CODEWITHSUNDEEP
tcl
Rahul
Rahul

Reputation: 11

How do i get the last non-zero element from a Tcl list?

I currently use

lindex $mylist end

to get the last element from list in tcl. Mostly mylist values are zeros and I need to get last non-zero element. I know I can put all non-zero elements into a different list or remove zeros from current list. But is there a way to do it without modifying mylist?

Upvotes: 0

Views: 445

Answers (2)

Schelte Bron
Schelte Bron

Reputation: 4813

As I mentioned in a comment to another answer, a simple way is to use lsearch to create a new (temporary) list with all zeroes removed, and then get the last element from that list:

lindex [lsearch -all -inline -not -integer $mylist 0] end

I assumed the other elements in the list are also integers. If that is not the case, change the -integer option to -real or -exact.

This may not be the fastest method. But you didn't indicate if that is a concern. At least it will use less memory than creating a reverse list and then using lsearch to find the first non-zero element. Especially since you indicated the list contains mostly zeroes.

Memory use will not be too bad anyway, because the new (temporary) list will just be a list of pointers to the non-zero elements of the original list. So it doesn't matter if the non-zero elements are short integers too, or happen to be long strings.

Upvotes: 2

Donal Fellows
Donal Fellows

Reputation: 137567

The obvious way is to scan backwards through the list:

proc lastNonZero list {
    set idx [llength $list]
    while {[incr idx -1] >= 0} {
        if {[lindex $list $idx] != 0} {
            break
        }
    }
    return $idx
}

Upvotes: 2

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