Biostatistics with R

The for loop

The for loop is useful for iteratively executing a group of instructions. The general format is given by

for(variation in a sequence) expression

A sequence inside the pair of braces next to for is considered. For every element in the sequence, the expression will be evaluated. The expression can be either a simple statement or a set of complex statements which may or may not involve the iterated elements. See the example below:

for(i in 1:10) { print(i) }

In the above code, the 1:10 creates a seqence from 1 to 10. The keyword in refers to individual elements in this sequence. The letter i is a variable name which refers to the element in the sequence. Any name can be used instead of this. The statement for(i in 1:10) iterates through every element from 1 to 10, and the command print(i) is executed 10 times for every value of i in the sequence. Consequently, the values of i from 1 to 10 are printed. After the 10 executions, the for loop terminates, resulting in the following output:

[1] 1 [1] 2 [1] 3 [1] 4 [1] 5 [1] 6 [1] 7 [1] 8 [1] 9 [1] 10

for loop through a vector by element

The for loop in R can be used in two ways. In the first way, we can access the elements of a vector directly though an iteration inside a for statement. See the example below:

avec <- c(2.1, 3.2, 4.3, 5.4, 6.5, 7.6) for( num in avec) { num = num*10 print(num) }

In the above script, a vector avec is created with 6 numbers. The statement for( num in avec) assigns the elements of avec iteratively to the variable num. Inside the loop defined by a pair of curly braces, every value of num is multiplied by 10 and printed, resulting in the following output:

[1] 21 [1] 32 [1] 43 [1] 54 [1] 65 [1] 76

for loop through a vector by index

In the second method, elements of a vector can be iteratively accessed inside the for loop by the index generated inside. This method is adopted in the following script:

avec <- c(2.1, 3.2, 4.3, 5.4, 6.5, 7.6) for( i in 1:length(avec) ) { num = avec[i]*10 print(num) }

in the above example, length(avec) returns a number 6 which is the length of the vector as defined in the code. Thus, 1:length(avec) generates a sequence from 1 to 6. As we have seen before, the for loop iterates through this sequence assigning values 1 to 6 for the variable i . Inside the loop, avec[i]*10 accesses the values of vector avec using this index and multiplies it by 10. The resulting output is presented here:

[1] 21 [1] 32 [1] 43 [1] 54 [1] 65 [1] 76