As many QlikView developers have grown accustomed to, QlikView offers developers more than one way to accomplish a task. Knowing when to use each function is half of the battle. For example let’s take a look at Peek() vs Previous().
There are certainly some similarities between the two functions but there are also distinct differences that need to be taken into account when deciding which function to use.
Both allow you to look back at previously loaded rows in a table.
Both can be manipulated to look at not only the last row loaded but also previously loaded rows.
Previous() operates on the Input to the Load statement, whereas Peek() operates on the Output of the Load statement. (Same as the difference between RecNo() and RowNo().) This means that the two functions will behave differently if you have a Where-clause.
The Peek() function can easily reference any previously loaded row in the table using the row number in the function e.g. Peek(‘Employee Count’, 0) loads the first row. Using the minus sign references from the last row up. e.g. Peek(‘Employee Count’, -1) loads the last row. If no row is specified, the last row (-1) is assumed. The Previous() function needs to be nested in order to reference any rows other than the previous row e.g. Previous(Previous(Hires)) looks at the second to last row loaded before the current row.
So, when is it best to use each function?
The previous() and peek() functions could be used when a user needs to show the current value versus the previous value of a field that was loaded from the original file.
The peek() function would be better suited when the user is targeting either a field that has not been previously loaded into the table or if the user needs to target a specific row.
I wrote a technical brief that shows you how and why to use the Peek() and Previous() functions. You can see it here.