In the simplest case there is one loop variable, varname, and one list, list, that is a list of values to assign to varname. The body argument is a Tcl script. For each element of list (in order from first to last), lfilter assigns the contents of the element to varname as if the lindex command had been used to extract the element, then calls the Tcl interpreter to execute body. If execution of the script completes normally and produces a true value, that element of list is appended to an accumulator list. lmap returns the accumulator list.
In the general case there can be more than one value list (e.g., list1 and list2), and each value list can be associated with a list of loop variables (e.g., varlist1 and varlist2). During each iteration of the loop the variables of each varlist are assigned consecutive values from the corresponding list. Values in each list are used in order from first to last, and each value is used exactly once. The total number of loop iterations is large enough to use up all the values from all the value lists. If a value list does not contain enough elements for each of its loop variables in each iteration, empty values are used for the missing elements. For each iteration, the value that is appended when body yields a true value (i.e., anything accepted by Tcl_GetBooleanFromObj as a boolean) is the value that was assigned to the first variable of the first list of variables; all other values are not collected.
The break and continue statements may be invoked inside body, with the same effect as in the for and foreach commands. In these cases the body does not complete normally and the lead value is not appended to the accumulator list.
lfilter x {0 2 4 1 3 5 10 8 6 11 9 7} {
expr {($x % 2) == 1}
}
# The result is "1 3 5 11 9 7"
Select elements of a list according to a condition in another list:
lfilter x [lseq 10] y [lseq 0 .. 1 by 0.1] {
expr {$y > 0.3 && $y < 0.7}
}
# The result is "4 5 6"
Select the elements containing at least five characters:
set sentence "The quick brown fox jumps over the lazy dog."
lfilter word [split $sentence] {
expr {[string length $word] >= 5}
}
# The result is "quick brown jumps"
Select the elements that are true palindromes:
set usernames {tom bob steve dood}
lfilter word $usernames {
string equal $word [string reverse $word]
}
# The result is "bob dood"