Add nested maps example

Author: RulerOf

traversing-nested-maps/.tool-versions

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+terraform 1.0.5

traversing-nested-maps/README.md

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+# Traversing Nested Maps
+
+Terraform [used to use splat expressions](https://www.terraform.io/docs/language/expressions/splat.html#splat-expressions-with-maps) for this stuff (and I wish they still worked), but now this is all done with [`for` expressions](https://www.terraform.io/docs/language/expressions/for.html).
+
+Using [your example code as a guide](https://github.com/RulerOf/terraform-code-examples/traversing-nested-maps/main.tf), it's pretty straightforward to select the structure of the map where the `address_space` lists are the first element down, like this:
+
+    > local.subscriptions.MySecondSubcription.VirtualNetworks
+    {
+      "vNet1" = {
+        "address_space" = [
+          "10.20.0.0/24",
+          "192.168.3.0/24",
+        ]
+      }
+      "vNet2" = {
+        "address_space" = [
+          "10.30.0.0/24",
+          "192.168.4.0/24",
+        ]
+      }
+    }
+
+Now, using the `for` expression, we can cherry pick the `address_space` into a list by iterating over that level of the map like this:
+
+    > [ for net in local.subscriptions.MySecondSubcription.VirtualNetworks : net.address_space ]
+    [
+      [
+        "10.20.0.0/24",
+        "192.168.3.0/24",
+      ],
+      [
+        "10.30.0.0/24",
+        "192.168.4.0/24",
+      ],
+    ]
+
+If the subdivided lists aren't helpful, you can [`flatten()`](https://www.terraform.io/docs/language/functions/flatten.html) the output:
+
+    > flatten([ for net in local.subscriptions.MySecondSubcription.VirtualNetworks : net.address_space ])
+    [
+      "10.20.0.0/24",
+      "192.168.3.0/24",
+      "10.30.0.0/24",
+      "192.168.4.0/24",
+    ]
+
+### Credits
+
+Inspired by the question in [this Reddit thread](https://www.reddit.com/r/Terraform/comments/pms32s/deeply_nested_maps_help/).

traversing-nested-maps/main.tf

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+locals {
+  subscriptions = {
+    "MyFirstSubscription" = {
+      "VirtualNetworks" = {
+        "vNet1" = {
+          "address_space" = [
+            "10.0.0.0/24",
+            "192.168.0.0/24",
+          ]
+        }
+        "vNet2" = {
+          "address_space" = [
+            "10.10.0.0/24",
+            "192.168.1.0/24",
+          ]
+        }
+      }
+    }
+    "MySecondSubcription" = {
+      "VirtualNetworks" = {
+        "vNet1" = {
+          "address_space" = [
+            "10.20.0.0/24",
+            "192.168.3.0/24",
+          ]
+        }
+        "vNet2" = {
+          "address_space" = [
+            "10.30.0.0/24",
+            "192.168.4.0/24",
+          ]
+        }
+      }
+    }
+  }
+}