Merge pull request #2933 from hashicorp/docs-math-interpolation

More details about math in interpolation

CHANGELOG.md

@@ -31,7 +31,7 @@ IMPROVEMENTS:
   * provider/aws: Compute private ip addresses of ENIs if they are not specified [GH-2743]
   * provider/aws: Add `arn` attribute for DynamoDB tables [GH-2924]
   * provider/azure: Allow `settings_file` to accept XML string [GH-2922]
-  * provider/azure: Provide a simpler error when using a Platform Image without a 
+  * provider/azure: Provide a simpler error when using a Platform Image without a
       Storage Service [GH-2861]
   * provider/google: `account_file` is now expected to be JSON. Paths are still supported for
       backwards compatibility. [GH-2839]
@@ -573,7 +573,7 @@ FEATURES:
       less likely to become corrupt in a catastrophic case: terraform panic
       or system killing Terraform.
   * **Math operations** in interpolations. You can now do things like
-      `${count.index+1}`. [GH-1068]
+      `${count.index + 1}`. [GH-1068]
   * **New AWS SDK:** Move to `aws-sdk-go` (hashicorp/aws-sdk-go),
       a fork of the official `awslabs` repo. We forked for stability while
       `awslabs` refactored the library, and will move back to the officially

website/source/docs/configuration/interpolation.html.md

@@ -17,7 +17,7 @@ The interpolation syntax is powerful and allows you to reference
 variables, attributes of resources, call functions, etc.
 
 You can also perform simple math in interpolations, allowing
-you to write expressions such as `${count.index+1}`.
+you to write expressions such as `${count.index + 1}`.
 
 You can escape interpolation with double dollar signs: `$${foo}`
 will be rendered as a literal `${foo}`.
@@ -92,7 +92,7 @@ The supported built-in functions are:
       format. The syntax for the format is standard `sprintf` syntax.
       Good documentation for the syntax can be [found here](http://golang.org/pkg/fmt/).
       Example to zero-prefix a count, used commonly for naming servers:
-      `format("web-%03d", count.index+1)`.
+      `format("web-%03d", count.index + 1)`.
 
   * `formatlist(format, args...)` - Formats each element of a list
       according to the given format, similarly to `format`, and returns a list.
@@ -201,3 +201,34 @@ resource "aws_instance" "web" {
 
 With this, we will build a list of `template_file.web_init` resources which we can
 use in combination with our list of `aws_instance.web` resources.
+
+## Math
+
+Simple math can be performed in interpolations:
+
+```
+variable "count" {
+  default = 2
+}
+
+resource "aws_instance" "web" {
+  // ...
+  count = "${var.count}"
+
+  // tag the instance with a counter starting at 1, ie. web-001
+  tags {
+    Name = "${format("web-%03d", count.index + 1)}"
+  }
+}
+```
+
+The supported operations are:
+
+- *Add*, *Subtract*, *Multiply*, and *Divide* for **float** types
+- *Add*, *Subtract*, *Multiply*, *Divide*, and *Modulo* for **integer** types
+
+-> **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.