Editing TestExpr

{{Function
 |origin = NVSE
 |summary = TestExpr is passed any expression and returns True (1) if it succeeds, or False (0) if it fails- meaning it generates an [[NVSE Error]], and the error message is suppressed from appearing in the console. 

This is an alternative to validating one's data, which might require multiple lines. Essentially it is saying; <i>"I know this line of code might fail, so tell me if it does and I will handle it. In this case the error is normal, so don't alert the user"</i>. 

Comparable to the 'try', 'catch/except' found in some programming languages. Added by NVSE 4.1. 
 |name = TestExpr
 |returnType = Result:bool
 |arguments = {{FunctionArgument
   |Name = AnyLine
   |Type = expression
  }}}}
==Example==
Simple example, instead of bothering to check a key exists with [[Ar_HasKey]]:
<pre>
if TestExpr SomeArray[SomeKey]
    ; SomeArray has SomeKey
else
    ; SomeArray does not have SomeKey
endif
</pre>

Suppose you want to keep a tally of the number of times the player encounters every NPC of a particular name:
<pre>
array_var aTally
ref rActor
string_var actor_name

let aTally := Ar_Construct "stringmap"
; ...

let actor_name := $rActor

if TestExpr aTally[actor_name] += 1
   ; do nothing. The increment worked because the actor_name is in the array already
else
    aTally[actor_name] := 1 ; * first time we met actor_name, so create the entry
endif
</pre>

Suppose you are worried a ref variable might be empty for some reason, so your functions would fail:
<pre>
ref rActor
int iStrengthStat

if TestExpr iStrengthStat := rActor.GetAV "Strength"
    ; do nothing. The function worked, so rActor must be valid
else
    ; rActor isn't valid, so don't proceed further yet
    return
endif
</pre>

==Notes==
*You can assign variables using ':=', '+=', etc, using any NVSE aware command (such as <b>TestExpr</b>, [[Eval]], [[While]]...), not just with [[Let]].

==See Also==
*[[NVSE Error]]
*[[Let]]
*[[ToString|ToString($)]]
*[[Array Variable]]
*[[IsFormValid]]

[[Category:Functions_(NVSE)]]
[[Category:Debugging]]