For Loops

Flow Statements

Ben Weinstein

For loops

Loops are an important programming tool. The first loop we will learn is a for loop. for loops run for a certain number of steps, which you define, during which any statements in the loop are executed.

The basic syntax is

for(some sequence of steps)

{

execute some statements

}

Why use a for loop?

  1. We have a repeated process with indentical formatting, but different values.
  2. To avoid laborious typing into R

Our first loop

for(i in 1:5){

i starts at 1, R will execute some statements; i is incremented to i = 2 and statements are executed again; i is incremented to i=3 and statements are executed; and so on, until i = 25, at which point the loop executes the set of statements for the last time.

}

for (i in 1:5) {
    print(i)
}

## [1] 1
## [1] 2
## [1] 3
## [1] 4
## [1] 5

Syntax

Keep these straight: Be sure you distinguish between curly braces {}, parentheses ( ), and square brackets [ ]. The brackets are used to access elements of vectors, matrices, and dataframes. The parentheses are used to specify arguments to functions. Finally, the curly braces enclose statements to be executed within the body of a loop.

x <- 1:10
x[6]

## [1] 6

sum(x)

## [1] 55

for (i in 1:3) {
    print(i)
}

## [1] 1
## [1] 2
## [1] 3

Using a for loop

x <- 2
for (i in 1:4) {
    x <- x^2
}

Try it!

  • 1. Create a for loop that prints numbers 1 to 100
  • 2. Create a for loop that prints numbers 100 to 1
  • 3. Create a for loop that adds x + 1 to numbers 1:5
  • 4. Create a for loop that divides all even numbers from 0-20 by 10 (consider using seq())
  • 5. Bonus! What would be the final value here, witout trying it!
dogs <- 10
for (i in 1:5) {
    dogs <- dogs + 1
}
  • Why?

For Loops and Vectors

In the above examples, we used i directly in several mathematical operations. But it is more common to loop over elements of a vector to accomplish some particular task.

nameVector <- c("Charlie", "Helga", "Clancy", "Matilda", "Jones")
for (i in 1:length(nameVector)) {
    print(paste("Nice to meet you", nameVector[i], " "))
}

## [1] "Nice to meet you Charlie  "
## [1] "Nice to meet you Helga  "
## [1] "Nice to meet you Clancy  "
## [1] "Nice to meet you Matilda  "
## [1] "Nice to meet you Jones  "

What did that really do?

Consider the loop in pieces

# How many positions are in nameVector
length(nameVector)

## [1] 5


# how do we index a vector
nameVector[1]

## [1] "Charlie"

What did that really do?

paste("Nice to meet you", nameVector[1], sep = " ")

## [1] "Nice to meet you Charlie"

# loops are their own little environments, so print() so you can see them on
# your console.
nameVector <- c("Charlie", "Helga", "Clancy", "Matilda", "Jones")
for (i in 1:length(nameVector)) {
    print(paste("Nice to meet you", nameVector[i], " "))
}

## [1] "Nice to meet you Charlie  "
## [1] "Nice to meet you Helga  "
## [1] "Nice to meet you Clancy  "
## [1] "Nice to meet you Matilda  "
## [1] "Nice to meet you Jones  "

Try it!

  • 1. Create a vector of names of people in your row, write them a nice message
  • 2. Explain why the following code is wrong

for(x in 1:10) { print(sum(i)) }

  • 3. Create a vector of 1:10 and a vector of 31:40. Add the first elements of each vector together.

Syntax of lists

One can think of lists as vertical vectors.

# For a vector
x <- c(1, 2, 3, 4, 5)
x[3]

## [1] 3

# For a List
a <- list(1, 2, 3, 4, 5)
a[[3]]

## [1] 3

Why use a list? A Brief Tangent

A list is a generic vector containing other objects. For example, x is a list containing three vectors n, s, b, and a numeric 3.

n = c(2, 3, 5)
s = c("aa", "bb", "cc", "dd", "ee")
b = c(TRUE, FALSE, TRUE, FALSE, FALSE)
x = list(n, s, b, 3)  # x contains copies of n, s, b
x

## [[1]]
## [1] 2 3 5
## 
## [[2]]
## [1] "aa" "bb" "cc" "dd" "ee"
## 
## [[3]]
## [1]  TRUE FALSE  TRUE FALSE FALSE
## 
## [[4]]
## [1] 3

List Slicing

We retrieve a list slice with the single square bracket "[]" operator. 

x[2]

## [[1]]
## [1] "aa" "bb" "cc" "dd" "ee"

# With an index vector, we can retrieve a slice with multiple members.
x[c(2, 4)]

## [[1]]
## [1] "aa" "bb" "cc" "dd" "ee"
## 
## [[2]]
## [1] 3

List Reference

In order to reference a list member directly, we have to use the double square bracket "[[]]" operator. 

x[[2]]

## [1] "aa" "bb" "cc" "dd" "ee"

# We can modify its content directly.
x[[2]][1] = "ta"
x[[2]]

## [1] "ta" "bb" "cc" "dd" "ee"

How do we save loop outputs

Instead of printing to the screen, we usually want to create an object with the outputs on the loop. In general, we do do this either with a vector, or with a list.

# Create an blank output
outputs <- list()

for (x in 1:5) {
    outputs[[x]] <- x * 10
}
outputs

## [[1]]
## [1] 10
## 
## [[2]]
## [1] 20
## 
## [[3]]
## [1] 30
## 
## [[4]]
## [1] 40
## 
## [[5]]
## [1] 50

Try it!

  • 1. Create a for loop that computes x * 2 for 1:100, and place the output in a vector
  • 2. Create a for loop that computes x * 2 for 1:100, and place the output in a list
  • 3. How do we get the 47th position in question 1 and 2?
  1. What does this code do?
output <- list()
for (x in 1:10) {
    output[1] <- sum(x + x^2)
}
  1. What does this code do?
output2 <- list()
for (x in 1:11) {
    output2[x + 1] <- sum(x + x^2)
}

Flow Statements

Often, we want to control for loops to account for variables, options, and logical statements

Let's begin with the if statement:

for (x in 1:5) {
    if (x > 3) {
        print(paste(x, "is greater than 3"))
    }
    if (x <= 3) {
        print(paste(x, "is less than or equal to 3"))
    }
}

## [1] "1 is less than or equal to 3"
## [1] "2 is less than or equal to 3"
## [1] "3 is less than or equal to 3"
## [1] "4 is greater than 3"
## [1] "5 is greater than 3"

Controlling Flow - Break

Often we need to handle logical cases within a loop; we can end a loop running based on an if and break statement

for (x in 1:5) {
    if (x > 3) {
        break
    }
    if (x <= 3) {
        print(paste(x, "is less than or equal to 3"))
    }
}

## [1] "1 is less than or equal to 3"
## [1] "2 is less than or equal to 3"
## [1] "3 is less than or equal to 3"

Controlling Flow - Next

Sometimes we don't want to break the statement, just skip a troublesome object or R that we know will cause an error. We can continue within a loop running based on an if and next statement. Here we want to skip 4. 

for (x in 1:5) {
    if (x == 4) {
        next
    }
    if (x > 3) {
        print(paste(x, "is greater than 3"))
    }
    if (x <= 3) {
        print(paste(x, "is less than or equal to 3"))
    }
}

## [1] "1 is less than or equal to 3"
## [1] "2 is less than or equal to 3"
## [1] "3 is less than or equal to 3"
## [1] "5 is greater than 3"

Try it!

  1. Create a for loop that computes x * 2 for 1:100, and place the output in a vector. However, make just the values of x=32 and x=67 x *3

  2. Create a for loop that computes x * 2 for 1:100, and place the output in a list. However, break the loop after 51 iterations

  3. Create a for loop that computes x * 2 for 1:100, and place the output in a list. However, skip x= 71, 74. How can you show that you suceeded?

  1. Why does this code fail?
for (x in 1:5) {
    if (x <= 4) {
        print(paste(x, "is less than or equal to 4"))
    }
    # Kill the loop after x=3
    if (x > 3) {
        break
    }
}
  1. Create a for loop that computes x * 2 for 1:100, and place the output in a list. However, break the loop when the square root of the output of a statement is greater than 8.4. What is the last x value reported?