Convert Data from Web Service - MATLAB & Simulink (original) (raw)

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This example shows how to download data from a web service and use a function as a content reader with webread.

The National Geophysical Data Center (NGDC) provides various geophysical and space weather data via a web service. Among other data sets, the NGDC aggregates sunspot numbers published by the American Association of Variable Star Observers (AAVSO). Usewebread to download sunspot numbers for every year since 1945.

api = 'http://www.ngdc.noaa.gov/stp/space-weather/'; url = [api 'solar-data/solar-indices/sunspot-numbers/' ... 'american/lists/list_aavso-arssn_yearly.txt']; spots = webread(url); whos('spots')

Name Size Bytes Class Attributes

spots 1x1269 2538 char

The NGDC web service returns the sunspot data as text. By default,webread returns the data as a character array.

ans =

    American

Year SSN 1945 32.3 1946 99.9 1947 170.9 1948 166.6

webread can use a function to return the data as a different type. You can use readtable with webread to return the sunspot data as a table.

Create a weboptions object that specifies a function forreadtable.

myreadtable = @(filename)readtable(filename,'HeaderLines',1, ... 'Format','%f%f','Delimiter','space','MultipleDelimsAsOne',1); options = weboptions('ContentReader',myreadtable);

For this data, call readtable with severalName,Value input arguments to convert the data. For example,Format indicates that each row has two numbers. Spaces are delimiters, and multiple consecutive spaces are treated as a single delimiter. To callreadtable with these input arguments, wrapreadtable and the arguments in a new function,myreadtable. Create a weboptions object withmyreadtable as the content reader.

Download sunspot data and return the data as a table.

spots = webread(url,options); whos('spots')

Name Size Bytes Class Attributes

spots 76x2 2932 table

Display the sunspot data by column and row.

spots(1:4,{'Year','SSN'})

ans = 

Year     SSN 
____    _____

1945     32.3
1946     99.9
1947    170.9
1948    166.6

Plot sunspot numbers by year. Use table functions to select sunspot numbers up to the year 2013. Convert the Year and SSN columns to arrays and plot them.

rows = spots.Year < 2014; vars = {'Year','SSN'}; spots = spots(rows,vars); year = spots.Year; numspots = spots.SSN; figure plot(year,numspots); title('Sunspot Data'); xlabel('Year'); ylabel('Number of Sunspots'); xlim([1940 2015]) ylim([0 180])

Sunspot data plotted as the number of sunspots by year

Aggregated data and web service courtesy of the NGDC. Sunspot data courtesy of the AAVSO, originally published in AAVSO Sunspot Counts: 1943-2013, AAVSO Solar Section (R. Howe, Chair).