R Programming – FOR loops, WHILE loop & REPEAT loop

FOR loops

For loops allow a certain operation to be repeated a fixed number of times and this is opposed to the While loop where the rep nr is not prefixed.
The syntax looks like this: for (name in vector) {commands}

for (i in 1:15) {print (i)}
for (z in 1:15) {print (z)}

variable name does not matter although you will see i quite often to skip certain elements in the loop: next

for(i in 1:12){
if(i==3)
next
print (i)}

to break/stop the loop: break

for(i in 1:12){
if(i==3)
break
print (i)}

Let us create the Fibonacci sequence, in this case the first 20 nunbers.
This is a famous sequence of numbers which is used e.g. in stock market predictions.
It starts with 2 x 1 and continues with the summation of the 2 preceding elements.

# with the first line we create an empty vector
Fibonacci = c()
# here we define the first two numbers
Fibonacci[1] = Fibonacci[2] = 1
# and here we create the loop to calculate positions 3 to 20
for (i in 3:20) {Fibonacci[i] = Fibonacci[i-2] + Fibonacci[i-1]}
Fibonacci

WHILE loop

if we want to repeat statements, but we do not yet know about the pattern of repetition.
as long as the condition holds the calculation is performed
x meets condition (command in curly brackets is performed)

syntax: while (condition) {commands}

x = 4
while(x < 5){
x = x+10
print(x)
}

# y does not meet condition
y = 6
while(y < 5){
y = y+10
print(y)
}

REPEAT loop
This loop is used infrequently - however sometimes it can be useful.the idea is to repeat a certain statement endlessly (difference to FOR) with the break statement we terminate the loop

syntax: repeat {statements} if (condition) break