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website: update docs for precedence rules

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Mitchell Hashimoto 2016-11-15 15:29:38 -08:00
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website/source/docs/configuration/interpolation.html.md
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@ -381,19 +381,21 @@ The supported operations are:
- *Add* (`+`), *Subtract* (`-`), *Multiply* (`*`), and *Divide* (`/`) for **float** types
- *Add* (`+`), *Subtract* (`-`), *Multiply* (`*`), *Divide* (`/`), and *Modulo* (`%`) for **integer** types
Operator precedences is the standard mathematical order of operations:
*Multiply* (`*`), *Divide* (`/`), and *Modulo* (`%`) have precedence over
*Add* (`+`) and *Subtract* (`-`). Parenthesis can be used to force ordering.
```
"${2 * 4 + 3 * 3}" # computes to 17
"${3 * 3 + 2 * 4}" # computes to 17
"${2 * (4 + 3) * 3}" # computes to 42
```
You can use the [terraform console](/docs/commands/console.html) command to
try the math operations.
-> **Note:** Since Terraform allows hyphens in resource and variable names,
it's best to use spaces between math operators to prevent confusion or unexpected
behavior. For example, `${var.instance-count - 1}` will subtract **1** from the
`instance-count` variable value, while `${var.instance-count-1}` will interpolate
the `instance-count-1` variable value.
-> **Note:** Operator precedence is not the usual one where *Multiply* (`*`),
*Divide* (`/`), and *Modulo* (`%`) have precedence over *Add* (`+`) and *Subtract* (`-`).
The operations are made in the order they appear. Parenthesis can be used to force ordering :
```
"${2 * 4 + 3 * 3}" # computes to 33
"${3 * 3 + 2 * 4}" # computes to 44
"${(2 * 4) + (3 * 3)}" # computes to 17
"${(3 * 3) + (2 * 4)}" # computes to 17
```