Version 19 of incr

increment a variable http://www.purl.org/tcl/home/man/tcl8.4/TclCmd/incr.html

incr variable ?increment?

The variable must exist before the incr call, and must have an integer value. The increment must also be an integer and defaults to 1 if not specified, with the result being that

 incr i

and

 set i [expr {$i + 1}]

are nearly equivalent. The difference of course has to do when the assumptions are violated - if i isn't an integer, then the second expression works whereas the first raises an error that you have to catch unless you want the program to terminate.

Note that the increment can be a negative number - allowing you to 'increment' decrementally ;-).

Work is underway to add more 64 bit support into Tcl. Even when this happens, the increment value is supposedly going to continue to be limited to 32 bits due to the far reaching ramifications of changes needed.

RS had a half-baked idea these days - extend incr to also work on characters (like in C), so we could write:

 for {set letter "A"} {$letter<="Z"} {incr letter} {...}

rmax pointed rightly out that ambiguities come up as soon as the character is a digit: set i 9; incr i would make it 10, not ":" as expected - so maybe with a -char switch which still looks better than

 set letter [format %c [expr {[scan $letter %c]+1}]]

Is there anything peculiar that happens with this in regards to encoding? Will the incrementing be of the utf8 byte value? Are there multi-byte character issues with regards to incrementing? What happens at wrap around time if only 8 bits are being used? Wrap to 0? Is this a 7 or 8 bit value? - RS: To Tcl (since 8.1), all characters are Unicodes and potentially 16 bits wide (this may have to change to 32), which mostly are stored as UTF characters, 1..3 bytes wide. I would propose that incr -char reacts like the format..expr..scan code above, just faster, and might throw an error if crossing 0xFFFF. Incrementing strings of more than a char wide seems of little use to me, could raise an error as well.

Are all 16 bit bytes valid unicode characters? (No. Some codes (0xFFFE, 0xFEFF, others?) are explicitly not characters, and others are non-spacing diacriticals that shouldn't be used on their own.)
When I was talking about multi-byte characters, I was referring to the 2 and 3 byte long sequences for a unicode character. However, now that you mention it, I would like to be able to say

 set name "/tmp/filename0"
 incr name

and have the file name go from filename0 to filename9, then filenamea-z, etc.

Here is a corresponding decrement function:

 proc decr { int { n 1 } } {
     if { [ catch {
        uplevel incr $int -$n
     } err ] } {
        return -code error "decr: $err"
     }
     return [ uplevel set $int ]
 }

RS wonders what the advantages against

 proc decr {var {n 1}} {uplevel 1 [list incr $var -$n]}

are (or against just writing incr i -1)?

Martin Lemburg: I love readable code and using decr var 1 is more readable than incr var -1, or am I wrong?

Isn't it worth to discuss readability of code?

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