# Control Flow


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This chapter describes the control flow commands.

[TOC]

## if, elseif, else {#if}

Syntax:

```soy
{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:

```soy
{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:

```soy
{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:

```soy
{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:

```soy
{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:

```soy
{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](functions.md#range), for
example

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

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