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Introduction to D3
Selection

D3 “injects” data visualizations onto a web page’s Document Object Model and associates data with a set of selected DOM elements. So before we go too far, let’s understand the first part of the process- the selection.

For D3 to inject or bind data to elements, a D3 selection of elements needs to be created. D3 selections are created with the .selectAll() or .select() methods.

Both methods take one CSS3 selector string as their parameter. CSS3 selectors are strings include anything from an HTML element name such as h1 to a class prepended with a ., or an id prepended with the # symbol.

  • The .select() method returns the first element that matches a selector string.
  • The .selectAll() returns an array-like structure with grouped nodes for all the elements that match the selector.

Each element in the selection includes a reference to its parent node in the DOM.

Take a look at the following examples:

d3.select('#visualization'); d3.selectAll('div');
  • The .select() element only selects the first element that matches the selector, an element with an id of visualization.
  • The .selectAll() element returns all of the elements that match the div selector in the DOM

Note: To create a selection, the elements do NOT need to exist in the DOM. Say you want to visualize 500 rows of data and associate them with 500 circle elements. Those elements don’t need to exist inside your web page in order for d3 to “theoretically” select them. Think of selecting theoretical elements as a “prepping your stage” step. You do this so that you can eventually create those elements and bind data to each in the later steps. We will re-visit this concept in later exercises.

Instructions

1.

Let’s selectAll()of the div elements and display some text. Notice there are three empty <div> elements inside index.html Copy and paste this code inside main.js:

d3.selectAll('div') .text('Select All');

We will go over the text() method in later steps. For now, take a second to observe that there are now three divs inside the body. What do they display? Can you start inferring what these two methods do?

2.

Now, instead of using selectAll(), let’s use .select(). Paste in the following snippet below the code inside main.js:

d3.select('div') .text('Select');

What does the first div display and how is it unlike the rest? Notice how only the first div element has changed the text that displays.

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