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Working With Arrays

Examples using arrays within Murex

Creating Arrays

Arrays can be defined with %[ ... ].

The syntax is a superset of JSON. So that any JSON array can be a Murex array when prefixed with %. For example

%["Monday", "Tuesday", "Wednesday", "Thursday", "Friday"]

…however while this is readable, this is not convenient when working in the interactive command line. We want something that’s a little more comfortable for “write many, read once” type environments like a shell REPL.

So Murex makes the parser-defined punctuation optional:

%[Monday Tuesday Wednesday Thursday Friday]

That’s better, however surely the computer already knows what days there are in a week? I think we can improve this syntax further…

%[Monday..Friday]

The .. describes a range. So we are saying “return everyday from Monday to Friday, inclusive”.

» %[Monday..Friday]
[
    "Monday",
    "Tuesday",
    "Wednesday",
    "Thursday",
    "Friday"
]

It’s not just days of the week that can be completed like this. Most forms of dates, number bases and other familiar sequences can be.

You can also have multiple parts of arrays expanded:

» %[[2024..2026]\\, [spring..winter]]
[
    "2024, spring",
    "2024, summer",
    "2024, autumn",
    "2024, winter",
    "2025, spring",
    "2025, summer",
    "2025, autumn",
    "2025, winter",
    "2026, spring",
    "2026, summer",
    "2026, autumn",
    "2026, winter"
]

Did you notice how the seasons are lower case this time? Murex respects the text case of the ranges. For example:

» %[MON..WED]
[
    "MON",
    "TUE",
    "WED",
]

» %[mon..wed]
[
    "mon",
    "tue",
    "wed",
]

Returning to our multi-part array, perhaps you want fall instead of autumn? (here you do need to include a comma)

» %[[2024..2026]\\, [spring,summer,fall,winter]]
[
    "2024, spring",
    "2024, summer",
    "2024, fall",
    "2024, winter",
    "2025, spring",
    "2025, summer",
    "2025, fall",
    "2025, winter",
    "2026, spring",
    "2026, summer",
    "2026, fall",
    "2026, winter"
]

Multi-Dimensional Arrays

But what if they were supposed to be nested rather than flattened arrays? Well that’s not a problem either. Just make sure your first nested array is also prefixed with %:

» %[%[2024..2026] [Spring..Winter]]
[
    [
        2024,
        2025,
        2026
    ],
    [
        "Spring",
        "Summer",
        "Autumn",
        "Winter"
    ]
]

Streaming Arrays

Ok, that’s great, but marshalling a data structure and passing it to functions requires allocating that entire data structure to memory. Whereas shells best excel when they’re streaming data because it allows processes to perform concurrent operations across massive data sets.

If that’s a problem for you too, then Murex has you sorted: a.

The a builtin returns a line separated list. And thus can be operated on via your traditional pipes and UNIX core utilities.

Lets say, for some reason, you wanted to run a job against every IPv4 address available. Marshalling that into a data structure just to run commands sequentially would be rather silly. So lets stream that array using a:

a [0..255].[0..255].[0..255].[0..255] | foreach $ip { ping -c 1 -t 1 $ip }

If you do happen to run this and wonder how to cancel the underlying a and foreach routines, you can press ctrl+\ – which is a Murex shortcut to kill everything in that shell session.

This is obviously an absurd example because nobody in their right mind would want to ping every valid IPv4 address. But it does demonstrate the advantages of streaming lists rather than creating arrays.

Accessing Array Values

There are two main ways to access values inside an array:

square brackets for immutable copies: $my_array[index]
dot notation, which allows being written to: $my_array.index

Why two? Because they support different features.

Square Brackets (immutable)

With square brackets you can select more than just a single element. For example, if you wanted the first element you can reference it the same way you’d reference arrays in any other language:

$my_array[0]

Murex arrays begin at 0.

If you wanted to count from the end of the array, you can use negative values:

$my_array[-1]

Watch out here because negative indexes count from 1 because -0 isn’t a valid number.

Multiple Elements

However what if you wanted multiple elements from the array, like 2nd and 4th? Then just specify multiple elements inside the square brackets:

$my_array[1 4]

Ranges

That’s handy, but if I actually want a range of elements? Well then you can use the range .. operator like before:

$my_array[1..4]

And if you don’t know the size of your array, you can ignore the index value entirely. For example:

Everything from and including the 2nd element:

$my_array[2..]

Everything up to and including the 4th element:

$my_array[..4]

Ranges are indexed from 1. Yes, I know that’s stupid and confusing.

Elements From A Pipeline

The square brackets can also be used as a function too. Which means any kind of array or list can be queried from stdin, you don’t have to first convert it to a variable.

» %[mon..fri] | [1 4]
[
    "tue",
    "fri"
]

This is especially helpful if say something writes JSON, YAML, or any other structured document, and you only want specific values. For example you want the last container in your cloud infrastructure, and you know your cloud CLI tool (cloud-api for our made up purposes here) returns JSON:

cloud-api list-containers | :json: [-1]

Making Changes

That’s all great, but what if I want to make a change to the host array?

Well this is where dot notation comes in…

Dot Notation (mutable)

Dot notation is a lot more limited in what you can do because it’s designed for making careful edits of the underlying data structure. So it can only be used with variables.

Assignment

You can edit an element, for example renaming Wednesday to Humpday:

» $days = %[Monday..Friday]

» $days.2 = "Humpday"

» $days
[
    "Monday",
    "Tuesday",
    "Humpday",
    "Thursday",
    "Friday"
]

Remember: arrays are zero based

Printing

You can also use dot notation to return a value, just like you would with the square braces solution above. But dot notation doesn’t support any special magic and still only works on variables:

$my_array.2

Appending Arrays

There are several ways to append an array. You can create a copy of that array and append via the pipeline:

$my_array | append foo bar

You can also prepend using prepend)

…or…

$my_array ~> %[foo bar]

However if you want to update a variable in place, you can use the merge operator, <~:

$my_array <~ %[foo bar]

Fin

Now you’re an expert in arrays. Hooray \o/

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