R Language Loops For While Repeat Complete Guide

1. for Loop

The for loop is used when the number of iterations is known beforehand.

Syntax:

for (variable in sequence) {
  # code to execute
}

Example 1: Print numbers from 1 to 5

# Using a numeric sequence
for (i in seq(1, 5)) {
  print(i)
}

# Or simply using a vector
for (i in 1:5) {
  print(i)
}

Explanation:

• In this example, the loop runs five times with i taking the values 1, 2, 3, 4, and 5 sequentially.
• Each time the loop runs, it prints the current value of i.

Example 2: Iterate through a vector of strings

# Define a vector of strings
fruits <- c("apple", "banana", "cherry", "date")

# Iterate through each element in the vector
for (fruit in fruits) {
  cat("I like", fruit, "\n")
}

Explanation:

• This loop iterates over each fruit in the fruits vector.
• Inside the loop, the cat function is used to concatenate and print the string "I like" followed by the name of each fruit.

2. while Loop

The while loop keeps executing as long as a specified condition is TRUE.

Syntax:

while (condition) {
  # code to execute
}

Example 1: Count until the number reaches 5

# Initialize counter variable
counter <- 1

# Continue looping while counter is less than or equal to 5
while (counter <= 5) {
  print(counter)
  counter <- counter + 1
}

Explanation:

• This loop starts with counter = 1.
• It checks if the condition counter <= 5 is true and executes the loop body.
• Inside the loop, it prints the value of counter and then increments it by 1.
• When counter becomes 6, the condition becomes false, and the loop stops.

Example 2: Simulate rolling a die until a six appears

# Simulate rolling a die
die_roll <- sample(1:6, 1)

# Keep rolling until a six appears
while (die_roll != 6) {
  cat("Rolled a", die_roll, ", rolling again...\n")
  die_roll <- sample(1:6, 1)
}

# Congratulations message when you roll a six
cat("Rolled a", die_roll, ", success!\n")

Explanation:

• The sample function randomly selects a number between 1 and 6, simulating a dice roll.
• The loop continues as long as the die_roll is not equal to 6.
• After each iteration, a new dice roll is simulated.
• Once a six is rolled, the loop exits, and a success message is printed.

3. repeat Loop

The repeat loop repeats indefinitely until an explicit call to break causes it to terminate.

Syntax:

repeat {
  # code to execute
  if (condition) {
    break
  }
}

Example 1: Count until the number reaches 5

# Initialize counter variable
counter <- 1

# Begin infinite loop
repeat {
  print(counter)
  counter <- counter + 1
  
  # Break out of the loop when counter exceeds 5
  if (counter > 5) {
    break
  }
}

Explanation:

• The repeat loop starts without any initial conditions.
• It prints the value of counter and increments it within each iteration.
• The loop includes an if condition that checks if counter has exceeded 5.
• When the condition is true, break terminates the loop.

Example 2: Roll a die until a six appears

# Begin infinite loop to simulate die rolls
repeat {
  die_roll <- sample(1:6, 1)  # Simulate rolling a die
  
  cat("Rolled a", die_roll, "\n")
  
  # Break out of the loop when a six is rolled
  if (die_roll == 6) {
    cat("Six appeared, exiting loop.\n")
    break
  }
}

Explanation:

• Similar to the previous while loop example, this loop uses repeat to simulate continuous dice rolls.
• After each roll, it checks if the result is a six.
• If a six is rolled, the break statement exits the loop.

Nested Loops

Loops can also be nested, where one loop is placed inside another loop.

Example: Nested for loops to create a multiplication table

# Create a multiplication table for numbers 1 through 5
for (i in 1:5) {
  for (j in 1:5) {
    product <- i * j
    cat(i, "*", j, "=", product, "\t")
  }
  cat("\n")  # Move to the next line after each row
}

Explanation:

• The outer for loop (i) runs from 1 to 5.
• The inner for loop (j) also runs from 1 to 5 for each value of i.
• For each combination of i and j, the product is calculated and printed in a tabular format.
• After completing each inner loop iteration, a new line is started for the outer loop's next value.

Summary

• for loop: Best for iterating over a fixed sequence or vector.

  • Example: Printing numbers from 1 to 5, iterating over a list of elements.

• while loop: Continues until a specified condition becomes false.

  • Example: Rolling a die until a six appears, processing data until a certain threshold is met.

• repeat loop: Repeats indefinitely until an explicit break statement is executed.

  • Example: Rolling a die until a six appears, waiting for specific user input to terminate.