Prelookup
Calculates the number and fraction of the interval to be interpolated.
Description
Block Prelookup calculates a number and a fraction for the interval that define how its input value relates to a set of reference points.
The block Prelookup works best with the Interpolation Using Prelookup block. Feed the resulting number and fraction values into the Interpolation Using Prelookup block to interpolate an n-dimensional table. These two blocks have distributed algorithms. When combined together, they perform the same operation as the integrated algorithm in block n-D Lookup Table. However, the blocks Prelookup and Interpolation Using Prelookup provide more flexibility and more efficient modelling and code generation than the block n-D Lookup Table.
Supported block operations
To use a block Prelookup*you must specify a set of reference point values. You can choose whether to specify the datum values directly in the dialogue box or to transmit them via the *bp input port by setting the parameters to Source value Dialog or Input port.
Typically, the dataset of reference points corresponds to one dimension of tabular data in the block Interpolation Using Prelookup.
The block Prelookup generates a pair of outputs for each input value by calculation:
-
the reference point number whose value is less than or equal to and forms an interval containing .
-
the fraction of the interval (fraction) in the range , representing the normalised position of on the interval between the reference point with the found number and the next reference point.
The number is counted from zero or from one depending on the value of the parameters Indexing Mode:
-
Zero-based indexing- indexing of reference points starts from0. Example: if the set of reference points is[0 5 10 20 50 100]and the input value of is55, the number is4and the interval fraction is0.1. -
One-based indexing- indexing of the reference points starts with1. Example: if the set of reference points is[0 5 10 20 50 50 100]and the input value of is55, then the number is5and the interval fraction is0.1.
The interval fraction may be negative or greater than 1 in case the input signal is out of range.
Ports
Input
#
u
—
input signal u
vector
Details
Block Prelookup accepts signals with real values of any numeric data type supported by Engee except Bool.
| Data types |
|
| Complex numbers support |
No |
#
u
—
input signal u
scalar | vector | matrix
Details
Block Prelookup accepts signals with real values of any numeric data type supported by Engee except Bool.
| Data types |
|
| Complex numbers support |
No |
#
bp
—
reference points
vector
Details
Block Prelookup accepts as reference points signals with real values of any numeric data type supported by Engee except Bool.
Dependencies
To use this port, set parameter Source value Input port.
| Data types |
|
| Complex numbers support |
No |
Output
#
k
—
number of the interval containing the input signal u
scalar | vector | matrix
Details
The number is a real integer defining the interval containing the input signal .
The number counts from zero or from one depending on the value of the parameters Indexing Mode.
| Data types |
|
| Complex numbers support |
No |
#
f
—
fraction representing the normalised position of the input in the interval
scalar | vector | matrix
Details
The fraction represents the normalised position of the input in the interval .
| Data types |
|
| Complex numbers support |
No |
Parameters
Breakpoints data
#
Source —
source of values for the reference points
Dialog | Input port
Details
Source of values for the datums:
-
Dialog- The reference point values are specified in the Value parameters. -
Input port- The reference point values will be obtained via the corresponding input point.
| Values |
|
| Default value |
|
| Program usage name |
|
| Tunable |
No |
| Evaluatable |
No |
#
Value —
datums
Array of real numbers
Details
Reference points. The set of anchor points must be a strictly monotonically increasing vector containing two or more elements.
Dependencies
To use this parameter, set the parameters to Source value Dialog.
| Default value |
|
| Program usage name |
|
| Tunable |
Yes |
| Evaluatable |
Yes |
#
Indexing Mode —
number value reference method
Zero-based indexing | One-based indexing
Details
The number value is counted from zero if the value is set to Zero-based indexing, or from one if the value is set One-based indexing.
| Values |
|
| Default value |
|
| Program usage name |
|
| Tunable |
No |
| Evaluatable |
No |
Algorithm
#
Output selection —
output signals
Index and fraction | Index only
Details
Options for selection:
-
Index only- outputs only the number, no share. -
Index and fraction- Outputs the number and fraction as separate signals.
| Values |
|
| Default value |
|
| Program usage name |
|
| Tunable |
No |
| Evaluatable |
No |
#
Index search method —
interval number search method
Evenly spaced points | Linear search | Binary search
Details
Each search method has speed advantages in different situations:
-
For evenly spaced anchor points (e.g., 10, 20, 30, and so on), optimal speed is achieved by choosing
Evenly spaced pointstable numbers for the calculation. This algorithm uses only the first two anchor points of the set to determine the offset and distance between the remaining points. -
For non-uniformly distributed landmark points, it is recommended:
-
Linear search- if the input values for do not change much between counting steps. -
Binary search- if the input values for change by more than one or two table intervals per count step.
-
A suboptimal choice of number lookup method can lead to poor performance of models that rely heavily on lookup tables.
| Values |
|
| Default value |
|
| Program usage name |
|
| Tunable |
No |
| Evaluatable |
No |
#
Extrapolation method —
method of processing input values that are out of range
Clip | Linear
Details
The method used by the block to extrapolate values for all inputs outside the range of the dataset of reference points.
Read more about extrapolation methods here: Methods of approximation of function values
| Values |
|
| Default value |
|
| Program usage name |
|
| Tunable |
No |
| Evaluatable |
No |