DSL reference: unset statements

You can clear a map key by assigning the empty string as its value: $x="" or @x="". Using unset you can remove the key entirely. Examples:

 cat 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

 mlr put 'unset $x, $a' data/small
 b=pan,i=1,y=0.7268028627434533
 b=pan,i=2,y=0.5221511083334797
 b=wye,i=3,y=0.33831852551664776
 b=wye,i=4,y=0.13418874328430463
 b=pan,i=5,y=0.8636244699032729

This can also be done, of course, using mlr cut -x. You can also clear out-of-stream or local variables, at the base name level, or at an indexed sublevel:

 mlr put -q '@sum[$a][$b] += $x; end { dump; unset @sum; dump }' data/small
 {
   "sum": {
     "pan": {
       "pan": 0.3467901443380824
     },
     "eks": {
       "pan": 0.7586799647899636,
       "wye": 0.38139939387114097
     },
     "wye": {
       "wye": 0.20460330576630303,
       "pan": 0.5732889198020006
     }
   }
 }
 {}

 mlr put -q '@sum[$a][$b] += $x; end { dump; unset @sum["eks"]; dump }' data/small
 {
   "sum": {
     "pan": {
       "pan": 0.3467901443380824
     },
     "eks": {
       "pan": 0.7586799647899636,
       "wye": 0.38139939387114097
     },
     "wye": {
       "wye": 0.20460330576630303,
       "pan": 0.5732889198020006
     }
   }
 }
 {
   "sum": {
     "pan": {
       "pan": 0.3467901443380824
     },
     "wye": {
       "wye": 0.20460330576630303,
       "pan": 0.5732889198020006
     }
   }
 }

If you use unset all (or unset @* which is synonymous), that will unset all out-of-stream variables which have been defined up to that point.