Version 16 of Tcl and octal numbers

Updated 2003-03-12 20:45:19

Purpose: discuss the common pitfall of Tcl and octal numbers

---

Since Tcl does many things in an obvious manner, non-unix/c people are frequently surprised when they try this kind of code:

 # somehow get today's date and time into MMM DDD YYY HH MM SS variables

 set newtime [expr $HH + 1]

and get an error at 8am. The problem - Tcl tries to do

  expr 08 + 1

and complains. 08, you see, isn't really the string of the decimal number that comes after 7. Instead, it is an error. Tcl sees the leading 0 and treats the digits after it as representing a base 8 number. But there are no 8s (or 9s) in a base 8 number. So it generates an error.

---

The fix is to use:

    scan $HH %d HH

which strips hazardous leading zeros. This is also safer than [string trimleft $HH 0] which can fail if $HH ever ends up containing "00" for example.

DKF

---

glennj: one potential pitfall of [scan] is that it might mask potential errors:

        set n 09blah42
        incr n

fails as expected with the error message:

        expected integer but got "09blah42"
        while evaluating {incr n}

However:

        set n 09blah42
        scan $n %d n
        incr n ;# ==> n is now 10

Application writers might actually want to trap an invalid entry like that.

[2003-03-12] I see Kevin Kenny contributed the following to c.l.t

     proc forceInteger { x } {
        set count [scan $x %d%s n rest]
        if { $count <= 0 || ( $count == 2 && ![string is space $rest] ) } {
            return -code error "not an integer: \"$x\""
        }
        return $n
     }
     % forceInteger x
     not an integer: "x"
     % forceInteger 123
     123
     % forceInteger 08
     8

This also covers my preceding concern:

     % forceInteger 09blah42
     not an integer: "09blah42"

---

[Refer to http://phaseit.net/claird/comp.lang.tcl/fmm.html#zero ]

[Explain improved diagnostic in 8.3.]

---

The question recently came up how do I display the octal value of a character in Tcl? and RS replied:

 'the complete sequence is "format 0%o [scan a %c]"'

(but only with Tcl more recent than 8.2 or so; older scan works slightly differently).

---

http://www.tcl.tk/cgi-bin/tct/tip/114.html proposes modifying Tcl in a future release so that numbers beginning with 0 will not be interpreted by default as being expressed in octal. The proposer believes that far more users stumble upon this feature by accident than use it intentionally.