$ so if the domain function is very oscillatory and/or has funky derivatives then it may have high errors.