Why does [expr {NaN}] results in "domain error: argument not in valid range"

[Yusuke Yamasaki]

Hi, 

I wanted to handle NaN or Inf safely in a long math expression.

For example, sqrt(-1) results in domain error so I wrote a user defined function under tcl::mathfunc namespace.

proc is_computable_double {val} {

expr {[string is double -strict $val] && ![is_nan $val]}

}

proc is_nan {val} {

string match -nocase [string trimleft $val +-] "nan"

}

proc sqrtf {a} {

if {[is_computable_double $a] && $a >= 0} {

expr {sqrt($a)}

} else {

return NaN

}

}

proc tcl::mathfunc::sqrtf {a} {

::sqrtf $a

}

# https://gist.github.com/yyamasak/1b8fa0ea5090053fbb576c2d20b35b0c

tcl::mathfunc::sqrtf -1; # => NaN
expr {sqrtf(-1)}; # => domain error: argument not in valid range

I noticed that expr never returns NaN. In such a case, it causes a domain error.

Is there any way to avoid it?

I had a danger expression like this.

set tau [expr {$uxy / (sqrt($uxx) * sqrt($uyy))}]

I made it safer by chaining some procedures.

set tau [divf $uxy [mulf [sqrtf $uxx] [sqrtf $uyy]]]

But I wanted to rewrite it as follows.

set tau [expr {divf($uxy,mulf(sqrtf($uxx),sqrtf($uyy)))}]

Above is a relatively short example but I have much longer expressions in my actual code.

So I wanted to make use of [expr] parser instead of nested procedures or something like tcl::mathfunc::sqrt...

Regards,

Yusuke

[Yusuke Yamasaki]

Hi, 

I came up with a solution.

proc myexpr {args} {

if {[catch {expr {*}$args} res]} {

if {$res eq "domain error: argument not in valid range"} {

return NaN

} else {

return -code error $res

}

} else {

return $res

}

}

I think this is enough for me.

Note: I found it's not a good idea to replace expr using rename.

rename expr _expr; proc expr {args} {...}

When this is done in wish, the history caller dies.

(bin) 23 % set errorInfo

can't read "history(nextid)": no such variable

    while executing

"_expr {*}$args"

    (procedure "expr" line 1)

    invoked from within

"expr {$history(nextid)-1}"

    (procedure "HistIndex" line 4)

    invoked from within

"HistIndex $event"

    (procedure "tcl::HistEvent" line 3)

    invoked from within

"tcl::HistEvent $event"

    (procedure "history" line 64)

    invoked from within

"history event [expr {[history nextid] -1}]"

Regards,

Yusuke

2018年5月15日火曜日 18時35分48秒 UTC+9 Yusuke Yamasaki: