Reply To: Mod 11 check digit

[Vince Angulo]

Haven’t seen Modulus 11 since my AS/400 days.  It’s easier to work if you reverse the input value.  This should work:

Code:

# create a list from the input value
set mInput [split $xlateInVals {}]

# reverse the list values to a new list
set mLength [llength $mInput]
set a [expr $mLength – 1]
set mReverse {}
foreach i $mInput {
lappend mReverse [lindex $mInput $a]
incr a -1
}

# create a list with enough repeating weight elements
# there are different weight values, AS/400 commonly used 2-7
set mCount [expr int($mLength / 6) + 1]
set mWeight {}
for {set j 0} {$j < $mCount} {incr j} {
lappend mWeight 2 3 4 5 6 7
}

# aggregate the products of the input and weight elements
set mSum 0
for {set k 0} {$k < $mLength} {incr k} {
set mSum [expr $mSum + ([lindex $mReverse $k] * [lindex $mWeight $k])]
}

# determine and evaluate the remainder
set mMod [expr $mSum % 11]
switch — $mMod {
0; {set mMod11 "0"}
1; {set mMod11 "X"}
default {set mMod11 [expr 11 – $mMod]}
}

# tag the check digit onto the input list and turn it back to a string
set xlateOutVals [join [lappend mInput $mMod11] {}]

Not exactly elegant, but functional…Maybe someone with stronger tcl chops can simplify it!!