# Aggregate Example
# Create bar plots with data processed by the 
# aggregate function.

# Set working directory.
setwd("rdatasets")

# Read dataset with columns in fixed with format.
# Specify widths, which are the column widths for each
# variable. Use dummy variables for columns which are not
# needed and which are discarded immediately.
students <- read.fwf("students.txt", 
  widths=c(10, 1, 5, 1, 4),
  col.names=c("dummy1", "gen", "dummy2", 
    "yr", "gpa"))[ , c(-1, -3)]

# Insert decimal point in gpa
students$gpa <- students$gpa / 1000                    
print(students)

# Compute group means by gender using aggregate function
means1 <- as.matrix(aggregate(students$gpa, 
  list(gender=students$gen), mean))

# Compute group means by year using aggregate function
means2 <- as.matrix(aggregate(students$gpa, 
  list(gender=students$yr), mean))

# Send graphs to PDF file.
pdf("aggregate.pdf")

# Create barplot of average GPA for each gender.
barplot(means1[ ,2], names.arg=c("Female", "Male"), 
  xlab="Gender", ylab="Average GPA",
  main="Average GPAs for Students at consolidated High")

# Create barplot of average GPA for each year
barplot(means2[ ,2], names.arg=c("Freshman", 
    "Sophomore", "Junior", "Senior"), 
  xlab="Gender", ylab="Average GPA", 
  main="Average GPAs for Students at consolidated High")

# Send graphics back to viewer.
dev.off( )