Differences

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--- rworkshop8 [2014/11/03 03:38]
johannabradie
+++ rworkshop8 [2014/11/03 16:18] (current)
johannabradie
@@ Line 62: / Line 62: @@
 </code>
-Curly brackets { } are used so that R knows to expect more input.  When using brackets, R waits to evaluate the command until the brackets have been closed.
+Curly brackets { } are used so that R knows to expect more input.  When using brackets, R waits to evaluate the command until the brackets have been closed.  If the curly brackets are not used, R may not behave as you are expecting.  For example, try:
-If the curly brackets are not used, R may not behave as you are expecting.  For example, try:
 <code rsplus>
@@ Line 82: / Line 80: @@
 }  # Since all brackets are now closed, R will evaluate the commands.
 </code>
+Note that if and if/else test a single condition.  If you want to test a vector of conditions (and get a vector of results), you can use ifelse:
+For example,
+<code rsplus>
+a<-1:10
+ifelse(a>5,"yes","no")
+</code>
+You can also use ifelse in a function to apply a function only under certain conditions:
+For example,
+<code rsplus>
+a<-(-4):5
+sqrt(ifelse(a>=0,a,NA))
+</code>
+**Exercise**
+<code rsplus>
+Paws<-"cat"
+Scruffy<-"dog"
+Sassy<-cat
+animals<-c(Paws,Scruffy,Sassy)
+</code>
+. Use an if statement to print "meow" if Paws is a "cat".
+<hidden>
+<code rsplus>
+if(Paws=="cat") {
+  print("meow")}
+</code>
+</hidden>
+. Use an if/else statement to print "woof" if you supply an object that is a "dog" and "meow" if it is not.  Try it out with Paws and Scruffy.
+<hidden>
+<code rsplus>
+if(Scruffy=="dog") {
+  print("woof")
+} else {
+  print ("meow")
+}
+if(Paws=="dog") {
+  print("woof")
+} else {
+  print ("meow")
+}
+</code>
+</hidden>
+. Use an ifelse statement to display "woof" for animals that are dogs and "meow" for animals that are cats.
+<hidden>
+<code rsplus>
+ifelse(animals=="dog","woof","meow")
+</code>
+</hidden>
 ==== Loops ====
@@ Line 147: / Line 208: @@
 }
 </file>
 The expression part of the loop can be almost anything and is usually a compound statement containing many commands.
@@ Line 162: / Line 222: @@
 //while loops// and //repeat loops// operate similarly to //for loops//.  Once you understand how //for loops// work, you should be able to use any type of loop.  You will see some examples of //while loops// and //repeat loops// in the next section.
+**Exercise**
+NEED TO WRITE IN AN EXERCISE
 ==== Loop Modifications ====
@@ Line 170: / Line 234: @@
 <file rsplus>
+# Print the CO2 concentrations for "chilled" treatments and keep count of how many replications there were.
+count=0
 for (i in 1:length(CO2[,1])) {
+  if (CO2$Treatment[i]=="nonchilled") next #Skip to next iteration if treatment is nonchilled
-repeat {
+  count=count+1
-    i <- i + 1
+  print(CO2$conc[i])
-    if (i %% 2 == 1) next  # skip this loop
-    print(i)
-    if (i == 10) break     # stop looping
 }
-</file>
+print(count) # The count and print command were performed 42 times.
+# This could be equivalently written using a repeat loop:
-<file rsplus>
+count=0
-i <- 0
+i=0
 repeat {
-    i <- i + 1
+      i <- i + 1
-    if (i %% 2 == 1) next  # skip this loop
+      if (CO2$Treatment[i]=="nonchilled") next  # skip this loop
-    print(i)
+      count=count+1
-    if (i == 10) break     # stop looping
+      print(CO2$conc[i])
-}
+      if (i == length(CO2[,1])) break     # stop looping
-</file>
+    }
-<file rsplus>
+print(count)
-i <- 0
-repeat {
+### This could also be written using a while loop:
-  i <- i + 1
-  if (i %% 2 == 1) next
-  print(i)
-  if (i == 10) break
-}
-</file>
-# -------------------------------------------------------------------
-# 9: The equivalent while loop:
 i <- 0
-while (i <= 10)
+count=0
+while (i < length(CO2[,1]))
 {
   i <- i + 1
-  if (i %% 2 == 0) print(i)
+  if (CO2$Treatment[i]=="nonchilled") next  # skip this loop
+  count=count+1
+  print(CO2$conc[i])
 }
+print(count)
+</file>
-# -------------------------------------------------------------------
+**Exercise**
-# 10: And the equivalent for loop:
-for (i in 1:10)
+NEED TO WRITE IN AN EXERCISE
-  if (i %% 2 == 0) print (i)
-# -------------------------------------------------------------------
+**Using flow control to make a complex plot**
-# 11: Or even more briefly, using seq() (read more in ?seq):
-for (i in seq(2, 10, by=2)) print(i)
+The idea here is that we have a dataset we want to plot, with concentration and uptake values, but each point has a type (Quebec or Mississippi) and a treatment ("chilled" or "nonchilled") and we want to plot the points differently for these cases.
+You can read more about mathematical typesetting with ?plotmath, and more about the way # that different colors, sizes, rotations, etc. are used in ?par.
+<file rsplus>
+head(CO2) # Look at the dataset
+unique(CO2$Type)
+unique(CO2$Treatment)
+# plot the dataset, showing each type and treatment as a different colour
+plot(x=CO2$conc, y=CO2$uptake, type="n", cex.lab=1.4,xlab="CO2 concentration", ylab="CO2 uptake") # Type "n" tells R to not actually plot the points.
+for (i in 1:length(CO2[,1]))
+{
+  if (CO2$Type[i]=="Quebec"&CO2$Treatment[i]=="nonchilled") {
+    points(CO2$conc[i],CO2$uptake[i],col="red",type="p") }
+  if (CO2$Type[i]=="Quebec"&CO2$Treatment[i]=="chilled") {
+    points(CO2$conc[i],CO2$uptake[i],col="blue") }
+  if (CO2$Type[i]=="Mississippi"&CO2$Treatment[i]=="nonchilled") {
+    points(CO2$conc[i],CO2$uptake[i],col="orange") }
+  if (CO2$Type[i]=="Mississippi"&CO2$Treatment[i]=="chilled") {
+    points(CO2$conc[i],CO2$uptake[i],col="green") }
+}
+</file>
+**Exercise**
+NEED TO WRITE IN AN EXERCISE
 <file rsplus>
 </file>