Is there a simple way to count the number of matching elements in a list? The answer is "Yes. Several ways."
RWT: And just to keep things interesting, we can compare them in the context of a little test harness that times how quickly they run. There are many times when a Tcl programmer might want to compare a couple of different techniques to see which is fastest. The [time] command can often help. (See the Tcl Performance page for more information on speed improvements.)
#!/bin/sh
# restart on the next line using tclsh \
exec tclsh "$0" "$@"
#----------------------------------------
# Define procs to test each method for
# counting identical list items. This
# enables the byte-code compiler to
# optimize the code.
#----------------------------------------
proc count_members1 list {
foreach member $list {
if {[info exists count($member)]} {
incr count($member)
} else {
set count($member) 1
}
}
}
proc count_members2 list {
foreach x $list {
if {[catch {incr count($x)}]} {set count($x) 1}
}
}
proc count_members3 list {
foreach x $list {
expr {[catch {incr count($x)}] && [set count($x) 1]}
}
}
proc count_members4 list {
foreach x $list {
lappend ulist($x) {}
}
foreach name [array names ulist] {
set count($name) [llength $ulist($name)]
}
}
proc count_members5 list {
foreach x $list {
append ulist($x) .
}
foreach name [array names ulist] {
set count($name) [string length $ulist($name)]
}
}
#----------------------------------------
# Create some test data. In this case,
# build a list of 10,000 items
#----------------------------------------
set items [list john paul jones mary]
for {set i 0} {$i<10000} {incr i} {
lappend data [lindex $items [expr {int(rand()*[llength $items])}]]
}
#----------------------------------------
# Print some information about our
# environment. This is very useful
# when consulting comp.lang.tcl.
#----------------------------------------
puts "[info patchlevel] over $tcl_platform(os) $tcl_platform(osVersion)."
#----------------------------------------
# Run the tests.
# Note that we have cleverly named
# the test procs so that [info] can
# easily find and execute them.
#----------------------------------------
foreach proc [info proc count_members*] {
puts ""
puts "$proc"
puts [time {$proc $data} 10]
}RS 2004-02-20: Note however that the above procs don't really yield their count - the local array is discarded on return. For practical use, I modified count_members4 which was among the fastest in my tests on WinXP, to return a list of {element count} pairs:
proc lcount list {
foreach x $list {lappend arr($x) {}}
set res {}
foreach name [array names arr] {
lappend res [list $name [llength $arr($name)]]
}
return $res
}
% lcount {yes no no present yes yes no no yes present yes no no yes yes}
{no 6} {yes 7} {present 2}The list is in hash (i.e., apparently no) order, but you can post-process it to
I don't have 8.5 at hand yet, but I expect this dict version to be a good solution too:
proc lcount list {
set count {}
foreach element $list {dict incr count $element}
set count
}gold Here's counting through a list of thrown dice combos for probability. The worker bee is lsearch -all $lister $facen $facen is the sum of a 2-dice throw like 7. Subroutine is invoked by a foreach procedure for probability, which would be number of thrown 7's over all possible throws ($lister). Drop (-all) and lose all, returning only one position of "7" in list. See Binomial Probability Slot Calculator Example.
console show
proc calculation { facen } {
# prob. subroutines for two 6-sided dice
set lister {2 3 4 5 6 7 3 4 5 6 7 8 4 5 6 /
7 8 9 5 6 7 8 9 10 6 7 8 9 10 11 7 8 9 10 11 12}
set ee [llength $lister ]
set kk [ llength [ lsearch -all $lister $facen ] ]
set prob [ expr { ($kk*1.) / $ee } ]
return $prob
}
set limit 12
for { set i 1 } { $i <= $limit } { incr i } {
lappend listxxx $i
lappend listxxx [ calculation $i ]
puts " $i [ calculation $i ] "
}
#end See Chart of proposed list functionality too.