Control Flow

## Learning Objectives - Master if, else if, else statements - Understand switch statements - Learn for, while, repeat loops - Use break and next (continue) - Apply family functions ## if Statement ### Basic if ```r age <- 18 if (age >= 18) { print("Adult") } ``` ### if-else ```r age <- 15 if (age >= 18) { print("Adult") } else { print("Minor") } ``` ### if-else if-else ```r score <- 85 grade <- if(score >= 90) { "A" } else if (score >= 80) { "B" } else if (score >= 70) { "C" } else if (score >= 60) { "D" } else { "F" } print(grade) # "B" ``` ### Inline ifelse ```r # Single line version age <- 20 status <- if(age >= 18) "adult" else "minor" ``` ## ifelse Function ### Vectorized Condition ```r # ifelse for vectors x <- 1:10 result <- ifelse(x > 5, "big", "small") # "small" "small" "small" "small" "small" "big" "big" "big" "big" "big" # Multiple conditions score <- 75 grade <- ifelse(score >= 90, "A", ifelse(score >= 80, "B", ifelse(score >= 70, "C", ifelse(score >= 60, "D", "F")))) ``` ### with NA Handling ```r x <- c(1, 2, NA, 4, 5) ifelse(x > 2, "big", "small") # "small" "small" NA "big" "big" # With NA replacement ifelse(is.na(x), 0, x) # 1 2 0 4 5 ``` ## switch Statement ### Basic switch ```r # By position day <- 2 day_name <- switch(day, "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday" ) print(day_name) # "Tuesday" ``` ### By Name ```r # By name (more readable) color <- "red" result <- switch(color, "red" = "Stop", "yellow" = "Slow down", "green" = "Go" ) print(result) # "Stop" ``` ### With Default ```r # No match returns NULL or last value result <- switch("purple", "red" = "Stop", "green" = "Go" ) print(result) # NULL # Use default as last value result <- switch("purple", "red" = "Stop", "green" = "Go", "Unknown color" ) print(result) # "Unknown color" ``` ## for Loop ### Basic for ```r # Loop through sequence for (i in 1:5) { print(i) } # 1, 2, 3, 4, 5 ``` ### Iterating Over Vectors ```r # Loop through vector fruits <- c("apple", "banana", "cherry") for (fruit in fruits) { print(fruit) } # Using index for (i in seq_along(fruits)) { print(paste(i, fruits[i])) } ``` ### Nested for ```r # Multiplication table for (i in 1:3) { for (j in 1:3) { cat(i, "x", j, "=", i * j, "\n") } } ``` ## while Loop ### Basic while ```r i <- 1 while (i <= 5) { print(i) i <- i + 1 } # 1, 2, 3, 4, 5 ``` ### while with Condition ```r # Read input until valid value <- 0 while (value <= 0 || value > 100) { value <- as.numeric(readline(prompt = "Enter 1-100: ")) } ``` ## repeat Loop ### Infinite Loop with break ```r # repeat is infinite loop (like while TRUE) repeat { print("This runs once") break # Must have break condition } ``` ### repeat for Menu ```r repeat { cat("1. New Game\n") cat("2. Load Game\n") cat("3. Quit\n") choice <- as.integer(readline(prompt = "Choose: ")) if (choice == 3) { print("Goodbye!") break } else if (choice == 1) { print("Starting new game...") break } } ``` ## break and next ### break - Exit Loop ```r # Find first even number numbers <- c(1, 3, 5, 6, 7, 8) for (num in numbers) { if (num %% 2 == 0) { print(paste("First even:", num)) break } } ``` ### next - Skip Iteration ```r # Skip even numbers for (i in 1:5) { if (i %% 2 == 0) { next # Skip to next iteration } print(i) } # 1, 3, 5 ``` ## Apply Family ### lapply - List Apply ```r # lapply returns a list nums <- list(a = 1:3, b = 4:6) result <- lapply(nums, mean) # $a: 2 # $b: 5 # Anonymous function result <- lapply(nums, function(x) x * 2) ``` ### sapply - Simplified Apply ```r # sapply simplifies to vector if possible nums <- list(a = 1:3, b = 4:6) result <- sapply(nums, mean) # a b # 2 5 # Vector mode result <- sapply(1:3, function(x) x^2) # 1 4 9 ``` ### apply - Array/Matrix Apply ```r mat <- matrix(1:9, nrow = 3) # Apply function to rows (1) or columns (2) apply(mat, 1, sum) # Row sums apply(mat, 2, mean) # Column means ``` ### vapply - Apply with Predefined Type ```r # vapply is faster and safer nums <- list(a = 1:3, b = 4:6) result <- vapply(nums, mean, numeric(1)) # a b # 2 5 ``` ### tapply - Table Apply ```r # Apply function by groups scores <- c(85, 90, 75, 95, 80) groups <- c("A", "A", "B", "B", "A") tapply(scores, groups, mean) # A B # 85 85 ``` ### mapply - Multiple Argument Apply ```r # Apply function to multiple vectors mapply(sum, 1:3, 10:12, 20:22) # 31 34 37 # Same as sum(1, 10, 20), sum(2, 11, 21), sum(3, 12, 22) ``` ## Common Patterns ### Sum 1 to N ```r # Vectorized (fast) sum(1:100) # Loop version total <- 0 for (i in 1:100) { total <- total + i } ``` ### Find in Vector ```r numbers <- c(3, 7, 2, 9, 4) target <- 9 for (num in numbers) { if (num == target) { print("Found!") break } } ``` ### Count Matches ```r numbers <- c(1, 2, 3, 2, 4, 2, 5) target <- 2 count <- 0 for (num in numbers) { if (num == target) { count <- count + 1 } } print(count) # 3 ``` ## Summary - if/else if/else for conditional logic - ifelse() for vectorized conditionals - switch() for discrete value matching - for loop for known iterations - while loop for condition-based iteration - repeat for infinite loops with break - next skips iteration, break exits loop - apply family: lapply, sapply, apply, vapply, tapply, mapply