Mutate a dictionary or map.

Question

As an example, in Javascript I can do this: var test = {a:1,b:2}
test.a = 'changed'
console.log(test) and it will output this: {a:"changed",b:2} Because I mutated the value in key 'a' of the test variable. How do I do this in an appian expression? Trying this: a!localVariables(
 local!test: {a:1,b:2},
 local!test["a"]: "changed",
 local!test
) 
 Gives me this error: Expression evaluation error at function 'localvariables': A variable is incorrectly defined. Parameter: 2. Expected syntax: localvariables(local!a, ..., expr) or localvariables(local!a:10, ..., expr) 
 So it seems you can't changed a key of a variable in an expression. Is there another way to do this? It seems like such an elementary operation...

Mike Schmitt · Accepted Answer

Your main mistake here is assuming Appian expressions are like lines of code that execute in order. Instead they should be thought of as a single code statement that will be flattened and evaluated when run. 
 Thus the technical answer to your question, "how can I make local!test.a == 'changed'", is to declare local!test.a as "changed" in the first place. This might seem like "the snarky answer" but it's also true. Diclaimer: when we start working in interfaces where users interact, this all changes. 
 The *real* answer to what you're getting at, though, is probably this: when you want to iteratively transform data in local variables, we declare subsequent local variables to handle iterative values. Assume you're fed your local!test value up front. Now you can call "a!update()" on it, feed in the property you want to swap out to a different value, then feed the different value. If you do this as the expression's output, you'll see a readout of the altered dictionary. If you want to keep that for later use, though, you can store that new value in 'local!test2' (etc).

Mike Schmitt · Answer

aj2000 said: They would be much more efficient when trying to transform large datasets imported from external APIs. 
 I'm not sure I really understand your general use case, at least up to your mention of large datasets from external APIs. 
 I've been doing this >10 years now and honestly I've never had much of an issue with the immutability of Appian data objects - it seems to me almost as if you're starting with a conclusion then working backwards to provide examples that meet it. For instance if you want "array of 10,000 integers" you could simply do enumerate(10000) - in my dev environment that takes 1ms. 
 For large datasets imported from external APIs i believe the "best practice" recommendation is to throw that data into a process instance and handle it there. In successive process nodes you can change any nature of any particiular item in an array individually or in groups, in a specific order. To be honest though, in all but the biggest and most unruly data sets, it's probably sufficient to just transmute incoming data using expression functionality - it just takes some practice at using Appian's functionality to do it.

Mutate a dictionary or map.

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