Chapter 7 Iteration & Functions

7.1 Learning Objectives

You will learn about functions and iteration by using simulation to calculate a power analysis for an independent samples t-test.

7.1.1 Basic

  1. Work with iteration functions rep, seq, and replicate
  2. Use arguments by order or name
  3. Write your own custom functions with function()
  4. Set default values for the arguments in your functions

7.1.2 Intermediate

  1. Understand scope
  2. Use error handling and warnings in a function

7.1.3 Advanced

The topics below are not (yet) covered in these materials, but they are directions for independent learning.

  1. Repeat commands and handle result using purrr::rerun(), purrr::map_*(), purrr::walk()
  2. Repeat commands having multiple arguments using purrr::map2_*() and purrr::pmap_*()
  3. Create nested data frames using dplyr::group_by() and tidyr::nest()
  4. Work with nested data frames in dplyr
  5. Capture and deal with errors using ‘adverb’ functions purrr::safely() and purrr::possibly()

7.2 Resources

In the next two lectures, we are going to learn more about iteration (doing the same commands over and over) and custom functions through a data simulation exercise, which will also lead us into more traditional statistical topics. Along the way you will also learn more about how to create vectors and tables in R.

7.3 Iteration functions

7.3.1 rep()

The function rep() lets you repeat the first argument a number of times.

Use rep() to create a vector of alternating "A" and "B" values of length 24.

##  [1] "A" "B" "A" "B" "A" "B" "A" "B" "A" "B" "A" "B" "A" "B" "A" "B" "A"
## [18] "B" "A" "B" "A" "B" "A" "B"

If you don’t specify what the second argument is, it defaults to times, repeating the vector in the first argument that many times. Make the same vector as above, setting the second argument explicitly.

##  [1] "A" "B" "A" "B" "A" "B" "A" "B" "A" "B" "A" "B" "A" "B" "A" "B" "A"
## [18] "B" "A" "B" "A" "B" "A" "B"

If the second argument is a vector that is the same length as the first argument, each element in the first vector is repeated than many times. Use rep() to create a vector of 11 "A" values followed by 3 "B" values.

##  [1] "A" "A" "A" "A" "A" "A" "A" "A" "A" "A" "A" "B" "B" "B"

You can repeat each element of the vector a sepcified number of times using the each argument, Use rep() to create a vector of 12 "A" values followed by 12 "B" values.

##  [1] "A" "A" "A" "A" "A" "A" "A" "A" "A" "A" "A" "A" "B" "B" "B" "B" "B"
## [18] "B" "B" "B" "B" "B" "B" "B"

What do you think will happen if you set both times to 3 and each to 2?

##  [1] "A" "A" "B" "B" "A" "A" "B" "B" "A" "A" "B" "B"

7.3.2 seq()

The function seq() is useful for generating a sequence of numbers with some pattern.

Use seq() to create a vector of the integers 0 to 10.

##  [1]  0  1  2  3  4  5  6  7  8  9 10

You can set the by argument to count by numbers other than 1 (the default). Use seq() to create a vector of the numbers 0 to 100 by 10s.

##  [1]   0  10  20  30  40  50  60  70  80  90 100

The argument length.out is useful if you know how many steps you want to divide something into. Use seq() to create a vector that starts with 0, ends with 100, and has 12 equally spaced steps (hint: how many numbers would be in a vector with 2 steps?).

##  [1]   0.000000   8.333333  16.666667  25.000000  33.333333  41.666667
##  [7]  50.000000  58.333333  66.666667  75.000000  83.333333  91.666667
## [13] 100.000000

7.4 Custom functions

In addition to the built-in functions and functions you can access from packages, you can also write your own functions (and eventually even packages!).

7.4.1 Structuring a function

The general structure of a function is as follows:

Here is a very simple function. Can you guess what it does?

## [1] 11

Let’s make a function that reports p-values in APA format (with “p = rounded value” when p >= .001 and “p < .001” when p < .001).

First, we have to name the function. You can name it anything, but try not to duplicate existing functions or you will overwrite them. For example, if you call your function rep, then you will need to use base::rep() to access the normal rep function. Let’s call our p-value function report_p and set up the framework of the function.

7.4.2 Arguments

We need to add one argument, the p-value you want to report. The names you choose for the arguments are private to that argument, so it is not a problem if they conflict with other variables in your script. You put the arguments in the parentheses after function in the order you want them to default (just like the built-in functions you’ve used before).

7.4.3 Argument defaults

You can add a default value to any argument. If that argument is skipped, then the function uses the default argument. It probably doesn’t make sense to run this function without specifying the p-value, but we can add a second argument called digits that defaults to 3, so we can round p-values to 3 digits.

Now we need to write some code inside the function to process the input arguments and turn them into a returned output. Put the output as the last item in function.

You might also see the returned output inside of the return() function. This does the same thing.

When you run the code defining your function, it doesn’t output anything, but makes a new object in the Environment tab under Functions. Now you can run the function.

## [1] "p = 0.049"
## [1] "p < .001"

7.4.4 Scope

What happens in a function stays in a function. You can change the value of a variable passed to a function, but that won’t change the value of the variable outside of the function, even if that variable has the same name as the one in the function.

## $`half(x)`
## [1] 5
## $x
## [1] 10

7.4.5 Warnings and errors

What happens when you omit the argument for p? Or if you set p to 1.5 or “a”?

You might want to add a more specific warning and stop running the function code if someone enters a value that isn’t a number. You can do this with the stop() function.

If someone enters a number that isn’t possible for a p-value (0-1), you might want to warn them that this is probably not what they intended, but still continue with the function. You can do this with warning().

## Error in report_p(): argument "p" is missing, with no default
## Error in report_p("a"): p must be a number
## Warning in report_p(-2): p-values are normally greater than 0
## Warning in report_p(2): p-values are normally less than 1
## [1] "p < .001"
## [1] "p = 2"

7.5 Iterating your own functions

First, let’s build up the code that we want to iterate.

7.5.1 rnorm()

Create a vector of 20 random numbers drawn from a normal distribution with a mean of 5 and standard deviation of 1 using the rnorm() function and store them in the variable A.

7.5.2 tibble::tibble()

A tibble is a type of table or data.frame. The function tibble::tibble() creates a tibble with a column for each argument. Each argument takes the form column_name = data_vector.

Create a table called dat including two vectors: A that is a vector of 20 random normally distributed numbers with a mean of 5 and SD of 1, and B that is a vector of 20 random normally distributed numbers with a mean of 5.5 and SD of 1.

7.5.3 t.test

You can run a Welch two-sample t-test by including the two samples you made as the first two arguments to the function t.test. You can reference one column of a table by its names using the format table_name$column_name

##  Welch Two Sample t-test
## data:  dat$A and dat$B
## t = -1.5937, df = 36.528, p-value = 0.1196
## alternative hypothesis: true difference in means is not equal to 0
## 95 percent confidence interval:
##  -0.9559243  0.1144139
## sample estimates:
## mean of x mean of y 
##  4.965341  5.386096

You can also convert the table to long format using the gather function and specify the t-test using the format dv_column~grouping_column.

##  Welch Two Sample t-test
## data:  score by group
## t = -1.5937, df = 36.528, p-value = 0.1196
## alternative hypothesis: true difference in means is not equal to 0
## 95 percent confidence interval:
##  -0.9559243  0.1144139
## sample estimates:
## mean in group A mean in group B 
##        4.965341        5.386096

7.5.4 broom::tidy()

You can use the function broom::tidy() to extract the data from a statistical test in a table format. The example below pipes everything together.

## # A tibble: 1 x 10
##   estimate estimate1 estimate2 statistic p.value parameter conf.low
##      <dbl>     <dbl>     <dbl>     <dbl>   <dbl>     <dbl>    <dbl>
## 1   -0.578      4.97      5.54     -1.84  0.0747      34.3    -1.22
## # … with 3 more variables: conf.high <dbl>, method <chr>,
## #   alternative <chr>

Finally, we can extract a single value from this results table using pull().

## [1] 0.256199

7.5.5 Turn into a function

First, name your function t_sim and wrap the code above in a function with no arguments.

Run it a few times to see what happens.

## [1] 0.0558203

7.5.6 replicate()

You can use the replicate function to run a function any number of times.

##            [,1]      [,2]        [,3]
## [1,]  0.2398579 1.0060960 -0.08836476
## [2,] -1.7685708 0.8362997  0.08114036
## [3,]  0.1457033 0.4557277  1.38814525
## [4,]  0.4462924 0.5616177 -0.02341062
## [5,]  0.5916637 1.4850093  0.98759269

Let’s run the t_sim function 1000 times, assign the resulting p-values to a vector called reps, and check what proportion of p-values are lower than alpha (e.g., .05). This number is the power for this analysis.

## [1] 0.304

7.5.7 Set seed

You can use the set.seed function before you run a function that uses random numbers to make sure that you get the same random data back each time. You can use any integer you like as the seed.

Make sure you don’t ever use set.seed() inside of a simulation function, or you will just simulate the exact same data over and over again.


Figure 7.1: @KellyBodwin

7.5.8 Add arguments

You can just edit your function each time you want to cacluate power for a different sample n, but it is more efficent to build this into your fuction as an arguments. Redefine t_sim, setting arguments for the mean and SD of group A, the mean and SD of group B, and the number of subjects per group. Give them all default values.

Test your function with some different values to see if the results make sense.

## [1] 0.5065619
## [1] 0.001844064

Use replicate to calculate power for 100 subjects/group with an effect size of 0.2 (e.g., A: m = 0, SD = 1; B: m = 0.2, SD = 1). Use 1000 replications.

## [1] 0.268

Compare this to power calculated from the power.t.test function.

##      Two-sample t test power calculation 
##               n = 100
##           delta = 0.2
##              sd = 1
##       sig.level = 0.05
##           power = 0.2902664
##     alternative = two.sided
## NOTE: n is number in *each* group

Calculate power via simulation and power.t.test for the following tests:

  • 20 subjects/group, A: m = 0, SD = 1; B: m = 0.2, SD = 1
  • 40 subjects/group, A: m = 0, SD = 1; B: m = 0.2, SD = 1
  • 20 subjects/group, A: m = 10, SD = 1; B: m = 12, SD = 1.5

7.6 Exercises

Download the exercises. See the answers only after you’ve attempted all the questions.