Google Apps Script – Iterating Through Ranges in Sheets the Right and Wrong Way

I was trying to rush out some Google Apps Script code to deal with a task on Google Sheets recently. Basically, I had to search through a heap of data and find certain values and do something too them.

My column was reaching across the page to something like Column BK and my rows were over 1000 deep. Running this code was taking forever!!!

My immediate instinct was:

What have I done wrong?

…and my instinct was right.

The Good and Bad Way to Search Through Code

So after looking at my code again, I discovered that for some reason I go it into my head that I should be searching each cell for the value I needed and then doing something with it.

Sounds logical right? It’s sorta what you are meant to do.

The problem is that I was calling the sever and asking for the range in each cell as I was looping through the entire document. This is super costly and inefficient in terms of time.

Google talks about this in their Google Apps Script Best Practice page under Batch Operations.

Also, if you do run a costly code like this, then you will get a little red light in your Script tool bar that represents your Execution Hints:

Google Apps Script Execution Hints

Clicking on Execution Hints and expanding the side bar with provide you with a far-too-deserving-polite dressing down about your slow and server costly code.

Method Range.getValue is heavily used

The Good

So instead of calling the server for each cell I need to get the full range of the data I am working on, search through it client-side to find what cells require modifying and then invoke the modifications.

Let’s look at a simple example:

Example

In this example I want to search through all the results over multiple quizzes and if there is a dash “-” or a zero “0” change the background accordingly.

Yeah! Yeah! I can do this with conditional formatting, but this is an example, damn it, Jim!!!

Below is a sample of the data I will use and here is the link.

Both Good and Bad examples have the same end result. The result should look like below:

First let’s set up the Google Apps Script code file by calling the user interface, sheet and range of data. (Lost? Create your first Apps Script)

Hopefully everything is self-explanatory here. We call the spreadsheet first and then look for the active sheet. Inside the active sheet we want the range of the data (rangeData) which will contain all the data in the range. We will use that data to get the last row and column number of the data. Finally we will call the server to get he range we want to work with (searchRange).

Once done, we will create our function, onOpen(). When it is called it will create a menu called Checker with the sub menu Bad Way and Good Way. This isn’t necessary, but it might be easier for you to physically test the difference in the slow method versus the fast (correct) one.

The Bad Way

As mentioned above in the Bad (slow) Way we call the server each time to look at what is in a cell.

As I loop across the columns and then the rows, I am using my search range to get the value of the cell in Line 25. This means I am contacting the server a total for 436 times. This significantly slows things down.

The Good Way

 

In the preferred approach I am taking the array that I created from searchRange.getValues() in Line 36  and searching through it before I make my calls to change the background when a dash or a zero occurs.

Why is this better?

I only make server call to collect the range data once. Then client-side (on my computer in this instance), I do all my searching before calling Google who collects all the changes in a cache until the loops are done before creating background colors all at once.

Super fast.

Speed comparison. 

Take a look at the speed differences over ten tests:

The Good Way is the clear winner. You can try it out for yourself if you have been playing along by going to <View><Execution Transcript> in the Script

Take Home

The take home from this is that, make as little calls to the server as possible. It significantly improves your speed.

The Full Code

 

 

 

 

 

 

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