Prelookup Index Search (Simulink Reference)

First stage of high-performance constant or linear interpolation that performs index search and interval fraction calculation for input on a breakpoint set

Library

Lookup Tables

Description

The PreLookup Index Search block calculates the indices and interval fractions for the input value in the Breakpoint data parameter. By using this combination of blocks, you can replace multiple Interpolation (n-D) blocks with one set of PreLookup Index Search blocks. In models that have many interpolation blocks simulation performance can be greatly increased.

To use this block, you must define a set of breakpoint values. In normal use, this breakpoint data set corresponds to one dimension of a Table data parameter in an Interpolation (n-D) using PreLookup block. The block generates a pair of outputs for each input value by calculating the index of the breakpoint set element that is less than or equal to the input value and the resulting fractional value that is a number 0 f < 1 that represents the input value's normalized position between the index and the next index value for in-range input.

For example, if the breakpoint data is

```
[ 0 5 10 20 50 100 ]
```

and the input value u is 55, the (index, fraction) pair is (4, 0.1), denoted as k and f on the block. Note that the index value is zero-based.

Note The interval fraction can be negative or greater than 1 for out-of-range input. See the documentation for the block's Process out of range input parameter for more information.

Data Type Support

The PreLookup Index Search block accepts signals of types double or single, but for any given block the inputs must all be of the same type. The Breakpoint data parameter must be of the same type as the inputs. The output data type is set to the input data type.

Parameters and Dialog Box

Breakpoint data: The set of numbers to search.
Index search method: Binary search, evenly spaced points, or linear search. Use linear search in combination with Begin index search using previous index result for higher performance than a binary search when the input values do not change much from one time step to the next. For large breakpoint sets, a linear search can be very slow if the input value changes by more than a few intervals from one time step to the next.
Begin index search using previous index result: Select this option if you want the block to start its search using the index that was found on the previous time step. For inputs that change slowly with respect to the interval size, you can realize a large performance gain.
Output only the index: If this block is not being used to feed an Interpolation (n-D) Using PreLookup block, the interval fraction output can be dropped and the resulting index value output is a uint32 instead.
Process out of range input: Specifies how to handle out-of-range input. Options include:

Clip to Range
If the input is less than the first breakpoint, return the index of the first breakpoint (i.e., 0) and 0 for the interval fraction. If the input is greater than the last breakpoint, return the index of the next-to-the-last breakpoint and 1 for the interval fraction. For example, suppose the range is [1 2 3] and you select this option. Then, if the input is 0.5, the block returns [0 0]; if the input is 3.5, the block returns [1 1].
Linear Extrapolation
If the input is less than the first breakpoint, return the index of the first breakpoint and an interval fraction representing the linear distance from the input to the first breakpoint. If the input is greater than the last breakpoint, return the index of the next-to-the-last breakpoint and an interval fraction that represents the linear distance from the next-to-the-last breakpoint to the input. For example, suppose the range is [1 2 3] and you select this option. Then, if the input is 0.5, the block returns [0 -0.5]; if the input is 3.5, the block returns [1 1.5].

Action for out of range input: Specifies whether to produce a warning or error message if the input is out of range. The options are None (the default, no warning or error message), Warning (display a warning message in the MATLAB command window and continue the simulation), Error (halt the simulation and display an error message in the Simulation Diagnostics Viewer).

Characteristics

Direct Feedthrough
Yes

Sample Time
Inherited from driving blocks

Scalar Expansion
Yes

Dimensionalized
Yes

Zero Crossing
No

Direct Feedthrough	Yes
Sample Time	Inherited from driving blocks
Scalar Expansion	Yes
Dimensionalized	Yes
Zero Crossing	No

Polynomial Product