Merge pull request #727 from KristinaRiemer/prob_decomp

Add code example to problem decomposition lecture

Author: ethanwhite

materials/decomposition-R.md

@@ -5,23 +5,118 @@ title: Problem Decomposition
 language: R
 ---
 
+### Approaches to solving problems
+
 * Real computational tasks are complicated. 
 * To accomplish them effectively you need to **think before you code**.
 
 * Decompose a coding problem:
-    1. Understand the problem.
-        * Identify inputs and outputs.
-    2. Break the problem down into a few pieces.
-        * Build an outline on paper or as comments.
-    3. Break those pieces into codeable chunks.
-    4. Code one chunk at a time.
+
+1. Understand the problem.
+    * Identify inputs and outputs.
+        * Input = Portal surveys csv
+        * Output = plot of yearly abundance for large and small rodents
+
+	```
+	# Overall goal: visualize how many large (>50g) and 
+	# small (<50g) mammals are collected each year
+	```
+
+2. Break the problem down into a few pieces.
+    * Bullet list on paper or comments in script. 
+
+	```
+	# Subtasks
+	    # Import data
+	    # Get column of small and large categories
+	    # Count number of each per year
+	    # Plot this
+	```
+
+3. Break those pieces into codeable chunks.
+
+	```
+	# TODO: function that returns size class from weight
+	```
+
+4. Code one chunk at a time.
+
+### Coding one chunk
+
+* Write function to return size classes
+
+```
+get_size_class()
+```
+
+* Start with if statement
+* Pseudocode
+    * Write code in English
+    * “If weight value is bigger than 50, return large; otherwise return small”
+
+```
+if(weight > 50){
+  size_class <- "large"
+} else {
+  size_class <- "small"
+}
+```
+
+* Test code using values with known output
+
+```
+weight <- 10
+```
+
+* Turn into function
+
+```
+get_size_class <- function(weight){
+  if(weight > 50){
+    size_class <- "large"
+  } else {
+    size_class <- "small"
+  }
+}
+get_size_class(100)
+```
+
+* Why doesn’t this work? 
+
+```
+get_size_class <- function(weight){
+  if(weight > 50){
+    size_class <- "large"
+  } else {
+    size_class <- "small"
+  }
+  return(size_class)
+}
+```
+
+* Generalize function
+* Easier to change values later
+* Always test after changing
+
+```
+get_size_class <- function(weight, threshold){
+  if(weight > threshold){
+    size_class <- "large"
+  } else {
+    size_class <- "small"
+  }
+  return(size_class)
+}
+get_size_class(100, 50)
+get_size_class(100, 150)
+```
+
+* Make one change at a time
 
 * Decompose a code chunk:
     1. Think about it.
     2. Write it.
     3. Test it ( *on it's own* ).
     4. Fix any problems.
 
-> Demo [decomposition-example.R]({{ site.baseurl }}/materials/decomposition-example.R).
-
-> Assign remaining exercises.
+> Rest of code in [decomposition-example.R]({{ site.baseurl }}/materials/decomposition-example.R).