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readall

Read all data in datastore

Description

example

data = readall(ds) returns all the data in the datastore specified by ds. If all the data in the datastore does not fit in memory, then readall returns an error.

example

data = readall(ds,'UseParallel',tf) reads the data in parallel (requires Parallel Computing Toolbox™).

Examples

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Create an ImageDatastore object containing four images.

imds = imageDatastore({'street1.jpg','street2.jpg','peppers.png','corn.tif'});

Read all the data in the datastore.

T = readall(imds);

Examine the output.

imout = imtile(T);
imshow(imout)

Figure contains an axes. The axes contains an object of type image.

Create a datastore from the sample file airlinesmall.csv, which contains tabular data.

ds = tabularTextDatastore('airlinesmall.csv','TreatAsMissing','NA');

Specify the variables of interest using the SelectedVariableNames property.

ds.SelectedVariableNames = {'DepTime','ArrTime','ActualElapsedTime'};

Read all the data in the datastore in parallel.

T = readall(ds,'UseParallel',true);

readall returns all the data in a table.

View information about the table. Only the selected variables are included in the output.

T.Properties
ans = 
  TableProperties with properties:

             Description: ''
                UserData: []
          DimensionNames: {'Row'  'Variables'}
           VariableNames: {'DepTime'  'ArrTime'  'ActualElapsedTime'}
    VariableDescriptions: {}
           VariableUnits: {}
      VariableContinuity: []
                RowNames: {}
        CustomProperties: No custom properties are set.
      Use addprop and rmprop to modify CustomProperties.

Create a datastore that maintains parity between the pair of images of the underlying datastores. For instance, create two separate image datastores, and then create a combined datastore representing the two underlying datastores.

Create an image datastore imds1 representing a collection of three images.

imds1 = imageDatastore({'street1.jpg','street2.jpg','peppers.png'}); 

Create a second datastore imds2 by transforming the images of imds1 to grayscale and then downsizing the images.

imds2 = transform(imds1,@(x) imresize(rgb2gray(x),0.5));

Create a combined datastore from imds1 and imds2.

imdsCombined = combine(imds1,imds2);

Read all of the data from the combined datastore. The output is a 3-by-2 cell array. The two columns represent all of the read data from the two underlying datastores imds1 and imds2, respectively.

dataOut = readall(imdsCombined)
dataOut=3×2 cell array
    {480x640x3 uint8}    {240x320 uint8}
    {480x640x3 uint8}    {240x320 uint8}
    {384x512x3 uint8}    {192x256 uint8}

Input Arguments

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Input datastore. You can use these datastores as input to the readall method.

Read in parallel, specified as true or false. If you specify true, readall reads all data from the datastore in parallel (requires Parallel Computing Toolbox). Parallel reading may result in improved performance when reading data, especially with remote data.

  • Datastore processing can be improved with the UseParallel property and theParallel Computing Toolbox. readall reads supported datastores faster on the local machine by using low overhead computing environments such as thread-based parallel pools. For more information on thread-based parallel pools see parpool (Parallel Computing Toolbox).

  • As a result of MATLAB's built-in multithreading, certain datastores (for example, imageDatastore) perform faster on the local machine when not using parallelism based on MATLAB workers. For more information see MATLAB Multicore.

Example: readall(ds,'UseParallel',true)

Output Arguments

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All data in the datastore, returned as a table or a cell array depending on the type of ds.

Type of DatastoreData type of dataDescription
TabularTextDatastore and SpreadsheetDatastoreTableThe SelectedVariableNames property determines the table variables.
ImageDatastoreCell array Each element in the cell array contains the image data for one image. The readall function supports all image types supported by the imread function. For more information on the supported image types, see imread.
KeyValueDatastoreTableThe table variable names are Key and Value.
FileDatastoreCell arrayEach element in the cell array contains the data read from one file using the custom read function specified by the ReadFcn property.
TransformedDatastoreVariesThe output is the same as the output returned by the underlying datastore specified by the UnderlyingDatastores property. For example, if the underlying datastore is an image datastore, then data is returned as a cell array where each element in the cell array contains the image data for one image.
CombinedDatastoreCell array

Each column of the cell array contains the result from calling readall on the corresponding underlying datastore specified by the UnderlyingDatastores property.

If the number of subsets of data in the underlying datastores differs, then readall only returns data while all underlying datastores have data. For example, suppose a combined datastore has two underlying datastores, one with m subsets of data and one with n subsets of data, where m > n. The output is a cell array with two columns and n rows.

Extended Capabilities

See Also

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Introduced in R2014b