Qlik subroutine parameters - passing by values vs by reference

Bitmetric Friday Qlik Test Prep #37 - Passing by reference vs passing by value

Every Friday at Bitmetric we’re posting a new Qlik certification practice question to our LinkedIn company page. Last Friday we asked the following Qlik Data Architect certification practice question about passing arguments to Qlik subroutine parameters:

This proved to be a harder question than expected, as only a single person got the right answer. Well done Vaibhav Sharma!

The correct answer is B: lines 4 and 8 both show the value 8

We’re modifying the parameter called pInput and somehow this also affects the variable vNumber? How is that possible? The explanation is that we passed the argument by reference instead of by value.

In short, when we pass by value we provide a copy of the variable, but when we pass by reference we provide the actual variable to the subroutine.

Read on to get a more in-depth explanation.

Sidebar: argument vs parameter
These terms are often used interchangeably, but they actually do have different meanings. A parameter is the name that is listed in the subroutine definition. In this example that’s pInput. An argument is the value that’s actually supplied. In this example that’s vNumber.

How variables are stored in memory

To explain the difference between passing by value and passing by reference we first need some basic knowledge of how memory works. We’ll use a simplified explanation for this, which should be enough to understand the basic concept.

You can consider memory as a collection of blocks that can hold data. Each block has an address, numbers 1 through 60in the illustration below. When you store a value into a variable, your operating system will automatically decide where to place this in memory. In the illustration below, the value we’ve set for the vNumber variable is stored at memory address 32.

Pass by value

When we pass by value, the value is copied to another location in memory. In the illustration below a copy of the value is placed at memory address 24. Subsequent operations, such as the multiplication in the subroutine, are performed on the copy. The original value is left untouched.

Pass by reference

When we pass by reference, we pass a reference to the memory address of the variable. What this means in our example is that both vNumber and vInput now reference the same memory space and value within that space.

Any operation that is performed to pInput updates the same value that is being referenced by vNumber, and vice versa. This explains the behavior that we see in the example script.

Passing by value vs passing by reference in Qlik

So how can we fix this script to ensure that vNumber is passed by value? By using Dollar-sign expansion! This passes the value of the variable to the subroutine, rather than a reference to the variable. See the updated script below, with a change on line 9.