Does Head evaluate its argument?From Using JSL to Develop Efficient, Robust Applications (EU 2018 415), I learned that there are some functions which evaluate their argument and others which don't:
I wondered about Head. Here everything looks logic ...
f= Expr(x=2);
Substitute(NameExpr(f),Expr(x), Expr(y));
Head(f);
Head is not like Substitute, I can directly use the function name as an argument - and Head returns the head of the expression: Assign()
After checking some other functions which "don't evaluate their argument":
Expr(x=2); // x=2
Name Expr(x=2); // x=2
let's check the result for
Head(x=2)
which is "2" - i.e. the result after evaluating the argument?!?! why is it not assign()?
Automation and ScriptingRe: Does Head evaluate it's argument?
Names Default To Here(1);
{a, b, c} = {1, 2, 3};
Head(Expr(Sum(a, b, c)));
HeadExpr(Sum(a, b, c)); // Deprecated
Head(Sum(a, b, c));
https://www.jmp.com/support/help/en/17.2/#page/jmp/advanced-expressions-macros-and-lists.shtml#
Re: Does Head evaluate it's argument?ah, ok, the scripting index is very clear.
So, the behavior nowadays is different than indicated by the list. No hidden secret behind it.
In general, it could make scripting a lot easier if the JSL editor used some highlighting to distinguish between arguments that are evaluated and arguments that aren't evaluated before the function is executed:
Advanced syntax highlighting in JSL Editor
Re: Does Head evaluate it's argument?same for Narg - the argument gets evaluated (1x, like with Name Expr()
(adapted from the scripting guide)
aExpr = {a+b,c,d,e+f+g};<BR />
Show(NArg(aExpr)); //4
Show(NArg(Arg(aExpr,4))); // 3
Show(NArg(Add(1,2,3,4))); // 0
Show(NArg(Expr(Add(1,2,3,4)))); //4
Show(Head(aExpr)); //List()
Show(Head(Arg(aExpr,4))); // Add()
Show(Head(Add(1,2,3,4))); // 10
Show(Head(Expr(Add(1,2,3,4)))); //AddRe: Does Head evaluate it's argument?this Name Expr-like behavior is especially useful for the cases with
- As Column(...)
- column reference via variable (col)
as an argument.
dt = Open("$SAMPLE_DATA/Big Class.jmp");
col = Name Expr(As Column("age"));
Head(As Column("age"));
Head(col);
dt << Recode Column(AsColumn("age"),{1},Target Column(AsColumn("age")));
dt << Recode Column(col,{1},Target Column(col));
dt << Graph Builder(Variables( X( As Column ("age") ) ),Elements( Points( X ) ));
dt << Graph Builder(Variables( X( col) ),Elements( Points( X ) ));
on the other hand: functions which don't work if a column is entered via As Column():
dt = Open("$SAMPLE_DATA/Big Class.jmp");
col = Name Expr(As Column("age"));
Distribution(Column(As Column("age")));The easy solution: wrap As Column with Name Expr:
Distribution(Column(Name Expr(As Column("age")))); ... or use Column() instead of As column where applicable:
Distribution(Column(Column("age")));
To distinguish between both cases, you can put a print(1); at the beginning of the argument:
dt << Graph Builder(Variables( X( Print(1);As Column ("age") ) ),Elements( Points( X ) )); // fails
Distribution(Column(Print(1);Column("age"))); // worksRe: Does Head evaluate it's argument?
@hogi wrote:
the argument gets evaluated (1x, like with Name Expr() )
sorry:
like with Name Expr()
Head():
means: "the expressionArg can be
1) a name holding an expression - and Head will use the expression,
2) an expression - and Head will evaluate it and will use the result as input
3) or a literal expression quoted by Expr() - and Head will take the literal expression".
Name Expr():
"the expressionArg can be
1) a name holding an expression - and Name Expr will use the expression,
2) an expression - and Name Expr will take the expression".
Name Expr(1+2); // 1 + 2
Head(1+2); // 3
Re: Does Head evaluate it's argument?Other functions like Head() [ID=7]
- Head()
- NArg()
- Arg()
- Is Empty()
- Print()
- Show()
- Write()
Re: Does Head evaluate it's argument?Functions which evaluate their arguments:
[ID=2]
almost all functions ...
most "is ..." functions:
- Is Expr()
- Is Matrix()
- Is List()
- Is Number()
- Is Namespace()
- Namespace Exists()
good to know:
- Equal() (==)
- Parse()
- Associative Array()
evaluate arguments > 1st one:
- Substitute Into()
- Insert Into()
- Remove From()
- Assign()
- Arg()
Re: Does Head evaluate it's argument?Functions which behave like Name Expr():
[ID=3]
"the expressionArg can be
1) a name holding an expression - and Name Expr will use the expression,
2) an expression - and Name Expr will take the expression".
- Name Expr()
- N Arg()
- Extract Expr() [both arguments] *)
*) so cool!
x= Expr(Print(1));
Extract Expr(x, x) // Print(1)
Re: Does Head evaluate it's argument?Other functions like Head()
[ID=7]
"the expressionArg can be
1) a name holding an expression - and Head will use the expression,
2) an expression - and Head will evaluate it and will use the result as input
3) or a literal expression quoted by Expr() - and Head will take the literal expression".
- Head()
- NArg()
- Arg()
- Is Empty()
- Print()
- Show()
- Write()
Re: Does Head evaluate it's argument?#ExpressionInspector
Re: Does Head evaluate it's argument?Another question:
Why does Type evaluate its argument?
Type(Char(1)) // String !!
