iu2 - started on Jan 7 2007

**1. Enjoying coding: Pressing letter keys, not arrow keys**

There is often a need to introduce a new variable inside a foreach loop. For example, given:

foreach x $list { puts $x }

in order to count the list's items during the loop I may do:

set c -1 foreach x $list { incr c puts "$c. $x" }

While not so bad I still prefer not introducing new variables because

- Having to go backwards to set new variables doesn't feel very pleasant. Coding feels good when it flows forward, especially inside loops. By coding I mean typing the code.
- If there is a need for more than one variable, it begins to feel messy.
- It makes me feel like coding in C... from tcl I would expect something else.

I think this is what mapping lists, filtering lists, lists comprehension and lambdas are all about: Make code typing flow forward. They also make the code clearer, because understanding syntax is easier than exploring algorithms.

This is a valid syntax (in python)

res = [2*x for x in lis]

but this is an algorithm:

res = [] for x in lis: res += [2*x]

and it remains an algorithm even if it is written like this

set res {}; foreach x $lis {lappend res $x}

This is better

proc mult2 x {expr $x*2} set res [struct::list map $res mult2]

but not as good as this one

set res [struct::list map $res {apply {x {expr $x*2}}}

LV Better in what sense? I look at this last item and have no idea what its purpose is, while if I look at the first tcl foreach example I have a clear idea of what it is doing... except that it isn't doing what the rest of the items is doing (there's nothing about multiplying anything by 2 in the foreach example...).

iu2 Oops,I should have written the first example like this

set res2 {}; foreach x $res {lappend res2 [expr {2*$x}]}

This form has three disadvantages:

- One has to read until the end of the line to get the idea of what this line is doing
- The list variable itself
**res**cannot be reused, so**res2**must be introduced - I feel that stacking up two commands in one line is a less favorable coding pattern...

while the last pattern has these three advantages:

- On can tell what's happening in one glimpse at the
**beginning**of the line - a list is formed out of another list. In order to understand exactly how it is formed - the 'apply' part is right there at the end. - The list variable itself can be reused, as shown in the example.
- One command in the line

I guess this is why there are so many pages here about looping, lambdas, etc.. Well, this is another one.. ;-)

Stu Jan 8 2007 - A proc facilitating the use and cleanup of 'temp' vars, counters, etc.

proc with {vars body} { foreach v $vars { lassign $v name default uplevel 1 [list set $name $default] } uplevel 1 $body foreach v $vars { lassign $v name default uplevel 1 [list unset -nocomplain $name] } }

Your example above, using *with*:

with {{c -1} x} { foreach x $list { incr c puts "$c. $x" } }

**2. Introducing a counter inside a foreach**

We start straight from eliminating *set c -1*

set list {a b c d e f g} foreach x $list c [struct::list iota [llength $list]] { puts "$c. $x" }

With a little help from

proc counters list { for {set c 0} {$c < [llength $list]} {incr c} { lappend res $c } return $res }

we can go

foreach x $list c [counters $list] { puts "$c. $x" }

This is the Python way

proc enumerate list { set c 0 foreach x $list { lappend res $c $x incr c } return $res } foreach {c x} [enumerate $list] { puts "$c. $x" }

but I prefer the previous one, which is more foreach-y.

**3. First iteration commands**

This code

foreach x $list first 1 { if {$first == 1} {set c 0} else {incr c} puts "$c. $x" }

introduces the variable *first*, which is set to 1 in the first iteration and then becomes *""* for all the rest. Since *first* is in *foreach*'s arguments list, it doesn't realy seem like going back and setting a new variable.

This code sums up a list

set numbers {1 2 3 4 5 6 7 8 9 10} foreach x $numbers first 1 { if {$first == 1} {set sum $x} else {incr sum $x} } puts $sum

Result:

55

and this one finds the maximum

set numbers2 {-2 -4 -1 -3 0 10 3} foreach x $numbers2 first 1 { if {$first == 1 || $x > $max} {set max $x} } puts $max

Result:

10

**4. Little functions giving a Common Lisp flavour**

Instead of *first*, let's call that variable *enablecl*, standing for *Enable Common Lisp*.

proc incrementing {var} { uplevel 1 [list if {$enablecl == 1} [list set $var 0] else [list incr $var]] } # test foreach x $list enablecl 1 { incrementing c1 puts "$c1. $x" }

Let's add more functions like that

proc summing {exp into var} { uplevel 1 [list if {$enablecl == 1} [list set $var $exp] else [list incr $var $exp]] } proc counting {cond into var} { uplevel 1 [list if {$enablecl == 1} [list set $var 0]] uplevel 1 [list if $cond [list incr $var]] }

and give them a try

set numbers {1 2 3 4 5 6 7 8 9 10 11} # test foreach x $numbers enablecl 1 { summing $x into sum1 summing [expr 2*$x] into sum2 counting 1 into count counting {int($x)/2*2 == $x} into even counting "int($x)/2*2 != $x" into odd } puts "$sum1, $sum2, $count steps, $even evens, $odd odds"

Result:

66, 132, 11 steps, 5 evens, 6 odds

Nice.

Maximum and minimum took me a while to figure out...

proc maximizing {exp into var} { uplevel 1 [list if {$enablecl == 1} [list set $var $exp]] uplevel [list if [concat $exp > $$var] [list set $var $exp]] } proc minimizing {exp into var} { uplevel 1 [list if {$enablecl == 1} [list set $var $exp]] uplevel [list if [concat $exp < $$var] [list set $var $exp]] } set numbers2 {-2 -4 -1 -3 0 10 3} # test foreach x $numbers2 enablecl 1 { maximizing $x into max minimizing $x into min puts $x,$max,$min } puts $max,$min

We advance towards list comprehension by introducing *appending*

proc appending {exp into var} { uplevel 1 [list if {$enablecl == 1} [list set $var [list $exp]] else [list lappend $var $exp]] } # test foreach x $numbers enablecl 1 { appending $x into nums appending "Number $x" into strings if {$x > 5} {appending [expr 2*$x] into twice} ;# a list comprehension... if {[set res [expr $x+1]] > 5} {appending $res into plus1} ;# and an improvement } puts [join $nums ,] puts [join $strings \n] puts [join $twice ", "] puts [join $plus1 ", "]

Adding a bit more syntax to *appending* makes it more interesting

proc appending {exp into var {if if} {cond 1}} { uplevel 1 [list if {$enablecl == 1} [list set $var {}]] set res [uplevel 1 [list subst $exp]] uplevel 1 [regsub -all -- {%\yr\y} [list if $cond [list lappend $var $res]] $res] } # test foreach x {1 2 3 4} y {5 6 7 8} enablecl 1 { appending [expr $x+$y] into sums2 if {%r > 8} } puts [join $sums2]

More stuff

proc toggling {into var} { uplevel 1 [list if {$enablecl == 1} [list set $var 0]] uplevel 1 [list if {$enablecl != 1} [list set $var [uplevel 1 [list expr 1-$$var]]]] } # test foreach x {1 2 3 4} enablecl 1 { toggling into togl puts "$x - $togl" }

This is a general form of updating a variable each iteration

proc updating {init exp into var} { uplevel 1 [list if {$enablecl == 1} [list set $var $init]] uplevel 1 [list set $var [uplevel 1 $exp]] } # test foreach x {1 2 3 4 5} enablecl 1 { updating 0 {expr $count+1} into count updating 0 {expr $sum2+$x} into sum2 updating 1 {expr $mult*$x} into mult updating {} {lappend mult2 [expr 2*$x]} into mult2 } # print result foreach x {count sum2 mult mult2} {puts "$x: [set $x]"}

Result:

count: 5 sum2: 15 mult: 120 mult2: 2 4 6 8 10

# another test foreach x {1 2 3 4 5 6 7 8 9 10 11 12} enablecl 1 { updating -1 {expr ($cyc+1)%3} into cyc puts "x: $x, cyc: $cyc" }

The last test leads to another idea

proc cycling {exp into var} { uplevel 1 [list if {$enablecl == 1} [list set $var -1]] uplevel 1 [list set $var [uplevel 1 [list expr ($$var+1)%$exp]]] } # test foreach x {1 2 3 4 5 6 7 8 9 10 11 12} enablecl 1 { cycling 3 into cyc puts "x: $x, cyc: $cyc" }

I often write lists as rows, each row having *col_count* items, so

foreach number $list enablecl 1 { puts -nonewline "$number " cycling $col_count into cyc if {$cyc == $col_count-1} {puts ""} }

because of that last example, I just can't resist extending *cycling* a little bit...

proc cycling {exp into var args} { uplevel 1 [list if {$enablecl == 1} [list set $var -1]] uplevel 1 [list set $var [uplevel 1 [list expr ($$var+1)%$exp]]] set condcount 0 foreach {on list what} $args { # if {[uplevel 1 [list set $var]] in $list} tcl 8.5 if {[lsearch $list [uplevel 1 [list set $var]]] > -1} {uplevel 1 $what; incr condcount} else { if {$on eq "else" && $condcount == 0} {uplevel 1 $list} } } } # test foreach x {1 2 3 4 5 6 7 8 9 10 11 12} enablecl 1 { puts -nonewline "$x " cycling 3 into cyc on 2 {puts ""} } # or even this foreach x {1 2 3 4 5 6 7 8 9 10 11 12} enablecl 1 { puts -nonewline "$x" cycling 3 into cyc on 2 {puts ""} else {puts -nonewline " "} } # another test foreach x {1 2 3 4 5 6 7 8 9 10 11 12} enablecl 1 { cycling 4 into cyc on 0 {puts "start: cyc=$cyc"} on 3 {puts "last: cyc=$cyc"} on {1 2} {puts "middle: cyc=$cyc"} puts $cyc }

**5. Helper functions with a helper variable**

The following example uses another variable besides *var*

proc cyclelist {hlpvar list into var} { uplevel 1 [list if {$enablecl == 1} [list set $hlpvar -1]] set len [llength $list] set index [uplevel 1 [list set $hlpvar]] set index [expr ($index+1)%$len] uplevel 1 [list set $hlpvar $index] uplevel 1 [list set $var [lindex $list $index]] } # test foreach x {1 2 3 4 5 6 7 8 9 10 11 12 13 14} enablecl 1 { cyclelist cyc1 {one two three four} into cyc2 puts "$x $cyc2 ($cyc1)" }

**6. Summary**

- With these little helpers foreach-ing can be more readable, more fun and more
**press letter keys, not arrow keys ;-)** - I'm sure more functions of this type can be introduced
- There still may be bugs in them, so bug fixes are welcome
- I'm not sure the
*uplevel 1 [list if...*way I use is the best. Shorter or more readable formats will be appreciated.

LES on same day:

set res [struct::list map $res {apply {x {expr $x*2}}}

You call that "readable"?

iu2 well, yes, beacuse I **recognize** a structure: map-list-apply.

RS Due to the path structure, the first part is a bit cluttered. If you seriously use map, it might help to

interp alias {} map {} struct::list map

and then code (remember to brace your expr-essions :^)

set res [map $res {apply {x {expr {$x*2}}}}]

NEM: And adding a constructor for lambdas helps a lot:

proc func {params body} { list ::apply [list $params [list expr $body] ::] } map $res [func x {$x*2}]

Unfortunately, the tcllib versions of map, filter and fold take arguments in an inconvenient order for interp alias, so e.g. we have to do:

proc ldouble xs { map $xs [func x {$x*2}] }

rather than simply:

def ldouble = map [func x {$x*2}]

aspect -- frankly, I find the use of list map and apply awkward in the above example. Using lmap it simplifies (without the need for lambda) to:

set res [lmap i $res {expr {$i*2}}]

.. or, using the version (which I prefer for succinctness, even if it is a little less flexible) from list map and list grep you get:

set res [lmap $res {expr {$_*2}}]

NEM I'm not sure which uses you find awkward? The *func* constructor I provide is a simple single-line proc, and the resultant map is just as tiny as lmap:

proc map {f xs} { set ys [list] foreach x $xs { lappend ys [{*}$f $x] } return $ys } map [func i {$i*2}] $res

It can also be used directly with existing commands. For instance, if we want to convert all words in a list to upper-case, we can simply do:

map {string toupper} {this is a list of words}

This is just one of the benefits of properly factoring out the construction of a callback from the control construct that uses it. Others are that we can use multiple different constructors in different situations: e.g. a plain lambda, a version that uses expr, a version that captures a closure, etc. Not to mention the benefits of byte-compilation that come from having the lambda as a single argument.

FM : sometimes I like to use this proc

proc except cond { uplevel [list if $cond {return -code continue}] } set L [list 0 1 2 3 4 5 6 7 8 9] foreach e $L { except {$e%2==1 || $e==0} lappend Res $e } set Res ; # 2 4 6 8