This may be a silly question, but I just can't figure out when to use .join and when to use .append in D3.
Like in this block, I don't know why to use join rather than append here.
elements.selectAll('circle')
.data(d=>d.values)
//.append('circle')
.join('circle')
.attr("class","dots")
.attr('r',2)
.attr('fill',d=>colorScale(d['track']))
.attr("cx", d=>dateScale(d['edate']))
.attr("cy", d=>valueScale(d['record_time']));
Can anybody help me to understand that?
TL;DR
selection.append by itself merely appends a single child element to every element in the selection it is called on (inheriting its parent's datum). selection.join() is data dependent: it conducts an enter/update/exit cycle so that the number of matching elements in the DOM matches the number of data array items.
The code you have suggests that you want to use .enter().append("circle") as opposed to merely .append("circle"): this completes the enter() portion of the enter/update/exit cycle that is also completed by using .join().
You can use join or individual enter/exit/update selections to achieve the same results, join is just a convenience, as stated in the docs:
This method is a convenient alternative to the explicit general update
pattern, replacing selection.enter, selection.exit, selection.append,
selection.remove, and selection.order. (docs)
Enter/Update/Exit
When you see selectAll() followed by .data() we are selecting all matching elements, for each one that exists, we bind an item from the data array to it. The use of .data() returns what is called the update selection: it contains existing elements (if any) with the newly supplied data bound to those existing items.
However, if the number of selected elements does not match the number of items¹, then .data() creates an enter selection or an exit selection. If we have excess data items, then we have an enter selection with one element for every item we need to add in order to have an equal number of DOM elements and data array items. Conversely, if we have excess DOM elements, then we have an exit selection.
Calling .enter() on the update selection returns the enter selection. This selection contains placeholders ("Conceptually, the enter selection’s placeholders are pointers to the parent element docs"), which we can use .append("tagname") with to add the elements we need.
Conversely, calling .exit() on the update selection returns the exit selection, which often is simply removed with .exit().remove();
This pattern generally looks something like this:
let circle = svg.selectAll("circle")
.data([1,2,3,4])
circle.exit().remove();
circle.enter()
.append("circle")
.attr...
circle.attr(...
First we select the all the circles, let's say there are 2 circles to select.
Second we remove excess elements using selection.exit() : however, since we have four data items and only two matching DOM elements there is nothing to remove, so the selection is empty and nothing is removed.
Third we add elements as required to ensure that the number of matching DOM elements is the same as the number of data array items. As we have four data items and only two matching DOM elements the enter selection contains two placeholders (pointers to the parent). We append circles to them and style them as we want.
Lastly we use the update selection containing the two pre-existing circles and style them as we want based on the new data.
Often we want to style new elements and existing elements the same, so we could use the merge method to combine the enter and update selections:
let circle = svg.selectAll("circle")
.data([1,2,3,4])
circle.exit().remove();
circle.enter()
.append("circle")
.merge(circle)
.attr(...
This simplifies our code a bit as we don't need to duplicate styling for both enter and update separately.
Join
So where does .join() come in? Its for convenience. In its simplest form: .join("elementTagName") .join replaces the above code with:
let circle = svg.selectAll("circle")
.data([1,2,3,4])
.join("circle")
.attr(...
Here the join method removes the exit selection and returns a merged update selection and enter selection (containing new circles), which we can now style as needed. It is essentially a short hand method that allows you to write more concise code, but is functionally the same² as the 2nd code block.
Your Code
In your code, you have a selection of one or more elements (a/some parent element/s). The bound datum contains a child array, which you wish to use to create child elements. In order to do so you provide to .data() that child data array:
parentElements.selectAll("circle")
.data(d=>d.values)
You can follow that up with .join(): this will do the enter/update/exit cycle for every parent element so that they each have the proper amount of circles and returns a selection of all these circles.
You cannot use just .append() because that would append one circle to every parent element, returning a selection of these circles. This is very unlikely to be the desired result.
Instead, as noted at the top of this answer, you can use .enter().append("circle") so that you are using the pattern correctly.
You only need the enter selection if you create the elements once and never update the data, otherwise, you'll need to use enter/update/exit methods to handle excess elements, excess data items, and updated elements.
Ultimately, the difference between join and enter/update/exit is a matter of code preference, style, succinctness, but otherwise, there is nothing that you can't do with one that you can't do with the other.
¹ Assuming the provision of only one parameter to .data() - the 2nd, optional, parameter is a keying function which matches DOM element and data item based on a key. DOM elements without a matching datum are placed in the exit selection, data array items without a matching DOM element are placed in the enter selection, the remainder are placed in the update selection.
² Assuming that .join() is not provided its second or third parameters, which allow more granular control of the enter/exit/update cycle.
I am new to d3 and I have objects that are connected to each other. Each of these objects has multiple fields that I want to display. I was wondering how can you do this in d3.
I’m not sure I understand the question, but what I think you’re going for is something like this:
Make your selection and bind the data.
let update = d3.selectAll(".object").data(objects);
Remove old elements (if necessary).
update.exit().remove();
Create an enter selection and append a child for each field.
let enter = update.enter()
.append("g").attr("class", "object");
enter.append("g").attr("class", "field1");
enter.append("g").attr("class", "field2");
Merge your enter selection back into update, then fill in the fields.
update = update.merge(enter);
update.select(".field1").text(d => d.field1);
update.select(".field2").text(d => d.field2);
I don't even know how to describe this :)
I have bunch of divs, with similar IDs that have random part added to each (the random part is different for each session). and deeply nested in one of them a bunch of radio input boxes, without anything I can tie to (also the whole tree under the div doesn't have unique attributes I can tie to).
I need the first radio button. I get the needed div with (//div[contains(#id,'div-question')])[2], and I thought I could follow it up with similar construct, but I can't figure out how. I Also tired following:
(//div[contains(#id,'div-question')])[2]//input[#type='radio' and position() = 1]
but it return me all radio buttons, not only the first one (I'm using FirePath from FireBug -- could it be it's bug?)
So, how do I join two //... searches?
//x[position()=1] returns every descendant x that is the first child of its parent. To select the first descendant x, you need (//x)[position()=1]. With a complex path it becomes easier to use the descendant axis explicitly rather than the // shorthand:
descendant::div[contains(#id,'div-question')][2]
/descendant::input[#type='radio'][1]
I have a combined bar / line chart. For each row in the input file, I create a group that contains multiple elements (lines, rectangles, texts):
var myGroups = svg.selectAll('g').data(myData)
myGroups.enter().append('g')
...
myGroups.append('line')
...
myGroups.append('polygon')
...
myGroups.append('text')
...
I currently just
svg.selectAll('*').remove()
and create everything from scratch every time the data are updated. However, I'd like to have a smooth transition for all elements.
I've gone through this tutorial several times, but I still don't understand how I can do it in my case.
The key is to handle all the selections, not just the enter selection:
var myGroups = svg.selectAll('g').data(myData);
// enter selection
var myGroupsEnter = myGroups.enter().append("g");
myGroupsEnter.append("line");
myGroupsEnter.append("polygon");
// ...
// update selection -- this will also contain the newly appended elements
myGroups.select("line").attr(...);
// ...
// exit selection
myGroups.exit().remove();
There are two things here that warrant further explanation. First, elements in the enter selection that have had new elements appended merge into the update selection. That is, there is no need to set attributes on the elements in the enter selection if the same thing happens in the update selection. This allows you to append new elements and update existing ones without duplicating code.
The second thing becomes important on subsequent calls with updated data. As the elements you're binding the data to are not the ones that are actually drawn, the new data needs to be propagated to them. This is what .select() does. That is, by doing myGroups.select("line"), you're propagating the new data bound to the g elements to their children line elements. So the code to set attributes is the same as for the enter case.
Now you can simply add transitions where desired before setting the new attributes.
I'm using kendo grid (which is awesome!) and one of my columns contains a description field. Does kendo come with support for expanding columns? i.e. if over a certain length display a '+' expand icon or will I need to use a third party library?
Thanks
If you want to expand a record, you might use detailTemplate and maybe also detailInit event. Remember that detailTemplate might be either a string (KendoUI template) or a function. This demo shows how to use it.
If you want to expand a column, you should use a template in the column (see it here) definition and program there the logic needed.
Grids do come with a resizing property. Not sure if this fits your needs though.
http://demos.kendoui.com/web/grid/column-resizing.html