DSL control structures¶
Pattern-action blocks¶
These are reminiscent of awk
syntax. They can be used to allow assignments to be done only when appropriate -- e.g. for math-function domain restrictions, regex-matching, and so on:
mlr cat data/put-gating-example-1.dkvp
x=-1 x=0 x=1 x=2 x=3
mlr put '$x > 0.0 { $y = log10($x); $z = sqrt($y) }' data/put-gating-example-1.dkvp
x=-1 x=0 x=1,y=0,z=0 x=2,y=0.3010299956639812,z=0.5486620049392715 x=3,y=0.4771212547196624,z=0.6907396432228734
mlr cat data/put-gating-example-2.dkvp
a=abc_123 a=some other name a=xyz_789
mlr put ' $a =~ "([a-z]+)_([0-9]+)" { $b = "left_\1"; $c = "right_\2" }' \ data/put-gating-example-2.dkvp
a=abc_123,b=left_abc,c=right_123 a=some other name a=xyz_789,b=left_xyz,c=right_789
This produces heteregenous output which Miller, of course, has no problems with (see Record Heterogeneity). But if you want homogeneous output, the curly braces can be replaced with a semicolon between the expression and the body statements. This causes put
to evaluate the boolean expression (along with any side effects, namely, regex-captures \1
, \2
, etc.) but doesn't use it as a criterion for whether subsequent assignments should be executed. Instead, subsequent assignments are done unconditionally:
mlr --opprint put ' $a =~ "([a-z]+)_([0-9]+)"; $b = "left_\1"; $c = "right_\2" ' data/put-gating-example-2.dkvp
a b c abc_123 left_abc right_123 some other name left_ right_ xyz_789 left_xyz right_789
Note that pattern-action blocks are just a syntactic variation of if-statements. The following do the same thing:
boolean_condition { body }
if (boolean_condition) { body }
If-statements¶
These are again reminiscent of awk
. Pattern-action blocks are a special case of if
with no elif
or else
blocks, no if
keyword, and parentheses optional around the boolean expression:
mlr put 'NR == 4 {$foo = "bar"}'
mlr put 'if (NR == 4) {$foo = "bar"}'
Compound statements use elif
(rather than elsif
or else if
):
mlr put ' if (NR == 2) { ... } elif (NR ==4) { ... } elif (NR ==6) { ... } else { ... } '
While and do-while loops¶
Miller's while
and do-while
are unsurprising in comparison to various languages, as are break
and continue
:
echo x=1,y=2 | mlr put ' while (NF < 10) { $[NF+1] = "" } $foo = "bar" '
x=1,y=2,3=,4=,5=,6=,7=,8=,9=,10=,foo=bar
echo x=1,y=2 | mlr put ' do { $[NF+1] = ""; if (NF == 5) { break } } while (NF < 10); $foo = "bar" '
x=1,y=2,3=,4=,5=,foo=bar
A break
or continue
within nested conditional blocks or if-statements will,
of course, propagate to the innermost loop enclosing them, if any. A break
or
continue
outside a loop is a syntax error that will be flagged as soon as the
expression is parsed, before any input records are ingested.
The existence of while
, do-while
, and for
loops in Miller's DSL means
that you can create infinite-loop scenarios inadvertently. In particular,
please recall that DSL statements are executed once if in begin
or end
blocks, and once per record otherwise. For example, while (NR < 10) will
never terminate. The NR
variable is only incremented
between records, and each DSL expression is invoked once per record: so, once
for NR=1
, once for NR=2
, etc.
If you do want to loop over records, see Operating on all records for some options.
For-loops¶
While Miller's while
and do-while
statements are much as in many other languages, for
loops are more idiosyncratic to Miller. They are loops over key-value pairs, whether in stream records, out-of-stream variables, local variables, or map-literals: more reminiscent of foreach
, as in (for example) PHP. There are for-loops over map keys and for-loops over key-value tuples. Additionally, Miller has a C-style triple-for loop with initialize, test, and update statements. Each is described below.
As with while
and do-while
, a break
or continue
within nested control structures will propagate to the innermost loop enclosing them, if any, and a break
or continue
outside a loop is a syntax error that will be flagged as soon as the expression is parsed, before any input records are ingested.
Single-variable for-loops¶
For maps, the single variable is always bound to the key of key-value pairs:
mlr --from data/small put -q ' print "NR = ".NR; for (e in $*) { print " key:", e, "value:", $[e]; } '
NR = 1 key: a value: pan key: b value: pan key: i value: 1 key: x value: 0.346791 key: y value: 0.726802 NR = 2 key: a value: eks key: b value: pan key: i value: 2 key: x value: 0.758679 key: y value: 0.522151 NR = 3 key: a value: wye key: b value: wye key: i value: 3 key: x value: 0.204603 key: y value: 0.338318 NR = 4 key: a value: eks key: b value: wye key: i value: 4 key: x value: 0.381399 key: y value: 0.134188 NR = 5 key: a value: wye key: b value: pan key: i value: 5 key: x value: 0.573288 key: y value: 0.863624
mlr -n put -q ' end { o = {"a":1, "b":{"c":3}}; for (e in o) { print "key:", e, "valuetype:", typeof(o[e]); } } '
key: a valuetype: int key: b valuetype: map
Note that the value corresponding to a given key may be gotten as through a computed field name using square brackets as in $[e]
for stream records, or by indexing the looped-over variable using square brackets.
For arrays, the single variable is always bound to the value (not the array index):
mlr -n put -q ' end { o = [10, "20", {}, "four", true]; for (e in o) { print "value:", e, "valuetype:", typeof(e); } } '
value: 10 valuetype: int value: 20 valuetype: string value: {} valuetype: map value: four valuetype: string value: true valuetype: bool
Key-value for-loops¶
For maps, the first loop variable is the key and the second is the value; for arrays, the first loop variable is the (1-up) array index and the second is the value.
Single-level keys may be gotten at using either for(k,v)
or for((k),v)
; multi-level keys may be gotten at using for((k1,k2,k3),v)
and so on. The v
variable will be bound to a scalar value (non-array/non-map) if the map stops at that level, or to a map-valued or array-valued variable if the map goes deeper. If the map isn't deep enough then the loop body won't be executed.
cat data/for-srec-example.tbl
label1 label2 f1 f2 f3 blue green 100 240 350 red green 120 11 195 yellow blue 140 0 240
mlr --pprint --from data/for-srec-example.tbl put ' $sum1 = $f1 + $f2 + $f3; $sum2 = 0; $sum3 = 0; for (key, value in $*) { if (key =~ "^f[0-9]+") { $sum2 += value; $sum3 += $[key]; } } '
label1 label2 f1 f2 f3 sum1 sum2 sum3 blue green 100 240 350 690 690 690 red green 120 11 195 326 326 326 yellow blue 140 0 240 380 380 380
mlr --from data/small --opprint put 'for (k,v in $*) { $[k."_type"] = typeof(v) }'
a b i x y a_type b_type i_type x_type y_type pan pan 1 0.346791 0.726802 string string int float float eks pan 2 0.758679 0.522151 string string int float float wye wye 3 0.204603 0.338318 string string int float float eks wye 4 0.381399 0.134188 string string int float float wye pan 5 0.573288 0.863624 string string int float float
Note that the value of the current field in the for-loop can be gotten either using the bound variable value
, or through a computed field name using square brackets as in $[key]
.
Important note: to avoid inconsistent looping behavior in case you're setting new fields (and/or unsetting existing ones) while looping over the record, Miller makes a copy of the record before the loop: loop variables are bound from the copy and all other reads/writes involve the record itself:
mlr --from data/small --opprint put ' $sum1 = 0; $sum2 = 0; for (k,v in $*) { if (is_numeric(v)) { $sum1 +=v; $sum2 += $[k]; } } '
a b i x y sum1 sum2 pan pan 1 0.346791 0.726802 2.073593 8.294372 eks pan 2 0.758679 0.522151 3.28083 13.12332 wye wye 3 0.204603 0.338318 3.542921 14.171684 eks wye 4 0.381399 0.134188 4.515587 18.062348 wye pan 5 0.573288 0.863624 6.4369119999999995 25.747647999999998
It can be confusing to modify the stream record while iterating over a copy of it, so instead you might find it simpler to use a local variable in the loop and only update the stream record after the loop:
mlr --from data/small --opprint put ' sum = 0; for (k,v in $*) { if (is_numeric(v)) { sum += $[k]; } } $sum = sum '
a b i x y sum pan pan 1 0.346791 0.726802 2.073593 eks pan 2 0.758679 0.522151 3.28083 wye wye 3 0.204603 0.338318 3.542921 eks wye 4 0.381399 0.134188 4.515587 wye pan 5 0.573288 0.863624 6.4369119999999995
You can also start iterating on sub-maps of an out-of-stream or local variable; you can loop over nested keys; you can loop over all out-of-stream variables. The bound variables are bound to a copy of the sub-map as it was before the loop started. The sub-map is specified by square-bracketed indices after in
, and additional deeper indices are bound to loop key-variables. The terminal values are bound to the loop value-variable whenever the keys are not too shallow. The value-variable may refer to a terminal (string, number) or it may be map-valued if the map goes deeper. Example indexing is as follows:
# Parentheses are optional for single key: for (k1, v in @a["b"]["c"]) { ... } for ((k1), v in @a["b"]["c"]) { ... } # Parentheses are required for multiple keys: for ((k1, k2), v in @a["b"]["c"]) { ... } # Loop over subhashmap of a variable for ((k1, k2, k3), v in @a["b"]["c"]) { ... } # Ditto for ((k1, k2, k3), v in @a { ... } # Loop over variable starting from basename for ((k1, k2, k3), v in @* { ... } # Loop over all variables (k1 is bound to basename)
That's confusing in the abstract, so a concrete example is in order. Suppose the out-of-stream variable @myvar
is populated as follows:
mlr -n put --jknquoteint -q ' begin { @myvar = { 1: 2, 3: { 4 : 5 }, 6: { 7: { 8: 9 } } } } end { dump } '
{ "myvar": { "1": 2, "3": { "4": 5 }, "6": { "7": { "8": 9 } } } }
Then we can get at various values as follows:
mlr -n put --jknquoteint -q ' begin { @myvar = { 1: 2, 3: { 4 : 5 }, 6: { 7: { 8: 9 } } } } end { for (k, v in @myvar) { print "key=" . k . ",valuetype=" . typeof(v); } } '
key=1,valuetype=int key=3,valuetype=map key=6,valuetype=map
mlr -n put --jknquoteint -q ' begin { @myvar = { 1: 2, 3: { 4 : 5 }, 6: { 7: { 8: 9 } } } } end { for ((k1, k2), v in @myvar) { print "key1=" . k1 . ",key2=" . k2 . ",valuetype=" . typeof(v); } } '
key1=3,key2=4,valuetype=int key1=6,key2=7,valuetype=map
mlr -n put --jknquoteint -q ' begin { @myvar = { 1: 2, 3: { 4 : 5 }, 6: { 7: { 8: 9 } } } } end { for ((k1, k2), v in @myvar[6]) { print "key1=" . k1 . ",key2=" . k2 . ",valuetype=" . typeof(v); } } '
key1=7,key2=8,valuetype=int
C-style triple-for loops¶
These are supported as follows:
mlr --from data/small --opprint put ' num suma = 0; for (a = 1; a <= NR; a += 1) { suma += a; } $suma = suma; '
a b i x y suma pan pan 1 0.346791 0.726802 1 eks pan 2 0.758679 0.522151 3 wye wye 3 0.204603 0.338318 6 eks wye 4 0.381399 0.134188 10 wye pan 5 0.573288 0.863624 15
mlr --from data/small --opprint put ' num suma = 0; num sumb = 0; for (num a = 1, num b = 1; a <= NR; a += 1, b *= 2) { suma += a; sumb += b; } $suma = suma; $sumb = sumb; '
a b i x y suma sumb pan pan 1 0.346791 0.726802 1 1 eks pan 2 0.758679 0.522151 3 3 wye wye 3 0.204603 0.338318 6 7 eks wye 4 0.381399 0.134188 10 15 wye pan 5 0.573288 0.863624 15 31
Notes:
-
In
for (start; continuation; update) { body }
, the start, continuation, and update statements may be empty, single statements, or multiple comma-separated statements. If the continuation is empty (e.g.for(i=1;;i+=1)
) it defaults to true. -
In particular, you may use
$
-variables and/or@
-variables in the start, continuation, and/or update steps (as well as the body, of course). -
The typedecls such as
int
ornum
are optional. If a typedecl is provided (for a local variable), it binds a variable scoped to the for-loop regardless of whether a same-name variable is present in outer scope. If a typedecl is not provided, then the variable is scoped to the for-loop if no same-name variable is present in outer scope, or if a same-name variable is present in outer scope then it is modified. -
Miller has no
++
or--
operators. -
As with all
for
/if
/while
statements in Miller, the curly braces are required even if the body is a single statement, or empty.
Begin/end blocks¶
Miller supports an awk
-like begin/end
syntax. The statements in the begin
block are executed before any input records are read; the statements in the end
block are executed after the last input record is read. (If you want to execute some statement at the start of each file, not at the start of the first file as with begin
, you might use a pattern/action block of the form FNR == 1 { ... }
.) All statements outside of begin
or end
are, of course, executed on every input record. Semicolons separate statements inside or outside of begin/end blocks; semicolons are required between begin/end block bodies and any subsequent statement. For example:
mlr put ' begin { @sum = 0 }; @x_sum += $x; end { emit @x_sum } ' ./data/small
a=pan,b=pan,i=1,x=0.346791,y=0.726802 a=eks,b=pan,i=2,x=0.758679,y=0.522151 a=wye,b=wye,i=3,x=0.204603,y=0.338318 a=eks,b=wye,i=4,x=0.381399,y=0.134188 a=wye,b=pan,i=5,x=0.573288,y=0.863624 x_sum=2.26476
Since uninitialized out-of-stream variables default to 0 for addition/subtraction and 1 for multiplication when they appear on expression right-hand sides (not quite as in awk
, where they'd default to 0 either way), the above can be written more succinctly as
mlr put ' @x_sum += $x; end { emit @x_sum } ' ./data/small
a=pan,b=pan,i=1,x=0.346791,y=0.726802 a=eks,b=pan,i=2,x=0.758679,y=0.522151 a=wye,b=wye,i=3,x=0.204603,y=0.338318 a=eks,b=wye,i=4,x=0.381399,y=0.134188 a=wye,b=pan,i=5,x=0.573288,y=0.863624 x_sum=2.26476
The put -q option suppresses printing of each output record, with only emit
statements being output. So to get only summary outputs, you could write
mlr put -q ' @x_sum += $x; end { emit @x_sum } ' ./data/small
x_sum=2.26476
We can do similarly with multiple out-of-stream variables:
mlr put -q ' @x_count += 1; @x_sum += $x; end { emit @x_count; emit @x_sum; } ' ./data/small
x_count=5 x_sum=2.26476
This is of course (see also here) not much different than
mlr stats1 -a count,sum -f x ./data/small
x_count=5,x_sum=2.26476
Note that it's a syntax error for begin/end blocks to refer to field names (beginning with $
), since begin/end blocks execute outside the context of input records.