Loops in R

A friend of mine wanted to quickly write a loop in R. Here is how to approach it

Necessity is the mother of codes.

I had learned most of R when I tried solving my analysis requirements. Today my friend approached me as she required me to help her write a loop for the following condition.

She mentions there are many rows, so she doesn’t want to go about in traditional ways she wants to use R. She has nine variables, each has to be multiplied by a number, then all those values need to be summed up to generate the final variable. Let’s break these requirements to understand the loop we will be writing.

Break the steps

1. each of 9 variables has to be multiplied by some number (could be identical or different)

2. all those multiplied new variables have to be summed up.

3. create a new variable with the final summed values.

Alright, let’s get into action.

I am creating a small dataset here for demonstration. The condition for the loop is the same for any number of variables. If you need to import your data, you will do it at this step.

Craeting a data frame

subject <- c("maths", "science", "english")
first_term_marks <- c(20,30,30)
second_term_marks <- c(15,20,25)

df <- data.frame(subject,first_term_marks,second_term_marks)

I created a data frame with three variables, 2 numbers and one nominal.

df # this is the full data

  subject first_term_marks second_term_marks
1   maths               20                15
2 science               30                20
3 english               30                25

nrow(df) # this is the number of rows in the data frame

[1] 3

Now. let’s loop :-)

a simple loop goes like this

A simple loop

for (i in 1:10) {# i have specified numbers from 1 to 10
  x=i*2 #this is our multiplication, see that we have stored it in object x
  print(x)#we are asking R to print our x values
}

[1] 2
[1] 4
[1] 6
[1] 8
[1] 10
[1] 12
[1] 14
[1] 16
[1] 18
[1] 20

But we want all her rows to be included not just 1-10. so instead we use, nrow(df)

Loop for multiplication and sum

for (i in 1:nrow(df)) { #from 1 to number of rows in the dataframe df
  df$marksnew_firstterm=first_term_marks*2 # multiply first term marks by 2
  df$marksnew_secondterm=second_term_marks*4 # multiply second term marks by 4
  df$total=df$marksnew_firstterm+df$marksnew_secondterm # give me a total of both the term marks
}

See that I have stored all the variables calculated as new variables in our data frame. so when we print our data frame now,

df

  subject first_term_marks second_term_marks marksnew_firstterm
1   maths               20                15                 40
2 science               30                20                 60
3 english               30                25                 60
  marksnew_secondterm total
1                  60   100
2                  80   140
3                 100   160

This will have all the calculated items. Let us say you want only the final output in the data frame and not the intermediate ones, then tweak the code in this way.

Retain only the final value

subject <- c("maths", "science", "english")
first_term_marks <- c(20,30,30)
second_term_marks <- c(15,20,25)

df_new <- data.frame(subject,first_term_marks,second_term_marks)
for (i in 1:nrow(df_new)) {
  marksnew_firstterm=first_term_marks*2
  marksnew_secondterm=second_term_marks*4
  df_new$total=marksnew_firstterm+marksnew_secondterm
}

Now when we print the data frame,

df_new

  subject first_term_marks second_term_marks total
1   maths               20                15   100
2 science               30                20   140
3 english               30                25   160

You see that it has only the total marks, and the intermediary values are not added. We did not make those intermediary variables part of the data frame. See that we skipped using df followed by $ and used only marksnew_firstterm. Using df$variablename creates the variable as part of the data frame, df

Happy looping!!