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Introduction to R for Ecologists
R語言在生態學的應用
EEB5082 (B44U1940)

• Study materials • Links • Class exercise • Individual work • Homework assignments Author: David Zelený recol:dplyr

Data manipulation with dplyr package

In this section we focus on the use of dplyr, one of the packages in tidyverse. We will see how to group and summarise the data, and how to reformat long to wide and wide to long format of the data frame (or tibble, as it is called in tidyverse). We will use two set of real ecological datasets for this purpose.

Piping data with %>% from magrittr

The piping operator %>% from the package margrittr is used to pipe data into the function and also the output of one function to the other function. So for example, instead of writing

result <- sum (log (sqrt (abs (-10:10))))

you can write the sequence using the pipe operator as

library (tidyverse)
result <- -10:10 %>% abs %>% sqrt %>% log1p %>% sum

Piping produces cleaner code, and allows to create a logical pipeline of the functions, allowing to better imagine how the data flow through it (from left to right).

If not specified, the data get piped to the first argument of the function:

cars %>% plot (xlab = 'Speed [mph]', ylab = 'Distance [ft]')

Alternatively, it can be specified and replaced by a placeholder, a dot (.):

cars %>% plot (., xlab = 'Speed [mph]', ylab = 'Distance [ft]')

In this way, the data can be piped also to other arguments (here xlab):

'Speed [mph]' %>% plot (cars, xlab = ., ylab = 'Distance [ft]')

The original package from which pipes come (magrittr) contains also compound assignment operator %<>%, which pipes the result of the pipeline back to the original variable:

a <- 1:10
a %<>% sin %>% abs
a
 0.8414710 0.9092974 0.1411200 0.7568025 0.9589243 0.2794155 0.6569866 0.9893582 0.4121185
 0.5440211

This piping operator is not imported to dplyr, so if you call dplyr directly (instead of tidyverse), you are not likely be able to use it.

Check magrittr reference for more details.

Group and summarise data (group_by and summarise from dplyr)

The two functions, group_by and summarise (or summarize if you wish to use American English) are used to aggregate the data in the data frame (or a tibble, how is it called in tidyverse language) possibly by grouping variable.

A simple example using the iris data set. Let's turn iris into a tibble using as_tibble function:

Sepal.Length Sepal.Width Petal.Length Petal.Width Species
1          5.1         3.5          1.4         0.2  setosa
2          4.9         3.0          1.4         0.2  setosa
3          4.7         3.2          1.3         0.2  setosa
4          4.6         3.1          1.5         0.2  setosa
5          5.0         3.6          1.4         0.2  setosa
6          5.4         3.9          1.7         0.4  setosa
iris_tb <- as_tibble (iris)
iris_tb
# A tibble: 150 x 5
Sepal.Length Sepal.Width Petal.Length Petal.Width Species
<dbl>       <dbl>        <dbl>       <dbl> <fct>
1          5.1         3.5          1.4         0.2 setosa
2          4.9         3            1.4         0.2 setosa
3          4.7         3.2          1.3         0.2 setosa
4          4.6         3.1          1.5         0.2 setosa
5          5           3.6          1.4         0.2 setosa
6          5.4         3.9          1.7         0.4 setosa
7          4.6         3.4          1.4         0.3 setosa
8          5           3.4          1.5         0.2 setosa
9          4.4         2.9          1.4         0.2 setosa
10          4.9         3.1          1.5         0.1 setosa
# ... with 140 more rows

As you can see, the tibble format returns the data in more compact format, which gets always printed on the screen. The first row are column names, below is the row summarising the types of variables (<dbl> means double precise real number, <fct> is a factor) and below the shorted table is summarised what else the tibble contains (# ... with 140 more rows).

Note that iris (and iris_tb) contains a grouping variable Species - the whole dataset represents four measurements of sepal and petal lenghts and widths on 150 plants of Iris from three species, Iris setosa, I. versicolor and I. virginica (each with 50 individuals). We may calculate mean of each column using the colMean function, but this is not going to be too informative, since each species is likely to have different mean values for different parameters. Here come the group_by and summarise functions:

mean_iris_SL <- iris_tb %>% group_by (Species) %>% summarise (mean_SL = mean (Sepal.Length))
mean_iris_SL
# A tibble: 3 x 2
Species    mean_SL
<fct>        <dbl>
1 setosa        5.01
2 versicolor    5.94
3 virginica     6.59

The iris_tb was piped into group_by function, which used the column Species to group the rows into three groups. Then, the function summarise, applied the mean function on Sepal.Length, and stored the result into variable mean_SL. Resulting tibble contains three rows - mean value of Sepal.Length for each species.

Similarly, we may include more than just a mean; here we take mean, standard deviation (sd) and the number of samples in each group (the function n(), which does not take arguments and simply counts numbers of rows in each group):

summary_iris_SL <- iris_tb %>% group_by (Species) %>% summarise (mean_SL = mean (Sepal.Length), sd_SL = sd (Sepal.Length), n_SL = n ())
summary_iris_SL
# A tibble: 3 x 4
Species    mean_SL sd_SL  n_SL
<fct>        <dbl> <dbl> <int>
1 setosa        5.01 0.352    50
2 versicolor    5.94 0.516    50
3 virginica     6.59 0.636    50

If we want to apply the same function on all columns (here mean), the function summarise_all is exactly for this:

mean_iris_all <- iris_tb %>% group_by (Species) %>% summarise_all (.funs = mean)
mean_iris_all
# A tibble: 3 x 5
Species    Sepal.Length Sepal.Width Petal.Length Petal.Width
<fct>             <dbl>       <dbl>        <dbl>       <dbl>
1 setosa             5.01        3.43         1.46       0.246
2 versicolor         5.94        2.77         4.26       1.33
3 virginica          6.59        2.97         5.55       2.03

Indeed, we the function summarise can be used without group_by, and then it works much like column-wise functions like colMeans, or generally as apply function with MARGIN = 2:

iris_tb[,-5] %>% summarise_all (mean)
# A tibble: 1 x 4
Sepal.Length Sepal.Width Petal.Length Petal.Width
<dbl>       <dbl>        <dbl>       <dbl>
1         5.84        3.06         3.76        1.20
iris_tb[,-5] %>% colMeans
Sepal.Length  Sepal.Width Petal.Length  Petal.Width
876.5        458.6        563.7        179.9
iris_tb[,-5] %>% apply (2, mean)
# or explicitly as iris_tb[,-5] %>% apply (MARGIN = 2, FUN = function (x) mean (x))
Sepal.Length  Sepal.Width Petal.Length  Petal.Width
5.843333     3.057333     3.758000     1.199333

(note that we needed to remove the fifth column from the original data (since it contains factor whcih we cannot be averaged) and that the colMeans and apply solution returns standard data frame, not a tibble (these are not tidyverse functions).

Long vs wide data format: transformation

Long format describes dataset in which the values of variables are gathered in two or three long columns, while wide format describes dataset in which the information is spread across many columns. Example of long format may be situation that we measure something (e.g. abundance) for each individual tree within several plots; then the data have one column for plot ID, one column for species name, and one column for the abundance. In this example, species name and abundance is so called key-value pair: key describes what is being measured, and value is the actually measured number. The same data can be shaped in wide format: each column is a single species name, each row is plot, and each cell of this data frame is the actual value of abundance of given species in given plot (in this case, the species abundance for species not occuring in given plot will be set to zero).

Long to wide

As an example of long data, we use the subset of 10 most dominant species occurring in 3 different plots sampled within the cloud forest of Tamanshan (塔曼山, highest peak of Taipei basin).

Load data stored as gist on GitHub (we use function read_delim from tidyverse package raedr for it):

long_format <- readr::read_delim ('https://gist.githubusercontent.com/zdealveindy/3078ac5f2852531604e703900c3d05a2/raw/90bb0c24baebfffb03cb9980aced88b2a784c2d3/long_format.txt', delim = '\t')
long_format
# A tibble: 22 x 3
Plot_ID Species_name     BA
<chr>   <chr>         <dbl>
1 L2L     EurGlab        98.2
2 L2L     PrunPhae      630.
3 L2L     SycoSine     1295
4 L2L     NeolAcum     2321.
5 L2L     QuerSess     9430.
6 L2R     QuerSten       11.3
7 L2R     SympMacr       11.3
8 L2R     PrunPhae       59.4
9 L2R     EurGlab        77.9
10 L2R     SycoSine      170.
# ... with 12 more rows

Use the function spread to spread the values in BA (the value argument) across multiple columns using Species_name as a key (argument fill = 0 makes sure that combinations of Species_name and Plot_ID which would become NA will be replaced by given value, here 0):

wide <- long_format %>% spread (key = Species_name, value = BA, fill = 0)
wide
# A tibble: 3 x 11
Plot_ID CarpRank DaphHima EurGlab NeolAcum PrunPhae PrunTran QuerSess QuerSten
<chr>      <dbl>    <dbl>   <dbl>    <dbl>    <dbl>    <dbl>    <dbl>    <dbl>
1 L2L           0         0    98.2    2321.    630.        0     9430.      0
2 L2R           0         0    77.9    1700      59.4       0    30627.     11.3
3 L2W        1166.     1190   719.      350.      1.3    2382.    1090    1625.
# ... with 2 more variables: SycoSine <dbl>, SympMacr <dbl>

Data in this format can be used for further analysis (e.g. using multivariate methods in the package vegan).

Wide to long

This is opposite situation - we have data spread across multiple columns, and wont to gather them into one key and one value column. We use the iris dataset here as an example:

iris_long <- as_tibble (iris) %>% gather (key = flower_feature, value = measurement, Sepal.Length:Petal.Width)
# A tibble: 600 x 3
Species flower_feature measurement
<fct>   <chr>                <dbl>
1 setosa  Sepal.Length           5.1
2 setosa  Sepal.Length           4.9
3 setosa  Sepal.Length           4.7
4 setosa  Sepal.Length           4.6
5 setosa  Sepal.Length           5
6 setosa  Sepal.Length           5.4
7 setosa  Sepal.Length           4.6
8 setosa  Sepal.Length           5
9 setosa  Sepal.Length           4.4
10 setosa  Sepal.Length           4.9
# ... with 590 more rows

Note that as_tibble function transforms the iris dataset into a tibble (although this may not be necessary here - if we didn't transform, result will be ordinary data frame). The sequence of variable names (Sepal.Length:Petal.Width) indicates which columns should be gathered into the key-value columns (if we did not specify, the function will use also the Plot_ID and gather it as a part of these two columns. Alternatively, we may code as_tibble (iris) %>% gather (key = flower_feature, value = measurement, -Species) (minus sign means to exclude this column from gathering).

Long format of data may be required by some analyses, and also may be more suitable e.g. for plotting (for example, try to use the formula version of boxplot function: boxplot (measurement ~ flower_feature, iris_long), or alternative solution using the lattice package for plotting: lattice::bwplot (measurement~Species|flower_feature, iris_long)).

recol/dplyr.txt · Last modified: 2019/02/01 20:53 by david

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