Write a Map Function - MATLAB & Simulink (2024)

Write a Map Function

Role of Map Function in MapReduce

mapreduce requires both an input map function that receives blocks of data and that outputs intermediate results, and an input reduce function that reads the intermediate results and produces a final result. Thus, it is normal to break up a calculation into two related pieces for the map and reduce functions to fulfill separately. For example, to find the maximum value in a data set, the map function can find the maximum value in each block of input data, and then the reduce function can find the single maximum value among all of the intermediate maxima.

This figure shows the Map phase of the mapreduce algorithm.

Write a Map Function- MATLAB & Simulink (1)

The Map phase of the mapreduce algorithmhas the following steps:

  1. mapreduce reads a single block of data using the read function on the input datastore, then calls the map function to work on the block.

  2. The map function then works on the individual block of data and adds one or more key-value pairs to the intermediate KeyValueStore object using the add or addmulti functions.

  3. mapreduce repeats this process for each of the blocks of data in the input datastore, so that the total number of calls to the map function is equal to the number of blocks of data. The ReadSize property of the datastore determines the number of data blocks.

The Map phase of the mapreduce algorithm is complete when the map function processes each of the blocks of data in the input datastore. The result of this phase of the mapreduce algorithm is a KeyValueStore object that contains all of the key-value pairs added by the map function. After the Map phase, mapreduce prepares for the Reduce phase by grouping all the values in the KeyValueStore object by unique key.

Requirements for Map Function

mapreduce automatically calls the map function for each block of data in the input datastore. The map function must meet certain basic requirements to run properly during these automatic calls. These requirements collectively ensure the proper movement of data through the Map phase of the mapreduce algorithm.

The inputs to the map function are data, info,and intermKVStore:

In addition to these basic requirements for the map function,the key-value pairs added by the map function must also meet theseconditions:

  1. Keys must be numeric scalars, character vectors, or strings. Numeric keys cannot be NaN, complex, logical, or sparse.

  2. All keys added by the map function must have the sameclass.

  3. Values can be any MATLAB® object, including allvalid MATLAB data types.

Note

The above key-value pair requirements may differ when usingother products with mapreduce. See the documentationfor the appropriate product to get product-specific key-value pairrequirements.

Sample Map Functions

Here are a few illustrative map functions used in mapreduce examples.

Identity Map Function

A map function that simply returns what mapreduce passes to it is called an identity mapper. An identity mapper is useful to take advantage of the grouping of values by unique key before doing calculations in the reduce function. The identityMapper mapper file is one of the mappers used in the example Tall Skinny QR (TSQR) Matrix Factorization Using MapReduce.

function identityMapper(data, info, intermKVStore) % This mapper function simply copies the data and add them to the % intermKVStore as intermediate values. x = data.Value{:,:}; add(intermKVStore,'Identity', x);end

Simple Map Function

One of the simplest examples of a nonidentity mapper is maxArrivalDelayMapper, which is the mapper for the example Find Maximum Value with MapReduce. For each chunk of input data, this mapper calculates the maximum arrival delay and adds a key-value pair to the intermediate KeyValueStore.

function maxArrivalDelayMapper (data, info, intermKVStore) partMax = max(data.ArrDelay); add(intermKVStore, 'PartialMaxArrivalDelay',partMax);end

Advanced Map Function

A more advanced example of a mapper is statsByGroupMapper, which is the mapper for the example Compute Summary Statistics by Group Using MapReduce. This mapper uses a nested function to calculate several statistical quantities (count, mean, variance, and so on) for each chunk of input data, and then adds several key-value pairs to the intermediate KeyValueStore object. Also, this mapper uses four input arguments, whereas mapreduce only accepts a map function with three input arguments. To get around this, pass in the extra parameter using an anonymous function during the call to mapreduce, as outlined in the example.

function statsByGroupMapper(data, ~, intermKVStore, groupVarName) % Data is a n-by-3 table. Remove missing values first delays = data.ArrDelay; groups = data.(groupVarName); notNaN =~isnan(delays); groups = groups(notNaN); delays = delays(notNaN); % Find the unique group levels in this chunk [intermKeys,~,idx] = unique(groups, 'stable'); % Group delays by idx and apply @grpstatsfun function to each group intermVals = accumarray(idx,delays,size(intermKeys),@grpstatsfun); addmulti(intermKVStore,intermKeys,intermVals); function out = grpstatsfun(x) n = length(x); % count m = sum(x)/n; % mean v = sum((x-m).^2)/n; % variance s = sum((x-m).^3)/n; % skewness without normalization k = sum((x-m).^4)/n; % kurtosis without normalization out = {[n, m, v, s, k]}; endend

See Also

mapreduce | tabularTextDatastore | add | addmulti | KeyValueStore

Related Topics

  • Write a Reduce Function
  • Getting Started with MapReduce

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Write a Map Function
- MATLAB & Simulink (2024)

FAQs

How to create a map in Matlab? ›

Set up a new map by using the newmap function. By default, map axes objects use an Equal Earth projection centered on the prime meridian and the equator. Display the land areas using the geoplot function. You can use map axes to create maps in any supported projected coordinate reference system (CRS).

Is there a map function in Matlab? ›

The map function then works on the individual block of data and adds one or more key-value pairs to the intermediate KeyValueStore object using the add or addmulti functions.

How to plot Simulink function in Matlab? ›

From the Simulink Editor, right-click the logging badge for a signal, and select Properties to open the Instrumentation Properties for the signal. Then, click the graphic representation of the Line. From the Instrumentation Properties, you can also select subplots where you want to plot the signal.

How to write formula in Simulink? ›

To insert an equation interactively:
  1. Go to the Insert tab and click Equation. A blank equation appears.
  2. Build your equation by selecting symbols, structures, and matrices from the options displayed in the Equation tab. ...
  3. Format your equation using the options available in the Text section.

How to write a function in MATLAB example? ›

Syntax for Function Definition
  1. function myOutput = myFunction(x) If your function returns more than one output, enclose the output names in square brackets.
  2. function [one,two,three] = myFunction(x) If there is no output, you can omit it.
  3. function myFunction(x) Or you can use empty square brackets.

How to make a map step by step? ›

How to Make a Map
  1. Choose a map template. Choose a map that fits your purpose. ...
  2. Label important locations and areas. Use text and graphics (such as push pins, arrows, and other symbols) to label the map with key information. ...
  3. Add a compass. ...
  4. Include a legend.

How do I create my map? ›

Create a map
  1. On your computer, sign in to My Maps.
  2. Click Create a new map.
  3. Go to the top left and click "Untitled map."
  4. Give your map a name and description.

Is map () a built in function? ›

map() The map() built-in function accepts a function and applies it to every item in an iterable. It outputs a map object.

Is A map a function? ›

In mathematics, a map or mapping is a function in its general sense. These terms may have originated as from the process of making a geographical map: mapping the Earth surface to a sheet of paper. A map is a function, as in the association of any of the four colored shapes in X to its color in Y.

What is mapping in MATLAB? ›

Mapping Toolbox™ provides algorithms and functions for transforming geographic data and creating map displays. You can visualize your data in a geographic context, build map displays from more than 60 map projections, and transform data from a variety of sources into a consistent geographic coordinate system.

How to setup Simulink in MATLAB? ›

Standard Installation
  1. Download and Install MATLAB. Download and install MATLAB, Simulink, and accompanying toolboxes and blocksets on a personal computer.
  2. Add Toolboxes and Products to MATLAB Installation. ...
  3. Get Update of Installed MATLAB Release. ...
  4. Get New MATLAB Release. ...
  5. Install Documentation for Temporary Offline Use.

How to use MATLAB code in Simulink? ›

You can integrate your MATLAB code into Simulink using the MATLAB Function block and MATLAB System block. Use MATLAB Function block to integrate simple functions. Use the MATLAB System block to integrate code that requires state dynamics, large streaming data interface, and interaction with the Simulink engine.

Can I create a function in MATLAB? ›

How to create a function in MATLAB ?
  • The function starts with the keyword function.
  • Returning variables of the function are defined in output_params.
  • function_name specifies the name of the function.
  • input_params are input arguments to the function.
May 26, 2021

How do I add a MATLAB function block to Simulink? ›

Add a MATLAB Function to your model to open the MATLAB Function Block Editor. In the Simulation tab, click Library Browser. Select Simulink > User-Defined Functions. Drag a MATLAB Function block to the Simulink canvas.

How do you write something in Simulink? ›

To show the Notes pane, in the Simulink Toolstrip, on the Modeling tab, click the arrow on the far right of the Design section. In the Design gallery, under Review, click Model Notes. Alternatively, press Ctrl+Shift+N. To write notes, you must create a file to store the notes, or load an existing notes file.

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