# 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](record-heterogeneity.md)). 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](reference-dsl-variables.md#built-in-variables) 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](operating-on-all-records.md) 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](reference-main-maps.md), 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](reference-main-arrays.md), 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](reference-main-maps.md), the first loop variable is the key and the
second is the value; for [arrays](reference-main-arrays.md), 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 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` or `num` 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/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=0.346791,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 += $x;
end {
emit @x_count;
emit @x_sum;
}
' ./data/small
x_count=5
x_sum=2.26476
This is of course (see also [here](reference-dsl.md#verbs-compared-to-dsl)) not much different than
mlr stats1 -a count,sum -f x ./data/small
x_count=5,x_sum=2.26476
Note that it's a syntax error for begin/end blocks to refer to field names (beginning with `$`), since begin/end blocks execute outside the context of input records.