switch.md

Switch Conditional (switch)

Blocks of cascading conditionals

Description

switch is a large block for simplifying cascades of conditional statements.

Usage

switch [value] {
  case | if { conditional } [then] { code-block }
  case | if { conditional } [then] { code-block }
  ...
  [ default { code-block } ]
} -> <stdout>

The first parameter should be either case or if -- the statements are subtly different and thus alter the behavior of switch.

then is optional ('then' is assumed even if not explicitly present).

Examples

Output an array of editors installed:

switch {
    if { which vi    } { out vi    }
    if { which vim   } { out vim   }
    if { which nano  } { out nano  }
    if { which emacs } { out emacs }
} -> format: json

A higher/lower game written using switch:

function higherlower {
  try {
    rand int 100 -> set rand
    while { $rand } {
      read guess "Guess a number between 1 and 100: "

      switch {
        case: { = $guess < $rand } then {
          out "Too low"
        }

        case: { = $guess > $rand } then {
          out "Too high"
        }

        default: {
          out "Correct"
          let rand=0
        }
      }
    }
  }
}

String matching with switch:

read name "What is your name? "
switch $name {
    case "Tom"   { out "I have a brother called Tom" }
    case "Dick"  { out "I have an uncle called Dick" }
    case "Sally" { out "I have a sister called Sally" }
    default      { err "That is an odd name" }
}

Detail

Comparing Values vs Boolean State

By Values

If you supply a value with switch...

switch value { ... }

...then all the conditionals are compared against that value. For example:

switch foo {
    case bar {
        # not executed because foo != bar
    }
    case foo {
        # executed because foo != foo
    }
}

You can use code blocks to return strings too

switch foo {
    case {out bar} then {
        # not executed because foo != bar
    }
    case {out foo} then {
        # executed because foo != foo
    }
}

By Boolean State

This style of syntax could be argued as a prettier counterpart to if/else if. Only code blocks are support and each block is checked for its boolean state rather than string matching.

This is simply written as:

switch { ... }

When To Use case, if and default?

A switch command may contain multiple case and if blocks. These statements subtly alter the behavior of switch. You can mix and match if and case statements within the same switch block.

case

A case statement will only move on to the next statement if the result of the case statement is false. If a case statement is true then switch will exit with an exit number of 0.

switch {
    case { false } then {
        # ignored because case == false
    }
    case { true } then {
        # executed because case == true
    }
    case { true } then {
        # ignored because a previous case was true
    }
}

if

An if statement will proceed to the next statement even if the result of the if statement is true.

switch {
    if { false } then {
        # ignored because if == false
    }
    if { true } then {
        # executed because if == true
    }
    if { true } then {
        # executed because if == true
    }
}

default

default statements are only run if all case and if statements are false.

switch {
    if { false } then {
        # ignored because if == false
    }
    if { true } then {
        # executed because if == true
    }
    if { true } then {
        # executed because if == true
    }
    if { false } then {
        # ignored because if == false
    }
    default {
        # ignored because one or more previous if's were true
    }
}

default was added in Murex version 3.1

catch

catch has been deprecated in version 3.1 and replaced with default.

See Also

• Caught Error Block (catch): Handles the exception code raised by try or trypipe
• Define Variable (set): Define a variable (typically local) and set it's value
• Exit Block (break): Terminate execution of a block within your processes scope
• False (false): Returns a false value
• If Conditional (if): Conditional statement to execute different blocks of code depending on the result of the condition
• Logic And Statements (and): Returns true or false depending on whether multiple conditions are met
• Logic Or Statements (or): Returns true or false depending on whether one code-block out of multiple ones supplied is successful or unsuccessful.
• Loop While (while): Loop until condition false
• Not (!): Reads the stdin and exit number from previous process and not's it's condition
• Pipe Fail (trypipe): Checks for non-zero exits of each function in a pipeline
• True (true): Returns a true value
• Try Block (try): Handles non-zero exits inside a block of code
• let: Evaluate a mathematical function and assign to variable (deprecated)

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This document was generated from builtins/core/structs/switch_doc.yaml.