Functions

Learning Objectives

Following this assignment students should be able to:

• use, modify, and write custom functions
• use the output of one function as the input of another

Reading

• Topics

  • Functions

• Readings

  • Creating functions

Lecture Notes

1. Functions

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Exercises

1. Writing Functions (25 pts)

   Copy the following function into your assignment and replace the ________ with variables names for the input and output.

   convert_pounds_to_grams <- function(________) {
       grams = 453.6 * pounds
       return(________)
   }
   

   Use the function to calculate how many grams there are in 3.75 pounds.

   Expected outputs for Writing Functions: 1

2. Use and Modify (25 pts)

   The length of an organism is typically strongly correlated with its body mass. This is useful because it allows us to estimate the mass of an organism even if we only know its length. This relationship generally takes the form:

   mass = a * length^b

   Where the parameters a and b vary among groups. This allometric approach is regularly used to estimate the mass of dinosaurs since we cannot weigh something that is only preserved as bones.

   The following function estimates the mass of an organism in kg based on its length in meters for a particular set of parameter values, those for Theropoda (where a has been estimated as 0.73 and b has been estimated as 3.63; Seebacher 2001).

   get_mass_from_length_theropoda <- function(length){
     mass <- 0.73 * length ^ 3.63
     return(mass)
   }
   

   1. Use this function to print out the mass of a Spinosaurus that is 16 m long based on its reassembled skeleton.
   2. Create a new version of this function called get_mass_from_length() that estimates takes length, a and b as arguments and uses the following code to estimate the mass mass <- a * length ^ b. Use this function to estimate the mass of a Sauropoda (a = 214.44, b = 1.46) that is 26 m long.
   Expected outputs for Use and Modify: 1

3. Default Arguments (25 pts)

   This is a follow up to Use and Modify.

   Allowing a and b to be passed as arguments to get_mass_from_length() made the function more flexible, but for some types of dinosaurs we don’t have specific values of a and b and so we have to use general values that can be applied to a number of different species.

   Rewrite your get_mass_from length() function from Use and Modify so that its arguments have default values of a = 39.9 and b = 2.6 (the average values from Seebacher 2001).

   1. Use this function to estimate the mass of a Sauropoda (a = 214.44, b = 1.46) that is 22 m long (by setting a and b when calling the function).
   2. Use this function to estimate the mass of a dinosaur from an unknown taxonomic group that is 16m long. Only pass the function length, not a and b, so that the default values are used.
   Expected outputs for Default Arguments: 1

4. Combining Functions (25 pts)

   This is a follow up to Default Argument.

   Measuring things using the metric system is the standard approach for scientists, but when communicating your results more broadly it may be useful to use different units (at least in some countries). Write a function called convert_kg_to_pounds that converts kilograms into pounds (pounds = 2.205 * kg). Use that function and your get_mass_from_length() function from Default Arguments to estimate the weight, in pounds, of a 12 m long Stegosaurus with a = 10.95 and b = 2.64 (The estimated a and b values for Stegosauria from Seebacher 2001).

   Expected outputs for Combining Functions: 1

Assignment submission & checklist