Theory, Examples & Exercises
As an example how to detect missing values in a matrix, let's use Danube meadow dataset with Ellenberg indicator values for individual species (this is a dataset with species attributes, with species in rows and tabulated Ellenberg indicator values in columns):
danube.ell <- read.delim ('https://raw.githubusercontent.com/zdealveindy/anadat-r/master/data/danube.ell.txt', row.names = 1)
A simple way to know mising values if the data are in
data.frame format is to use
Light Temp Cont Moist React Nutr Min. :4.000 Min. :4.000 Min. :2.000 Min. : 2.000 Min. :3.000 Min. :1.000 1st Qu.:6.250 1st Qu.:5.000 1st Qu.:3.000 1st Qu.: 4.000 1st Qu.:6.500 1st Qu.:4.000 Median :7.000 Median :5.000 Median :3.000 Median : 5.000 Median :7.000 Median :5.000 Mean :6.889 Mean :5.435 Mean :3.901 Mean : 5.524 Mean :6.851 Mean :4.938 3rd Qu.:7.000 3rd Qu.:6.000 3rd Qu.:5.000 3rd Qu.: 6.000 3rd Qu.:7.000 3rd Qu.:6.000 Max. :8.000 Max. :6.000 Max. :7.000 Max. :10.000 Max. :8.000 Max. :9.000 NA's :4 NA's :48 NA's :23 NA's :10 NA's :47 NA's :13
The bottom row in the output of
summary shows the number of missing values in each variable. But which values are missing?
row col Chenopodium album 22 1 Festuca rubra 34 1 Melandrium diurnum 58 1 Vicia sepium 93 1 Achillea millefolium 1 2 Ajuga reptans 2 2 Alopecurus pratensis 4 2 Angelica sylvestris 5 2 Anthriscus silvestris 6 2 ...
Here, the function
is.na transforms the
danube.ell data frame into logical values - TRUE for the values which are missing and FALSE for those which are present. The function
which searches for TRUE values in the data frame, and by setting argument
arr.ind = TRUE the function returns the coordinates (row x column) of each missing value.
If you want to visualize how many holes your dataset has, use the function
image, which draws the “heatmap” of values in matrix-like object (here it has to be really a
data.frame, which is why I transformed the
danube.ell data frame into matrix by
Since I am interested to see only two types of values, those which have some value and those which have missing value (NA), I also used function
is.na to transform original real values into logical TRUE/FALSE values depending on whether given element is missing or not. The color palette used in
image function by default is
heat.colors, in which low values are red and high values are yellow (or almost white). In this case, high values are missing values, and are displayed as yellow in the resulting heat map. I also transposed the matrix using function
t, otherwise, the columns would be drawn horizontally. But note that still, the drawing starts from the left bottom corner, not from the left top, so the matrix visualization is upside down compare to the values in the original matrix.