Conditional Statements and Loops

# ─── IF STATEMENT ────────────────────────────────────
score <- 75

if (score >= 60) {
  cat("PASS\n")
}

# ─── IF-ELSE ─────────────────────────────────────────
if (score >= 90) {
  cat("Grade: A\n")
} else if (score >= 80) {
  cat("Grade: B\n")
} else if (score >= 70) {
  cat("Grade: C\n")
} else if (score >= 60) {
  cat("Grade: D\n")
} else {
  cat("Grade: F\n")
}
# Output: Grade: C

# ─── INLINE IF (one-liner) ───────────────────────────
result <- if (score >= 60) "PASS" else "FAIL"
cat(result, "\n")  # PASS

# ─── IFELSE() — VECTORIZED (the R way) ───────────────
scores <- c(92, 45, 78, 61, 88, 55, 70)
grades <- ifelse(scores >= 90, "A",
           ifelse(scores >= 80, "B",
           ifelse(scores >= 70, "C",
           ifelse(scores >= 60, "D", "F"))))
print(grades)  # "A" "F" "C" "D" "B" "F" "C"

# Key insight: ifelse() works on entire vector at once — no loop needed!

# Basic for loop
for (i in 1:5) {
  cat("Iteration:", i, "\n")
}

# Loop over a vector
fruits <- c("apple", "banana", "cherry", "date")
for (fruit in fruits) {
  cat("Processing:", toupper(fruit), "\n")
}

# Loop with index
students <- c("Alice", "Bob", "Carol", "David")
scores   <- c(92, 78, 85, 61)

for (i in seq_along(students)) {
  cat(sprintf("%-10s → %.0f → %s\n",
              students[i], scores[i],
              ifelse(scores[i] >= 70, "PASS", "FAIL")))
}

# Loop over data frame rows
employees <- data.frame(
  name   = c("Alice", "Bob", "Carol"),
  dept   = c("IT", "HR", "Finance"),
  salary = c(85000, 55000, 70000)
)

for (i in 1:nrow(employees)) {
  bonus <- employees$salary[i] * 0.10
  cat(sprintf("%s (%s): Base=$%g, Bonus=$%g\n",
              employees$name[i], employees$dept[i],
              employees$salary[i], bonus))
}

# ─── WHILE LOOP ──────────────────────────────────────
n <- 1
while (n <= 5) {
  cat("n =", n, "\n")
  n <- n + 1  # Always ensure termination condition!
}

# Practical: compound interest until target
balance <- 10000; rate <- 0.05; years <- 0; target <- 15000
while (balance < target) {
  balance <- balance * (1 + rate)
  years   <- years + 1
}
cat(sprintf("Reached $%.2f after %d years\n", balance, years))
# Reached $15078.68 after 9 years

# ─── BREAK and NEXT ──────────────────────────────────
# break: exit loop immediately
for (i in 1:10) {
  if (i == 5) break    # Stop when i reaches 5
  cat(i, "")
}  # 1 2 3 4

cat("\n")

# next: skip current iteration (like continue)
for (i in 1:10) {
  if (i %% 2 == 0) next  # Skip even numbers
  cat(i, "")
}  # 1 3 5 7 9

cat("\n")

# ─── REPEAT LOOP ─────────────────────────────────────
x <- 1
repeat {
  cat(x, "")
  x <- x + 1
  if (x > 5) break  # Must have break or it runs forever!
}  # 1 2 3 4 5

# ─── GRADE EVALUATION SYSTEM ─────────────────────────

evaluate_student <- function(name, math, science, english) {
  # Calculate average
  avg <- mean(c(math, science, english))

  # Assign grade
  grade <- if (avg >= 90)      "A (Excellent)"
            else if (avg >= 80) "B (Good)"
            else if (avg >= 70) "C (Average)"
            else if (avg >= 60) "D (Below Average)"
            else                "F (Fail)"

  # Subject performance
  subjects <- c(Math=math, Science=science, English=english)
  strongest <- names(which.max(subjects))
  weakest   <- names(which.min(subjects))

  list(name=name, avg=avg, grade=grade,
       strongest=strongest, weakest=weakest, scores=subjects)
}

# Class evaluation
class_data <- list(
  list("Alice",  92, 88, 76),
  list("Bob",    78, 82, 88),
  list("Carol",  85, 90, 84),
  list("David",  60, 55, 70),
  list("Eve",    95, 97, 89)
)

cat("=== GRADE EVALUATION REPORT ===\n\n")
results <- list()

for (student in class_data) {
  result <- evaluate_student(student[[1]], student[[2]], student[[3]], student[[4]])
  results[[student[[1]]]] <- result

  cat(sprintf("Student: %-8s | Avg: %5.1f | Grade: %s\n",
              result$name, result$avg, result$grade))
  cat(sprintf("  Strongest: %-10s | Needs work: %s\n",
              result$strongest, result$weakest))
}

# Class summary
all_avgs <- sapply(results, function(r) r$avg)
cat(sprintf("\n─── CLASS SUMMARY ───\n"))
cat(sprintf("Class Average: %.1f\n", mean(all_avgs)))
cat(sprintf("Highest:       %.1f (%s)\n", max(all_avgs), names(which.max(all_avgs))))
cat(sprintf("Lowest:        %.1f (%s)\n", min(all_avgs), names(which.min(all_avgs))))
cat(sprintf("Pass Rate:     %.0f%%\n", mean(all_avgs >= 60) * 100))