Looping over Objects in R Programming

One of the biggest issues with the “for” loop is its memory consumption and its slowness in executing a repetitive task. When it comes to dealing with a large data set and iterating over it, a for loop is not advised. 

In this article we will discuss How to loop over a list in R Programming Language provides many alternatives to be applied to vectors for looping operations that are pretty useful when working interactively on a command line.

function and its variants:

• apply()
• lapply()
• sapply()
• tapply()
• mapply()

Let us see what each of these functions does.

apply(): This function applies a given function over the margins of a given array.

apply(): This function applies a given function over the margins of a given array.

apply(array, margins, function, ...) array = list of elements margins = dimension of the array along which the function needs to be applied function = the operation which you want to perform

# R program to illustrate
# apply() function

# Creating a matrix
A = matrix(1:9, 3, 3)
print(A)

# Applying apply() over row of matrix
# Here margin 1 is for row 
r = apply(A, 1, sum)
print(r)

# Applying apply() over column of matrix
# Here margin 2 is for column
c = apply(A, 2, sum)
print(c)

Output:

[, 1] [, 2] [, 3]
[1, ]    1    4    7
[2, ]    2    5    8
[3, ]    3    6    9

[1] 12 15 18
[1]  6 15 24

lapply(): This function is used to apply a function over a list. It always returns a list of the same length as the input list.

lapply(list, function, ...)
list = Created list
function = the operation which you want to perform

# R program to illustrate
# lapply() function

# Creating a matrix
A = matrix(1:9, 3, 3)

# Creating another matrix
B = matrix(10:18, 3, 3) 

# Creating a list
myList = list(A, B)

# applying lapply()
determinant = lapply(myList, det)
print(determinant)

Output:

[[1]]
[1] 0

[[2]]
[1] 5.329071e-15

sapply(): This function is used to simplify the result of lapply(), if possible. Unlike lapply(), the result is not always a list. The output varies in the following ways:-

sapply(list, function, ...)
list = Created list
function = the operation which you want to perform

# R program to illustrate
# sapply() function

# Creating a list
A = list(a = 1:5, b = 6:10)

# applying sapply()
means = sapply(A, mean)
print(means)

Output:

a b
3 8

A vector is returned since the output had a list with elements of length 1.

• If output is a list containing elements having length 1, then a vector is returned.
• If output is a list where all the elements are vectors of same length(>1), then a matrix is returned.
• If output contains elements which cannot be simplified or elements of different types, a list is returned.

tapply(): This function is used to apply a function over subset of vectors given by a combination of factors.

tapply(vector, factor, function, ...)
vector = Created vector
factor = Created factor
function = the operation which you want to perform

# R program to illustrate
# tapply() function

# Creating a factor
Id = c(1, 1, 1, 1, 2, 2, 2, 3, 3)

# Creating a vector
val = c(1, 2, 3, 4, 5, 6, 7, 8, 9)

# applying tapply()
result = tapply(val, Id, sum)
print(result)

Output:

1  2  3 
10 18 17 

How does the above code work?

mapply(): It's a multivariate version of lapply(). This function can be applied over several list simultaneously.

mapply(function, list1, list2, ...)
function = the operation which you want to perform
list1, list2= Created lists.

# R program to illustrate
# mapply() function

# Creating a list
A = list(c(1, 2, 3, 4))

# Creating another list
B = list(c(2, 5, 1, 6))

# Applying mapply()
result = mapply(sum, A, B)
print(result)

Output:

[1] 24

Loop Through List & Display All Sub-Elements on Same Line

my_list <- c(1, 2, 3, 4, 5)

for (element in my_list) {
  cat(element, " ")
}

Output:

1  2  3  4  5  

Loop Through List & Display All Sub-Elements on Different Lines

my_list <- c(1, 2, 3, 4, 5)

for (element in my_list) {
  cat(element, "\n")
}

Output:

1 
2 
3 
4 
5

Loop Through List and Only Display Specific Values

my_list <- c(1, 2, 3, 4, 5)

for (element in my_list) {
  if (element %% 2 == 0) {  # Display only even values
    cat(element, "\n")
  }
}

Output:

2 
4 

First we creates a list my_list with values from 1 to 5. It then iterates through each element using a for loop, and if the element is even (determined by element %% 2 == 0), it is printed on a new line using cat. The output displays only the even values (2 and 4) from the list.