For Loop

For loop is extremely important in R programming language. We will illustrate multiple examples in this notebook.

The basic structure of a for loop is:

for (val in sequence)
{
statement
}

Example1: Print out everything in a vector

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

for (i in number){
  print(i)
}

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

This i in print(i) comes from for i part in the for loop. For loop is frequently used with if statemenets.

Example2: Print out even numbers in a vector

number <- c(2, 3, 6, 10, 11)

for (i in number){
  if(i %% 2 == 0){
    print(i)
  }
}

## [1] 2
## [1] 6
## [1] 10

One note:

a %% b

will compute the remainder of a divided by b.

In this code, we did not write the else statement. Thus, this program won’t do anything if the number is not divisible by 2.

Example3: Count the number of even numbers in a list

count <- 0

number <- c(2, 3, 6, 10, 11)

for (i in number){
  if (i %% 2 == 0){
    count <- count + 1
  }
}
count

## [1] 3

There are 3 even numbers in number vector.

Example4: Use the same print function for multiple times

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

## [1] "R"
## [1] "R"
## [1] "R"
## [1] "R"
## [1] "R"

1:5 is the same as c(1,2,3,4,5). We can specify the range of for loop by writing in this method.

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