control-flow.md

Control Flow

This chapter describes the control flow commands.

[TOC]

if, elseif, else {#if}

Syntax:

{if <expression>}
  ...
{elseif <expression>}
  ...
{else}
  ...
{/if}

Use these commands for conditional output. It works exactly like you'd expect!

As soon one expression evaluates to true, the compiler processes that sub-block and skips the rest. The else sub block is evaluated when none of the if and elseif conditions are true.

For example:

{if round($pi, 2) == 3.14}
  {$pi} is a good approximation of pi.
{elseif round($pi) == 3}
  {$pi} is a bad approximation of pi.
{else}
  {$pi} is nowhere near the value of pi.
{/if}

Example output (for pi = 2.71828):

2.71828 is a bad approximation of pi.

switch, case, default {#switch}

Syntax:

{switch <expression>}
  {case <expression_list>}
    ...
  {case <expression_list>}
    ...
  {default}
    ...
{/switch}

The switch command is used for conditional output based on the value of an expression, and works how you'd expect.

Each case can have one or more expressions (use a comma-separated list if you have more than one), and the case matches when any one of its expressions matches the value of the switch expression. {default} executes only when none of the expressions from the case statements match the switch expression.

For example:

{switch $numMarbles}
  {case 0}
    You have no marbles.
  {case 1, 2, 3}
    You have a normal number of marbles.
  {default}  // 4 or more
    You have more marbles than you know what to do with.
{/switch}

Example output (for numMarbles = 2):

You have a normal number of marbles.

for, ifempty {#for}

Syntax:

{for <local_var> in <data_ref>}
  ...
{ifempty}
  ...
{/for}

The for command iterates over a list. The iterator local_var is a local variable that is defined only in the block. Within the block, you can use three special functions that only take the iterator as their argument:

• isFirst($var) returns true only on the first iteration.
• isLast($var) returns true only on the last iteration.
• index($var) returns the current index in the list. List indices are 0-based.

The optional ifempty command is for a fallback section that is executed when the list is empty.

For example:

{for $operand in $operands}
  {if not isFirst($operand)} + {/if}
  {$operand}
{ifempty}
  0
{/for}

Example output (for operands = ['alpha', 'beta', 'gamma']):

alpha + beta + gamma

Indexed iteration {#for-indexed}

Occasionally, instead of iterating over a list, you may need to use a numerical loop. For this you can use the range(...) function, for example

{for $i in range(5)}
  {$i}{if not isLast($i)}, {/if}
{/for}

Example output: 0, 1, 2, 3, 4