DSL reference: 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"; $c = "right_\2"
}' \
data/put-gating-example-2.dkvp
a=abc_123,b=left_\1,c=right_\2
a=some other name
a=xyz_789,b=left_\1,c=right_\2
This produces heteregenous output which Miller, of course, has no problems with (see Record-heterogeneity). But if you want homogeneous output, the curly braces can be replaced with a semicolon between the expression and the body statements. This causes put to evaluate the boolean expression (along with any side effects, namely, regex-captures \1, \2, etc.) but doesn’t use it as a criterion for whether subsequent assignments should be executed. Instead, subsequent assignments are done unconditionally:
mlr put '$x > 0.0; $y = log10($x); $z = sqrt($y)' data/put-gating-example-1.dkvp
x=1,y=0,z=0
x=2,y=0.3010299956639812,z=0.5486620049392715
x=3,y=0.4771212547196624,z=0.6907396432228734
mlr put '
$a =~ "([a-z]+)_([0-9]+)";
$b = "left_\1";
$c = "right_\2"
' data/put-gating-example-2.dkvp
a=abc_123,b=left_\1,c=right_\2
a=xyz_789,b=left_\1,c=right_\2
If-statements¶
These are again reminiscent of awk. Pattern-action blocks are a special case of if with no elif or else blocks, no if keyword, and parentheses optional around the boolean expression:
mlr put 'NR == 4 {$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 as NR is only incremented between records.
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.
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.
Key-only for-loops¶
The key variable is always bound to the key of key-value pairs:
mlr --from data/small put '
print "NR = ".NR;
for (key in $*) {
value = $[key];
print " key:" . key . " value:".value;
}
'
NR = 1
key:a value:pan
key:b value:pan
key:i value:1
key:x value:0.3467901443380824
key:y value:0.7268028627434533
a=pan,b=pan,i=1,x=0.3467901443380824,y=0.7268028627434533
NR = 2
key:a value:eks
key:b value:pan
key:i value:2
key:x value:0.7586799647899636
key:y value:0.5221511083334797
a=eks,b=pan,i=2,x=0.7586799647899636,y=0.5221511083334797
NR = 3
key:a value:wye
key:b value:wye
key:i value:3
key:x value:0.20460330576630303
key:y value:0.33831852551664776
a=wye,b=wye,i=3,x=0.20460330576630303,y=0.33831852551664776
NR = 4
key:a value:eks
key:b value:wye
key:i value:4
key:x value:0.38139939387114097
key:y value:0.13418874328430463
a=eks,b=wye,i=4,x=0.38139939387114097,y=0.13418874328430463
NR = 5
key:a value:wye
key:b value:pan
key:i value:5
key:x value:0.5732889198020006
key:y value:0.8636244699032729
a=wye,b=pan,i=5,x=0.5732889198020006,y=0.8636244699032729
mlr -n put '
end {
o = {1:2, 3:{4:5}};
for (key in o) {
print " key:" . key . " valuetype:" . typeof(o[key]);
}
}
'
key:1 valuetype:int
key:3 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 $[key] for stream records, or by indexing the looped-over variable using square brackets.
Key-value for-loops¶
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 to a scalar value (a string or a number) if the map stops at that level, or to a map-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 0.3467901443380824 0.7268028627434533 string string int float float
eks pan 2 0.7586799647899636 0.5221511083334797 string string int float float
wye wye 3 0.20460330576630303 0.33831852551664776 string string int float float
eks wye 4 0.38139939387114097 0.13418874328430463 string string int float float
wye pan 5 0.5732889198020006 0.8636244699032729 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 0.3467901443380824 0.7268028627434533 2.0735930070815356 8.294372028326142
eks pan 2 0.7586799647899636 0.5221511083334797 3.280831073123443 13.123324292493772
wye wye 3 0.20460330576630303 0.33831852551664776 3.5429218312829507 14.171687325131803
eks wye 4 0.38139939387114097 0.13418874328430463 4.515588137155445 18.06235254862178
wye pan 5 0.5732889198020006 0.8636244699032729 6.436913389705273 25.747653558821092
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 0.3467901443380824 0.7268028627434533 2.0735930070815356
eks pan 2 0.7586799647899636 0.5221511083334797 3.280831073123443
wye wye 3 0.20460330576630303 0.33831852551664776 3.5429218312829507
eks wye 4 0.38139939387114097 0.13418874328430463 4.515588137155445
wye pan 5 0.5732889198020006 0.8636244699032729 6.436913389705273
You can also start iterating on sub-hashmaps 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-hashmap as it was before the loop started. The sub-hashmap 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 0.3467901443380824 0.7268028627434533 1
eks pan 2 0.7586799647899636 0.5221511083334797 3
wye wye 3 0.20460330576630303 0.33831852551664776 6
eks wye 4 0.38139939387114097 0.13418874328430463 10
wye pan 5 0.5732889198020006 0.8636244699032729 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 0.3467901443380824 0.7268028627434533 1 1
eks pan 2 0.7586799647899636 0.5221511083334797 3 3
wye wye 3 0.20460330576630303 0.33831852551664776 6 7
eks wye 4 0.38139939387114097 0.13418874328430463 10 15
wye pan 5 0.5732889198020006 0.8636244699032729 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
intornumare 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=0.3467901443380824,y=0.7268028627434533
a=eks,b=pan,i=2,x=0.7586799647899636,y=0.5221511083334797
a=wye,b=wye,i=3,x=0.20460330576630303,y=0.33831852551664776
a=eks,b=wye,i=4,x=0.38139939387114097,y=0.13418874328430463
a=wye,b=pan,i=5,x=0.5732889198020006,y=0.8636244699032729
a=zee,b=pan,i=6,x=0.5271261600918548,y=0.49322128674835697
a=eks,b=zee,i=7,x=0.6117840605678454,y=0.1878849191181694
a=zee,b=wye,i=8,x=0.5985540091064224,y=0.976181385699006
a=hat,b=wye,i=9,x=0.03144187646093577,y=0.7495507603507059
a=pan,b=wye,i=10,x=0.5026260055412137,y=0.9526183602969864
x_sum=4.536293840335763
Since uninitialized out-of-stream variables default to 0 for addition/substraction 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=0.3467901443380824,y=0.7268028627434533
a=eks,b=pan,i=2,x=0.7586799647899636,y=0.5221511083334797
a=wye,b=wye,i=3,x=0.20460330576630303,y=0.33831852551664776
a=eks,b=wye,i=4,x=0.38139939387114097,y=0.13418874328430463
a=wye,b=pan,i=5,x=0.5732889198020006,y=0.8636244699032729
a=zee,b=pan,i=6,x=0.5271261600918548,y=0.49322128674835697
a=eks,b=zee,i=7,x=0.6117840605678454,y=0.1878849191181694
a=zee,b=wye,i=8,x=0.5985540091064224,y=0.976181385699006
a=hat,b=wye,i=9,x=0.03144187646093577,y=0.7495507603507059
a=pan,b=wye,i=10,x=0.5026260055412137,y=0.9526183602969864
x_sum=4.536293840335763
The put -q option is a shorthand which suppresses printing of each output record, with only emit statements being output. So to get only summary outputs, one could write
mlr put -q '
@x_sum += $x;
end { emit @x_sum }
' ../data/small
x_sum=4.536293840335763
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=10
x_sum=4.536293840335763
This is of course not much different than
mlr stats1 -a count,sum -f x ../data/small
x_count=10,x_sum=4.536293840335763
Note that it’s a syntax error for begin/end blocks to refer to field names (beginning with $), since these execute outside the context of input records.