Nested for Loops in R

1. for Loop in R Language
2. Use Nested for Loop to Iterate Over Matrix Elements in R

This article will introduce the nested for loops in R.

for Loop in R Language

The for loop is available in R language with similar heuristics as in most programming languages. It repeats the given code block multiple times. The for loop syntax is as follows.

for (item in set) {}

item is an object that stores the iterated element from the set. The for loop doesn’t return output, so we need to call the print function to output the word value on each iteration.

vec1 <- c("ace", "spades", "king", "spades", "queen", "spades", "jack",
           "spades", "ten", "spades")

for (word in vec1) {
  print(word)
}

Output:

[1] "ace"
[1] "spades"
[1] "king"
[1] "spades"
[1] "queen"
[1] "spades"
[1] "jack"
[1] "spades"
[1] "ten"
[1] "spades"

We can also implement the for loop, where the index is exposed as a variable. In this case, the length function is utilized to calculate the size of the vec1 vector and iterate from the first element to the end. Note that 1: notation is important, and it specifies the beginning of the range. The following example code creates a string vector copied to another vector of the same size using the for loop.

vec1 <- c("ace", "spades", "king", "spades", "queen", "spades", "jack",
           "spades", "ten", "spades")

vec2 <- vector(length = length(vec1))

for (i in 1:length(vec1)) {
  vec2[i] <- vec1[i]
}
vec2

Output:

[1] "ace"    "spades" "king"   "spades" "queen"  "spades" "jack"   "spades" "ten"   
[10] "spades"

Use Nested for Loop to Iterate Over Matrix Elements in R

Nested loops can be implemented using the for loop structure. This can be utilized to iterate over matrix elements and initialize them with random values. Note that the general notation is the same as the previous example, except that the end of the range is calculated with the nrow and ncol functions. nrow and ncol return the number of rows or columns of the array, respectively.

mat1 <- matrix(0, nrow = 5, ncol = 5)
for (i in 1:nrow(mat1)) {
  for (j in 1:ncol(mat1)) {
    mat1[i, j] <- sample(1:100, 1, replace=TRUE)
  }
}
mat1

Output:

[,1] [,2] [,3] [,4] [,5]
[1,]   13   14   13   67   98
[2,]   28   50   23   55    9
[3,]    3   65   99   17   93
[4,]   18    6   20   50   46
[5,]   51   76   33   26    3

Although, the nested loop structure works fine in the previous example code. Matrix initialization is better done using the sample function chained directly as the first argument of the matrix function, as shown in the following snippet.

mat2 <- matrix(sample(1:100, 25, replace = TRUE), ncol = 5)
mat2

Output:

[,1] [,2] [,3] [,4] [,5]
[1,]   85   19   26   53   88
[2,]   44   50   66   96   56
[3,]   42   46   37   19   66
[4,]   43   23   13   32   67
[5,]   56   51   21    2   56