Part 19 Repetitive execution

Loops are used to repeat a specific block of code.

Structure of the for loop:

for(i in vector_expression){
    action_command
}

3 main elements:

• i is the loop variable: it is updated at each iteration.
• vector_expression: value attributed to i at each iteration (the number of iterations is the length of vector_expression).
• action_command: what action to take at each iteration.

Note the usage of curly brakets {} to start and end the loop!

• Example:

for(i in 2:5){
    # prints the value of i at each iteration
    print(i)
    # multiplies i by 2 at each iteration
    y <- i*2
    # prints the value of y at each iteration
    print(y)
}

• Example of a for loop that iterates over a character vector:

# Character vector
myfruits <- c("apple", "pear", "grape")

# The for loop prints the current element j, and the number of characters of element j
for(j in myfruits){
    print(j)
    print(nchar(j))
}

• Example of a for loop that iterates over each row of a matrix, and prints the minimum value of that row :

# Matrix of 50 rows, 16 columns 
mymat <- matrix(rnorm(800), 
    nrow=50)
    
# For loop over mymat rows
    # 1:nrow(mymat): ranges from 1 to the number of rows in the matrix: 1, 2, 3, 4, ...., 48, 49, 50
    
for(i in 1:nrow(mymat)){
    # prints the value of i at each iteration
    print(i)
    # prints the minimum value of the ith row of mymat at each iteration
    print(min(mymat[i,]))
}

• If statement in For loop:

You can combine for loops and if statements:

# Matrix
mymat <- matrix(rnorm(800), 
        nrow=50)
    
# Loop over rows of mymat and print row if its median value is > 0
for(i in 1:nrow(mymat)){
    # extract the current row
    rowi <- mymat[i,]
    # if median of row is > 0, print row
    if(median(rowi) > 0){
        print(rowi)
    }
}

You can also have a look at this tutorial, to know more about loops.