Loop expressions

Loop (or iteration) evaluates an expression repeatedly by either iterating over a vector (for) or checking whether a condition is violated (while).

Such language constructs largely reduce the redundancy of the code if the same task is run over and over again each time with some changes in input.

Using the for loop

The for loop evaluates an expression by iterating over a given vector or list. The syntax of a for loop is as follows:

for (var in vector) { 
  expr 
} 

Then, expr will be evaluated iteratively, with var taking the value of each element of vector in turn. If vector has n elements, then the preceding loop is equivalent to evaluating:

var <- vector[[1]] 
expr 
var <- vector[[2]] 
expr 
... 
var <- vector[[n]] 
expr 

For example, we can create a loop to iterate over 1:3 with iterator variable i. In each iteration, we will show text on the screen to indicate the value of i:

for (i in 1:3) {
cat("The value of...