Do not Repeat Yourself - DRY rule

vs. Write Everything Twice - WET rule


Following the WET rule will:

  • Increase the difficulty of change
  • Decrease clarity
  • Leads to opportunities for inconsistency

    To prevent duplication and follow the DRY rule, we can write custom functions.

Functions are ‘self contained’ sets of commands that accomplish a specific task.
Functions usually ‘take in’ data or parameter values (these inputs are called ‘function arguments’), process it, and ‘return’ a result. You’ve already used several functions in this tutorial; for example rnorm(n, mean, sd), where n, mean, and sd are inputs and the result is a random sample from the normal distribution. The only difference here is that you are writing the function yourself. Once a function is written, it can be used over and over again by calling its name, just like other functions such as rnorm().
To write your own function, use the function function:

AwesomeFunctionName <- function(argument1, argument2,…) {  
  do stuff here  
}

To build up a function, start by writing the “stuff” to test that it works outside the function.

YOUR TURN:
Create a function that draws a histogram of nrep mean(rnorm(100))
Modify your function to draw a histogram of nrep mean(rnorm(n))

Note that it is useful to define nrep outside of the function, so users of your script can more easily change that value e.g. from a low number (to verify the script runs without error) to a large number (to obtain reliable results).

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