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Introduction to loops

Gaston Sanchez

Learning Objectives

  • Why do you need loops?
  • Get to know the For loop
  • Get to know the While loop
  • Get to know the Repeat loop

About Loops

  • Many times we need to perform a procedure several times
  • We perform the same operation several times as long as some condition is fulfilled
  • For this purpose we use loops
  • The main idea is that of iteration
  • R provides three basic paradigms: for, repeat, while

Motivation example

Consider a numeric vector with prices of five items:

prices <- c(2.50, 2.95, 3.45, 3.25)

prices
## [1] 2.50 2.95 3.45 3.25

Printing prices “manually”

Say you are interested in printing each price individually. You can manually display them one by one, by typing the same command several times:

cat("Price 1 is", prices[1])
cat("Price 2 is", prices[2])
cat("Price 3 is", prices[3])
cat("Price 4 is", prices[4])
## Price 1 is 2.5

## Price 2 is 2.95

## Price 3 is 3.45

## Price 4 is 3.25

Printing prices with a for loop

Or you can use a loop structure in which you tell the computer to display the prices a given number of times, but using one command instead of typing it various times:

for (i in 1:4) {
  cat("Price", i, "is", prices[i], "\n")
}
## Price 1 is 2.5 
## Price 2 is 2.95 
## Price 3 is 3.45 
## Price 4 is 3.25

Let’s make it less simple by creating a vector of prices with the names of the associated coffees:

coffee_prices <- c(
  expresso = 2.50,
  latte = 2.95,
  mocha = 3.45, 
  cappuccino = 3.25)

coffee_prices
##   expresso      latte      mocha cappuccino 
##       2.50       2.95       3.45       3.25

Without using a loop, you can display, via cat(), the prices one-by-one; (this, of course, involves a lot of repetition)

cat("Expresso has a price of", coffee_prices[1])
cat("Latte has a price of", coffee_prices[2])
cat("Mocha has a price of", coffee_prices[3])
cat("Capuccino has a price of", coffee_prices[4])
## Expresso has a price of 2.5

## Latte has a price of 2.95

## Mocha has a price of 3.45

## Capuccino has a price of 3.25

Printing coffee prices with a for loop

for (i in 1:4) {
  cat(names(coffee_prices)[i], "has a price of", 
      prices[i], "\n")
}
## expresso has a price of 2.5 
## latte has a price of 2.95 
## mocha has a price of 3.45 
## cappuccino has a price of 3.25

For Loops

  • Often we want to repeatedly carry out some computation a fixed number of times.
  • For instance, repeat an operation for each element of a vector.
  • In R this can be done with a for loop.
  • for loops are used when we know exactly how many times we want the code to repeat

The anatomy of a for loop is as follows:

for (iterator in times) { 
  do_something
}

for() takes an iterator variable and a vector of times to iterate through.

value <- 2

for (i in 1:5) { 
  value <- value * 2 
  print(value)
}
## [1] 4
## [1] 8
## [1] 16
## [1] 32
## [1] 64

The vector of times does NOT have to be a numeric vector; it can be any vector

value <- 2
times <- c('one', 'two', 'three', 'four')

for (i in times) { 
  value <- value * 2 
  print(value)
}
## [1] 4
## [1] 8
## [1] 16
## [1] 32

However, if the iterator is used inside the loop in a numerical computation, then the vector of times will almost always be a numeric vector:

set.seed(4321)
numbers <- rnorm(5)

for (h in 1:length(numbers)) {
  if (numbers[h] < 0) {
    value <- sqrt(-numbers[h])
  } else {
    value <- sqrt(numbers[h])
  }
  print(value)
}
## [1] 0.6532667
## [1] 0.4728761
## [1] 0.8471168
## [1] 0.9173035
## [1] 0.3582698

For Loops and Next statement

Sometimes we need to skip a loop iteration if a given condition is met, this can be done with a next statement

for (iterator in times) { 
  expr1
  expr2
  if (condition) {
    next
  }
  expr3
  expr4
}

Example:

x <- 2

for (i in 1:5) {
  y <- x * i
  if (y == 8) {
    next
  }
  print(y)
}
## [1] 2
## [1] 4
## [1] 6
## [1] 10

Nested Loops

It is common to have nested loops

for (iterator1 in times1) { 
  for (iterator2 in times2) {
    expr1
    expr2
    ...
  }
}

Example: Nested loops

# some matrix
A <- matrix(1:12, nrow = 3, ncol = 4)

A
##      [,1] [,2] [,3] [,4]
## [1,]    1    4    7   10
## [2,]    2    5    8   11
## [3,]    3    6    9   12

Example: Nested Loops

# reciprocal of values less than 6
for (i in 1:nrow(A)) { 
  for (j in 1:ncol(A)) {
    if (A[i,j] < 6) A[i,j] <- 1 / A[i,j] 
  }
}

A
##           [,1] [,2] [,3] [,4]
## [1,] 1.0000000 0.25    7   10
## [2,] 0.5000000 0.20    8   11
## [3,] 0.3333333 6.00    9   12

About for Loops and Vectorized Computations

  • R loops have a bad reputation for being slow.

  • Experienced users will tell you: “tend to avoid for loops in R” (me included).

  • It is not really that the loops are slow; the slowness has more to do with the way R handles the boxing and unboxing of data objects, which may be a bit inefficient.

  • R provides a family of functions that are usually more efficient than loops (i.e. apply() functions).

  • For this course, especially if you have NO programming experience, you should ignore any advice about avoiding loops in R.

  • You should learn how to write loops, and understand how they work; every programming language provides some type of loop structure.

  • In practice, many (programming) problems can be tackled using some loop structure.

  • When using R, you may need to start solving a problem using a loop. Once you solved it, try to see if you can find a vectorized alternative.

  • It takes practice and experience to find alternative solutions to for loops.

  • There are cases when using for loops is not that bad.

Repeat Loop

repeat executes the same code over and over until a stop condition is met:

repeat { 
  # keep
  # doing
  # something
  if (stop_condition) {
    break
  }
}

The break statement stops the loops. If you enter an infinite loop, you can manually break it by pressing the ESC key.

value <- 2

repeat {
  value <- value * 2 
  print(value)
  if (value >= 40) {
    break
  }
}
## [1] 4
## [1] 8
## [1] 16
## [1] 32
## [1] 64

To skip a current iteration, use next

value <- 2

repeat {
  value <- value * 2
  print(value)
  if (value == 16) {
    value <- value * 2
    next
  }
  if (value > 80) break 
}
## [1] 4
## [1] 8
## [1] 16
## [1] 64
## [1] 128

While Loops

It can also be useful to repeat a computation until a condition is false. A while loop provides this form of control flow.

while (condition) { 
  # keep
  # doing
  # something
  # until
  # condition is FALSE
}

About while loops

  • while loops are backward repeat loops
  • while checks first and then attempts to execute
  • computations are carried out for as long as the condition is true
  • the loop stops when the condition is FALSE
  • If you enter an infinite loop, break it by pressing ESC key
value <- 2

while (value < 40) { 
  value <- value * 2 
  print(value)
}
## [1] 4
## [1] 8
## [1] 16
## [1] 32
## [1] 64

Loops: for, while, repeat

Let’s see one last example of a for loop, and how to achieve the same task with while and repeat loops.

Say you have a vector x <- c(2, 4, 6, 8, 10), and the goal is to obtain the sum of the elements in x; in other words get sum(x) but using loops.

# using a for loop
x <- c(2, 4, 6, 8, 10)

# initialize output
sumx <- 0

for (i in seq_along(x)) {
  print(paste('iteration:'), i)
  sumx <- sumx + x[i]
  print(paste('sum =', sumx))
}
## [1] "iteration:"
## [1] "sum = 2"
## [1] "iteration:"
## [1] "sum = 6"
## [1] "iteration:"
## [1] "sum = 12"
## [1] "iteration:"
## [1] "sum = 20"
## [1] "iteration:"
## [1] "sum = 30"

sumx
## [1] 30

Now let’s do it with a while loop

# initialize output
sumx <- 0

# initialize counter
i <- 1

# while loop
while (i <= length(x)) {
  print(paste('iteration:', i))
  sumx <- sumx + x[i]
  print(paste('sum =', sumx))
  i <- i + 1
}
## [1] "iteration: 1"
## [1] "sum = 2"
## [1] "iteration: 2"
## [1] "sum = 6"
## [1] "iteration: 3"
## [1] "sum = 12"
## [1] "iteration: 4"
## [1] "sum = 20"
## [1] "iteration: 5"
## [1] "sum = 30"

sumx
## [1] 30

And finally with a repeat loop:

# initialize output
sumx <- 0

# initialize counter
i <- 1

# repeat loop (visualizing iterations)
repeat {
  print(paste('iteration:', i))
  sumx <- sumx + x[i]
  print(paste('sum =', sumx))
  i <- i + 1
  if (i > length(x)) {
    break
  }
}
## [1] "iteration: 1"
## [1] "sum = 2"
## [1] "iteration: 2"
## [1] "sum = 6"
## [1] "iteration: 3"
## [1] "sum = 12"
## [1] "iteration: 4"
## [1] "sum = 20"
## [1] "iteration: 5"
## [1] "sum = 30"

sumx
## [1] 30

Repeat, While, For

  • If you don’t know the number of times something will be done, you can use either repeat or while
  • while evaluates the condition at the beginning
  • repeat executes operations until a stop condition is met
  • If you know the number of times that something will be done, use for
  • for needs an iterator and a vector of times

Questions

  • What happens if you pass NA as a condition to if()?
  • What happens if you pass NA as a condition to ifelse()?
  • What types of values can be passed as the first argument to switch()?
  • How do you stop a repeat loop executing?
  • How do you jump to next iteration of a loop?