# _murex_ Shell Docs

## Command Reference: `for`

> A more familiar iteration loop to existing developers

## Description

This `for` loop is fills a small niche where `foreach` or `formap` are
inappropiate in your script. It's generally not recommended to use `for`
because it performs slower and doesn't adhere to _murex_'s design
philosophy. However it does offer additional flexibility around recursion. 

## Usage

    for ( variable; conditional; incrementation ) { code-block } -> <stdout>

## Examples

    » for ( i=1; i<6; i++ ) { echo $i }
    1
    2
    3
    4
    5

## Detail

### Syntax

`for` is a little naughty in terms of breaking _murex_'s style guidelines due
to the first parameter being entered as one string treated as 3 separate code
blocks. The syntax is like this for two reasons:
  
1. readability (having multiple `{ blocks }` would make scripts unsightly
2. familiarity (for those using to `for` loops in other languages

The first parameter is: `( i=1; i<6; i++ )`, but it is then converted into the
following code:

1. `let i=0` - declare the loop iteration variable
2. `= i<0` - if the condition is true then proceed to run the code in
the second parameter - `{ echo $i }`
3. `let i++` - increment the loop iteration variable

The second parameter is the code to execute upon each iteration

### Better `for` loops

Because each iteration of a `for` loop reruns the 2nd 2 parts in the first
parameter (the conditional and incrementation), `for` is very slow. Plus the
weird, non-idiomatic, way of writing the 3 parts, it's fair to say `for` is
not the recommended method of iteration and in fact there are better functions
to achieve the same thing...most of the time at least.

For example:

    a: [1..5] -> foreach: i { echo $i }
    1
    2
    3
    4
    5
    
The different in performance can be measured. eg:

    » time { a: [1..9999] -> foreach: i { out: <null> $i } }
    0.097643108
    
    » time { for ( i=1; i<10000; i=i+1 ) { out: <null> $i } }
    0.663812496
    
You can also do step ranges with `foreach`:

    » time { for ( i=10; i<10001; i=i+2 ) { out: <null> $i } }
    0.346254973
    
    » time { a: [1..999][0,2,4,6,8],10000 -> foreach i { out: <null> $i } }
    0.053924326
    
...though granted the latter is a little less readable.

The big catch with using `a` piped into `foreach` is that values are passed
as strings rather than numbers.

### Tips when writing JSON inside for loops

One of the drawbacks (or maybe advantages, depending on your perspective) of
JSON is that parsers generally expect a complete file for processing in that
the JSON specification requires closing tags for every opening tag. This means
it's not always suitable for streaming. For example

    » ja [1..3] -> foreach i { out ({ "$i": $i }) }
    { "1": 1 }
    { "2": 2 }
    { "3": 3 }
    
**What does this even mean and how can you build a JSON file up sequentially?**

One answer if to write the output in a streaming file format and convert back
to JSON

    » ja [1..3] -> foreach i { out (- "$i": $i) }
    - "1": 1
    - "2": 2
    - "3": 3
    
    » ja [1..3] -> foreach i { out (- "$i": $i) } -> cast yaml -> format json
    [
        {
            "1": 1
        },
        {
            "2": 2
        },
        {
            "3": 3
        }
    ]
    
**What if I'm returning an object rather than writing one?**

The problem with building JSON structures from existing structures is that you
can quickly end up with invalid JSON due to the specifications strict use of
commas.

For example in the code below, each item block is it's own object and there are
no `[ ... ]` encapsulating them to denote it is an array of objects, nor are
the objects terminated by a comma.

    » config -> [ shell ] -> formap k v { $v -> alter /Foo Bar }
    {
        "Data-Type": "bool",
        "Default": true,
        "Description": "Display the interactive shell's hint text helper. Please note, even when this is disabled, it will still appear when used for regexp searches and other readline-specific functions",
        "Dynamic": false,
        "Foo": "Bar",
        "Global": true,
        "Value": true
    }
    {
        "Data-Type": "block",
        "Default": "{ progress $PID }",
        "Description": "Murex function to execute when an `exec` process is stopped",
        "Dynamic": false,
        "Foo": "Bar",
        "Global": true,
        "Value": "{ progress $PID }"
    }
    {
        "Data-Type": "bool",
        "Default": true,
        "Description": "ANSI escape sequences in Murex builtins to highlight syntax errors, history completions, {SGR} variables, etc",
        "Dynamic": false,
        "Foo": "Bar",
        "Global": true,
        "Value": true
    }
    ...
    
Luckily JSON also has it's own streaming format: JSON lines (`jsonl`). We can
`cast` this output as `jsonl` then `format` it back into valid JSON:

    » config -> [ shell ] -> formap k v { $v -> alter /Foo Bar } -> cast jsonl -> format json
    [
        {
            "Data-Type": "bool",
            "Default": true,
            "Description": "Write shell history (interactive shell) to disk",
            "Dynamic": false,
            "Foo": "Bar",
            "Global": true,
            "Value": true
        },
        {
            "Data-Type": "int",
            "Default": 4,
            "Description": "Maximum number of lines with auto-completion suggestions to display",
            "Dynamic": false,
            "Foo": "Bar",
            "Global": true,
            "Value": "6"
        },
        {
            "Data-Type": "bool",
            "Default": true,
            "Description": "Display some status information about the stop process when ctrl+z is pressed (conceptually similar to ctrl+t / SIGINFO on some BSDs)",
            "Dynamic": false,
            "Foo": "Bar",
            "Global": true,
            "Value": true
        },
    ...
    
#### `foreach` will automatically cast it's output as `jsonl` _if_ it's STDIN type is `json`

    » ja: [Tom,Dick,Sally] -> foreach: name { out Hello $name }
    Hello Tom
    Hello Dick
    Hello Sally
    
    » ja [Tom,Dick,Sally] -> foreach name { out Hello $name } -> debug -> [[ /Data-Type/Murex ]]
    jsonl
    
    » ja: [Tom,Dick,Sally] -> foreach: name { out Hello $name } -> format: json
    [
        "Hello Tom",
        "Hello Dick",
        "Hello Sally"
    ]

## See Also

* [`a` (mkarray)](../commands/a.md):
  A sophisticated yet simple way to build an array or list
* [`break`](../commands/break.md):
  terminate execution of a block within your processes scope
* [`foreach`](../commands/foreach.md):
  Iterate through an array
* [`formap`](../commands/formap.md):
  Iterate through a map or other collection of data
* [`if`](../commands/if.md):
  Conditional statement to execute different blocks of code depending on the result of the condition
* [`ja` (mkarray)](../commands/ja.md):
  A sophisticated yet simply way to build a JSON array
* [`let`](../commands/let.md):
  Evaluate a mathematical function and assign to variable (deprecated)
* [`set`](../commands/set.md):
  Define a local variable and set it's value
* [`while`](../commands/while.md):
  Loop until condition false