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Learn JS Data

Data manipulation, munging, and processing in JavaScript

Summarizing Data

With the data loaded, we want to take a quick look at what we have. D3 has a number of tools to use for quick data exploration.

To start, let's pretend we have loaded up a csv file - and have a dataset that looks something like:

var data = [
  {"city":"seattle", "state":"WA", "population":652405, "land_area":83.9},
  {"city":"new york", "state":"NY", "population":8405837, "land_area":302.6},
  {"city":"boston", "state":"MA", "population":645966, "land_area":48.3},
  {"city":"kansas city", "state":"MO", "population":467007, "land_area":315}
];

Min & Max

As it turns out, D3 comes to the rescue again, with d3.min and d3.max. Use the callback function to indicate which property (or computed value based on the properties) to access.

var minLand = d3.min(data, function(d) { return d.land_area; });
console.log(minLand);
=> 48.3
This code is using d3.js
var maxLand = d3.max(data, function(d) { return d.land_area; });
console.log(maxLand);
=> 315
This code is using d3.js

If you want both of them at the same time, you can use d3.extent

var landExtent = d3.extent(data, function(d) { return d.land_area; });
console.log(landExtent);
=> [48.3, 315]
This code is using d3.js

This returns an array with the first element the minimum value and the second element the maximum.

Summary Statistics

D3 provides a few basic tools to analyze your data, all using the same format as the min and max functions. Simply provide the property you would like to analyze, and you are good to go.

d3.mean

var landAvg = d3.mean(data, function(d) { return d.land_area; });
console.log(landAvg);
=> 187.45
This code is using d3.js

d3.median

var landMed = d3.median(data, function(d) { return d.land_area; });
console.log(landMed);
=> 193.25
This code is using d3.js

d3.deviation - for standard deviation

var landSD = d3.deviation(data, function(d) { return d.land_area; });
console.log(landSD);
=> 140.96553952414519
This code is using d3.js

Next Task

Iterating and Reducing

See Also