Converting Objects

Before you start: R storage types; type checks and conversion.
Setup: No data files are needed. Create the objects on this page in the order shown.
Your check: Explain type changes when combining objects or converting factor labels. Practise in the unit summary.

“Errors using inadequate data are much less than those using no data at all.” – Charles Babbage

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For vector conversions, see Unit 02: Converting Types of Data.

• Use is. to test for data type. Returns TRUE or FALSE
• Use as. to explicitly convert it.

	to vector	to matrix	to dataframe
from vector	c(x,y)	cbind(x,y) rbind(x,y)	data.frame(x,y)
from matrix	as.vector(mymatrix)		as.data.frame(mymatrix)
from dataframe		as.matrix(myframe)

Combining vectors into rows or columns

rbind() stacks the vectors as rows; cbind() places them side by side as columns. Predict the dimensions before running each call. The more detailed Unit 08 binding lesson later asks whether observations, names and units are compatible.

v1 <- 1:3
v2 <- 4:6
by_rows <- rbind(v1, v2)
by_columns <- cbind(v1, v2)
dim(by_rows)
dim(by_columns)

The dimensions are 2 3 and 3 2. Both remain numeric. A different type can change the result:

Check what a conversion changes

Before running this example, predict which columns can remain numeric after the conversion.

sites <- data.frame(site = c("A", "B"), height_cm = c(12, 18))
site_matrix <- as.matrix(sites)
str(sites)
typeof(site_matrix)
site_matrix

The data frame has a character column and a numeric column. The matrix needs one atomic type, so both columns become character; heights appear as "12" and "18". Returning to a data frame with as.data.frame(site_matrix) does not automatically recover the numeric type. Keep the original object and inspect the result before calculating.

Factor codes and labels

A factor stores codes that point into its levels. Numeric-looking labels need special care:

category <- factor(c("20", "10", "20"), levels = c("10", "20"))
as.numeric(category)
as.character(category)
as.numeric(as.character(category))

The three results are 2, 1, 2; "20", "10", "20"; and 20, 10, 20. Only use the final conversion if the labels really represent numeric quantities. For categories or identifiers, preserve the labels instead. This is why the same storage conversion can be appropriate for a measurement and inappropriate for a site code.