for loop

The structure is:

for(condition){
    do something
}

I am going to make the following very confusing statement:

DO NOT USE FOR LOOPS IN R UNLESS YOU CANNOT DO OTHERWISE

Why? Because -for- is very slow in R! There is much better, you will learn what later! If you cannot wait, look at the functions apply, lapply and sapply.

So why having a section about 'for' then? Because the concept of loop is one of the most important in programming and because sometimes you will need to use -for- even if it is not super efficient.

To understand the concept and utility of looping, try to code the following task yourself without looking at the code below. Create a vector of gene names and print each name on the console (use the function 'print'). To be consistent, re-use the following vector:

gene_names <- c("CHD8","MALAT1","HoxC","Nanog").

You probably ended doing something like this:

> ## Create the vector
> gene_names <- c("CHD8","MALAT1","HoxC","Nanog")
> ## Visualize it
> gene_names
> ## Display the first gene
> gene_names[1]
> ## Display the second gene
> gene_names[2]
> ## Display the third gene
> gene_names[3]
> ## Display the fourth gene
> gene_names[4]

Do you see the problem? That is very repetitive and a lot of code. Fortunately, a loop enables you to do this automatically:

> ## Let see first how the 'condition' works
> for(i in 1:4){
+ print(i)
}
> ## Now you have understood what it does, use it to access your gene names
>  for(i in 1:4){
+ print(gene_names[i])
+}

WOW! Super efficient isn't it?

Exercise 11:

• R has some pre-loaded data that you can use. Enter 'iris' in your console.
• To see only the first lines, try 'head(iris)'.
• For each row of iris, if the value of the 'Sepal.Length' is greater than 5, store the value "greater than 5" in the empty vector 'output'. Else, store the value "lesser than 5".