# For Loop in R with Examples for List and Matrix

A for loop is very valuable when we need to iterate over a list of elements or a range of numbers. Loop can be used to iterate over a list, data frame, vector, matrix or any other object. The braces and square bracket are compulsory.

In this tutorial, we will learn,

## For Loop Syntax and Examples

```For (i in vector) {
Exp
}
```

Here,

R will loop over all the variables in vector and do the computation written inside the exp. Let’s see a few examples.

Example 1: We iterate over all the elements of a vector and print the current value.

```# Create fruit vector
fruit <- c('Apple', 'Orange', 'Passion fruit', 'Banana')
# Create the for statement
for ( i in fruit){
print(i)
}
```

Output:

```##  "Apple"
##  "Orange"
##  "Passion fruit"
##  "Banana"
```

Example 2: creates a non-linear function by using the polynomial of x between 1 and 4 and we store it in a list

```# Create an empty list
list <- c()
# Create a for statement to populate the list
for (i in seq(1, 4, by=1)) {
list[[i]] <- i*i
}
print(list)
```

Output:

`##    1  4 9 16		`

The for loop is very valuable for machine learning tasks. After we have trained a model, we need to regularize the model to avoid over-fitting. Regularization is a very tedious task because we need to find the value that minimizes the loss function. To help us detect those values, we can make use of a for loop to iterate over a range of values and define the best candidate.

## For Loop over a list

Looping over a list is just as easy and convenient as looping over a vector. Let’s see an example

```# Create a list with three vectors
fruit <- list(Basket = c('Apple', 'Orange', 'Passion fruit', 'Banana'),
Money = c(10, 12, 15), purchase = FALSE)
for (p  in fruit)
{
print(p)
}
```

Output:

```##  "Apple" "Orange" "Passion fruit" "Banana"
##  10 12 15
##  FALSE
```

## For Loop over a matrix

A matrix has 2-dimension, rows and columns. To iterate over a matrix, we have to define two for loop, namely one for the rows and another for the column.

```# Create a matrix
mat <- matrix(data = seq(10, 20, by=1), nrow = 6, ncol =2)
# Create the loop with r and c to iterate over the matrix
for (r in 1:nrow(mat))
for (c in 1:ncol(mat))
print(paste("Row", r, "and column",c, "have values of", mat[r,c]))
```

Output:

```##  "Row 1 and column 1 have values of 10"
##  "Row 1 and column 2 have values of 16"
##  "Row 2 and column 1 have values of 11"
##  "Row 2 and column 2 have values of 17"
##  "Row 3 and column 1 have values of 12"
##  "Row 3 and column 2 have values of 18"
##  "Row 4 and column 1 have values of 13"
##  "Row 4 and column 2 have values of 19"
##  "Row 5 and column 1 have values of 14"
##  "Row 5 and column 2 have values of 20"
##  "Row 6 and column 1 have values of 15"
##  "Row 6 and column 2 have values of 10"
```