plot1.R

## plot1.R - ExData_Plotting1 - 8 May 2015
##
## This code is to be run in a directory along 
## with the "household_power_consumption.txt"
## file, which must be unzipped.
## It loads the entire file into memory as a
## data.frame, so it may not perform well on
## older machines
##

## First, load the file contents into memory
householdPowerConsumption <- read.table(file = "household_power_consumption.txt", sep = ";", na.strings = "?", header = TRUE)

## Copy to a second data.frame, merging the
## first two columns into a single, POSIXct
## column.
timeHouseholdPowerConsumption <- cbind(as.POSIXct(strptime(do.call(paste,c(householdPowerConsumption[,1:2],sep = " ")),format = "%d/%m/%Y %T")),householdPowerConsumption[,c(3,4,5,6,7,8,9)])

## Fix the ugly column name left by cbind
newColNames <- colnames(timeHouseholdPowerConsumption)
newColNames[1] <- "Time"
colnames(timeHouseholdPowerConsumption) <- newColNames

## Subset data.frame to relevant dates only
startDate <- as.POSIXct("2007-02-01")
stopDate  <- as.POSIXct("2007-02-03")
rowsInSubset <- (timeHouseholdPowerConsumption$Time >= startDate) & 
                (timeHouseholdPowerConsumption$Time < stopDate)
subsetHouseholdPowerConsumption <- timeHouseholdPowerConsumption[rowsInSubset,]

## Make Plot 1:
png("plot1.png")
hist(subsetHouseholdPowerConsumption$Global_active_power,
     main = "Global Active Power", 
     xlab = "Global Active Power (kilowatts)", 
     col = "red")
dev.off()