read

Read data in datastore

Description

example

data = read(ds) returns data from a datastore. Subsequent calls to the read function continue reading from the endpoint of the previous call.

[data,info] = read(ds) also returns information about the extracted data in info, including metadata.

Examples

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

ds = tabularTextDatastore('airlinesmall.csv','TreatAsMissing','NA','MissingValue',0);

Modify the SelectedVariableNames property to specify the variables of interest.

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

While there is data available to be read from the datastore, read one block of data at a time and analyze the data. In this example, sum the actual elapsed time.

sumElapsedTime = 0;
while hasdata(ds)
    T = read(ds);
    sumElapsedTime = sumElapsedTime + sum(T.ActualElapsedTime);
end

View the sum of the actual elapsed time.

sumElapsedTime
sumElapsedTime = 14531797

Create a datastore from the sample file, mapredout.mat, which is the output file of the mapreduce function.

ds = datastore('mapredout.mat');

Read a subset of data in the datastore.

T = read(ds)
T=1×2 table
     Key        Value  
    ______    _________

    {'AA'}    {[14930]}

Change the number of key-value pairs to read at a time, by changing the ReadSize property of the datastore.

ds.ReadSize = 5;

Read the next five key-value pairs in the datastore.

T = read(ds)
T=5×2 table
     Key        Value  
    ______    _________

    {'AS'}    {[ 2910]}
    {'CO'}    {[ 8138]}
    {'DL'}    {[16578]}
    {'EA'}    {[  920]}
    {'HP'}    {[ 3660]}

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 containing a mask of the bright regions of the three images. To create this datastore, first transform the images of imds1 to grayscale. Then convert each image to a binary mask by mapping bright pixels (with a value greater than 250) to white and dark pixels to black.

imds2 = transform(imds1,@(x) rgb2gray(x)>250);

Create a combined datastore from imds1 and imds2.

imdsCombined = combine(imds1,imds2);

Read the first subset of data from the combined datastore. The output is a 1-by-2 cell array. The two columns represent the first subset of data read from the two underlying datastores imds1 and imds2, respectively.

dataOut = read(imdsCombined)
dataOut=1×2 cell
    {480x640x3 uint8}    {480x640 logical}

Display the read data from the combined datastore as a pair of tiled images.

tile = imtile(dataOut);
imshow(tile)

Input Arguments

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

Output Arguments

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Output data, returned as a table or an array depending on the type of ds.

Type of DatastoreData type of dataDescription
TabularTextDatastore and SpreadsheetDatastoreTableThe SelectedVariableNames property determines the table variables.
ImageDatastoreInteger array

The dimensions of the integer array depend on the type of image:

  • For grayscale images, data is m-by-n.

  • For truecolor images, data is m-by-n-by-3.

  • For CMYK Tiff images, data is m-by-n-by-4.

If the ReadSize property is greater than 1, then data is a cell array of image data corresponding to each image. The read 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.
FileDatastoreVariesThe output is the same as the output returned by the custom read function, specified by the 'ReadFcn' value.
TransformedDatastoreVariesThe output is the same as the output of the transformation function @fcn specified in the transform method used to create the TransformedDatastore.
CombinedDatastoreVaries

Contains the horizontal concatenation of the output of read from the corresponding underlying datastores.

Information about read data, returned as a structure array or a cell array of structure arrays.

  • For MATLAB datastores and TransformedDatastore, info is a structure array that has fields with information about the datastore.

  • For CombinedDatastore, info is a cell array of structure arrays. Each element of the cell array contains a structure with the relevant fields of the corresponding underlying datastore.

Information in the structure array depends on the type of the input datastore. The structure array can contain the following fields.

Field NameDatastore TypesDescription
FilenameAllFilename is a fully resolved path containing the path string, name of the file, and file extension. For ImageDatastore objects whose ReadSize property is greater than 1, Filename is a cell array of file names corresponding to each image.
FileSizeAll

Total file size, in bytes.

For ImageDatastore objects whose ReadSize property is greater than 1, FileSize is a vector of file sizes corresponding to each image.

For MAT-files, the value of FileSize depends on the type of the datastore.

  • KeyValueDatastore and TallDatastore — The FileSize field contains the total number of key-value pairs in the file.

  • FileDatastore — The FileSize field contains the total file size in bytes.

FileTypeKeyValueDatastore only

The type of file from which data is read, either 'mat' for MAT-files or 'seq' for sequence files.

LabelImageDatastore only

Image label name. If the ReadSize property is greater than 1, then Label is a vector of label names corresponding to each image. If the Labels property is empty, then Label is an empty cell array.

NumCharactersReadTabularTextDatastore only

Number of characters read.

NumDataRowsSpreadsheetDatastore only

Vector containing number of rows read from each sheet.

OffsetKeyValueDatastore and TabularTextDatastore only

Starting position of the read operation, in bytes. For MAT-files, Offset is the index of the first key and value read.

SheetNamesSpreadsheetDatastore only

Names of sheets read.

SheetNumbersSpreadsheetDatastore only

Numbering associated with sheets read.

Introduced in R2014b