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Functions, for loops, and if/else constructs

Jamshaid Shahir June 17, 2019

Basic functions

So far we’ve been using a lot of functions that exist within R. Now we’ll learn how to build our own function.

myfunction <- function(arg1, arg2, ...) {
  
  #do some stuff
  return(object)
  
}

Make a simple function that just returns the argument that it is given.

first_function <- function (x) {
  
  return(x)
  
}

Now if you pass a value to first_function it should return that value:

first_function(9)
## [1] 9

A function that only returns the given value is not useful, but it does illustrate how you can add functions to your environment. Now make a function that squares a value and then adds one to it.

second_function <- function(x) {
  ans <- x^2 + 1
  return(ans)
  
}
second_function(9)
## [1] 82

Exercise 1

Suppose we wanted to look at the Gapminder dataset and calculate GDP. Write a function to do that, returning the dataset with the new GDP column

Exercise 2

Write a function that converts a Farhenheit temperature to Kelvin (remember that K = 5/9*(F - 32) + 273.15).

For loops

For loops are used to iterate through elements in a vector, repeating the same block of code each time. They take the basic form:

for(i in vector) {
  Do some operation to each element of the vector
}

Example:

for (year in c(2010,2011,2012,2013,2014,2015)){
  print(paste("The year is", year))
}
## [1] "The year is 2010"
## [1] "The year is 2011"
## [1] "The year is 2012"
## [1] "The year is 2013"
## [1] "The year is 2014"
## [1] "The year is 2015"

We can also store the output of the code block evaluated in each iteration of the for loop:

output <- c() # create an empty vector
for (year in c(2010,2011,2012,2013,2014,2015)){
  output <- c(output, paste("The year is", year)) # add to the empty vector each iteration
}
output
## [1] "The year is 2010" "The year is 2011" "The year is 2012"
## [4] "The year is 2013" "The year is 2014" "The year is 2015"

We can also do this another way by iterating through the indices with the length function. This is rather common in more traditional programming languages, such as MATLAB, C++, and Java.

output <- c() # create an empty vector
years <- c(2010,2011,2012,2013,2014,2015)
for (i in 1:length(years)){
  output[i] <- years[i] # add to the empty vector each iteration
}
output
## [1] 2010 2011 2012 2013 2014 2015

Exercise 3

Use a for loop to print out all the countries in the Americas from Gapminder (hint: use filter to just extract the American countries; also, how can you avoid repeats?)

This usually is not preferred if you want to keep the output of a for loop, because it can become inefficient. Instead, we could use a function from the apply family.

The levels tacked on make our output a little hard to read. We can suppress those by using the following function: as.character

head(as.character(gapminder$continent))
## [1] "Asia" "Asia" "Asia" "Asia" "Asia" "Asia"

For loop exercise

Use the following vector years and the function printYear inside a for loop to print a new line for each year in years.

years <- c(2010,2011,2012,2013,2014,2015)

printYear <- function(year) {
  paste("The year is", year)
}

If/else statements

In R, you create an if/else statement with the following syntax:

if (logical statment) { do something } else  { do something different }

For example:

if (1 > 2) { print("TRUE") } else { print("FALSE") }
## [1] "FALSE"

Not all if/else statements have to have an else clause. By default, if no else statement is given R does nothing.

x <- 1000
if (x > 100) print("'x' is greater than 100")
## [1] "'x' is greater than 100"

If/else statements can also be nested:

y <- 25
if (y < 100) {
  if (y > 10) {
    print("10 < y < 100")
  } else {
    print("y < 10 or y > 100")
  }
} else {
  print("y < 10 or y > 100")
}
## [1] "10 < y < 100"

Exercise 5

Without using filter, loop through the entire Gapminder dataset, and just print out countries in the American continent. For simplicity and to avoid repeats, just use data from the year 2002: gap2002 = filter(gapminder, year==2002)

Bonus

Write a function that takes as input a gapminder dataset from 2002 and a specified continent, and prints out the list of countrys

Apply family functions

Apply family functions similarly perform operations iteratively on slices of data (rows in a matrix, elements in a list). For many operations, the apply family can be more efficient than a for loop. You can read on about apply functions at Data Camp.

We can use sapply, which applies a function over a list or vector and returns the output as a vector, to get output as the above for loop:

years <- c(2010,2011,2012,2013,2014,2015)
sapply(years, function(x) paste("The year is", x))
## [1] "The year is 2010" "The year is 2011" "The year is 2012"
## [4] "The year is 2013" "The year is 2014" "The year is 2015"

We can use lapply, which applies a function over a list or vector and returns the output as a list.

years <- c(2010,2011,2012,2013,2014,2015)
lapply(years, function(x) paste("The year is", x))
## [[1]]
## [1] "The year is 2010"
## 
## [[2]]
## [1] "The year is 2011"
## 
## [[3]]
## [1] "The year is 2012"
## 
## [[4]]
## [1] "The year is 2013"
## 
## [[5]]
## [1] "The year is 2014"
## 
## [[6]]
## [1] "The year is 2015"

Exercise 4

Repeat the prior exercise using sapply and lapply