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Laurence MorganAbout 1 min


Iterate through a map or other collection of data


formap is a generic tool for iterating through a map, table or other sequences of data similarly like a foreach. In fact formap can even be used on array too.

Unlike foreach, formap's default output is str, so each new line will be treated as a list item. This behaviour will differ if any additional flags are used with foreach, such as --jmap.


formap writes a list:

<stdin> -> foreach variable { code-block } -> <stdout>

formap writes to a buffered JSON map:

<stdin> -> formap --jmap key value { code-block (map key) } { code-block (map value) } -> <stdout>


First of all lets assume the following dataset:

set json people={
    "Tom": {
        "Age": 32,
        "Gender": "Male"
    "Dick": {
        "Age": 43,
        "Gender": "Male"
    "Sally": {
        "Age": 54,
        "Gender": "Female"

We can create human output from this:

» $people -> formap key value { out "$key is $value[Age] years old" }
Sally is 54 years old
Tom is 32 years old
Dick is 43 years old

Please note that maps are intentionally unsorted so you cannot guarantee the order of the output produced even if the input has been superficially set in a specific order.

With --jmap we can turn that structure into a new structure:

» $people -> formap --jmap key value { $key } { $value[Age] }
    "Dick": "43",
    "Sally": "54",
    "Tom": "32"


  • --jmap Write a json map to STDOUT instead of an array


formap can also work against arrays and tables as well. However foreach is a much better tool for ordered lists and tables can look a little funky when when there are more than 2 columns. In those instances you're better off using [ (index) to specify columns and then tabulate for any data transformation.

Meta values

Meta values are a JSON object stored as the variable $.. The meta variable will get overwritten by any other block which invokes meta values. So if you wish to persist meta values across blocks you will need to reassign $., eg

%[1..3] -> foreach {
    meta_parent = $.
    %[7..9] -> foreach {
        out "$(meta_parent.i): $.i"

The following meta values are defined:

  • i: iteration number

See Also

  • [index]: Outputs an element from an array, map or table
  • break: Terminate execution of a block within your processes scope
  • for: A more familiar iteration loop to existing developers
  • foreach: Iterate through an array
  • json: JavaScript Object Notation (JSON)
  • set: Define a local variable and set it's value
  • tabulate: Table transformation tools
  • while: Loop until condition false

This document was generated from builtins/core/structs/formap_doc.yamlopen in new window.

Last update:
Contributors: Laurence Morgan,Laurence Morgan,Laurence Morgan