In Part 12, let’s see how to create histograms in R. Let’s create a simple histogram using the `hist()`

command, which is easy to use, but actually quite sophisticated. First, we set up a vector of numbers and then we create a histogram.

`B <- c(2, 4, 5, 7, 12, 14, 16)`

hist(B)

That was easy, but you need more from your histogram. Note that R decided on an appropriate bin width. OK. Now we create a histogram from all the data in an array.

`A <- structure(list(James = c(1L, 3L, 6L, 4L, 9L), Robert = c(2L, 5L, 4L, 5L, 12L), David = c(4L, 4L, 6L, 6L, 16L), Anne = c(3L, 5L, 6L, 7L, 6L)), .Names = c("James", "Robert", "David", "Anne"), class = "data.frame", row.names = c(NA, -5L))`

attach(A)

A

The trick is to transform the four variables into a single vector and make a histogram of all elements.

`B <- c(A$James, A$Robert, A$David, A$Anne)`

Let’s create a histogram of B in dark green and include axis labels.

`hist(B, col="darkgreen", ylim=c(0,10), ylab ="My frequency", xlab ="B values")`

However, controlling bin numbers can be problematic. Try setting the number of bins at 6 using the `breaks`

argument.

`hist(B, col = "red", breaks=6, xlim=c(0,max(B)), main="My Histogram", las=2, xlab = "B values", cex.lab = 1.3)`

You can see that R has taken the number of bins (6) as indicative only. However, setting up histogram bins as a vector gives you more control over the output. Now we set up the bins as a vector, each bin four units wide, and starting at zero.

`bins <- c(0, 4, 8, 12, 16)`

hist(B, col = "blue", breaks=bins, xlim=c(0,max(B)), main="My Histogram", las=2, xlab = "B values", cex.lab = 1.3)

Now we have four bins of the right width. That wasn’t so hard! In Blog 13 we will look at further plotting techniques in R.

See you later!

David

#### Annex: R codes used

[code lang=”r”]

# Create a vector of numbers.

B <- c(2, 4, 5, 7, 12, 14, 16)

hist(B)

# Generate a dataset.

A <- structure(list(James = c(1L, 3L, 6L, 4L, 9L), Robert = c(2L, 5L, 4L, 5L, 12L), David = c(4L, 4L, 6L, 6L, 16L), Anne = c(3L, 5L, 6L, 7L, 6L)), .Names = c("James", "Robert", "David", "Anne"), class = "data.frame", row.names = c(NA, -5L))

attach(A)

A

# Transform the four variables into a single vector.

B <- c(A$James, A$Robert, A$David, A$Anne)

# Make a histogram.

hist(B, col="darkgreen", ylim=c(0,10), ylab ="My frequency", xlab ="B values")

# Make a histogram.

hist(B, col = "red", breaks=6, xlim=c(0,max(B)), main="My Histogram", las=2, xlab = "B values", cex.lab = 1.3)

# Set number of bins and make a histogram.

bins <- c(0, 4, 8, 12, 16)

hist(B, col = "blue", breaks=bins, xlim=c(0,max), main="My Histogram", las=2, xlab = "B values", cex.lab = 1.3)

[/code]