R If Statement

An if statement runs a block of code only when a specific condition is TRUE. It gives your program the ability to make decisions. Without if statements, a program runs the same code every time regardless of the data. With if statements, it adapts to different situations.

Syntax

if (condition) {
  # code runs only when condition is TRUE
}

How It Flows

         Program reaches if()
                 │
                 ▼
         Evaluate condition
                 │
        ┌────────┴────────┐
        ▼                 ▼
      TRUE              FALSE
        │                 │
   Run the code       Skip the code
   inside { }         inside { }
        │                 │
        └────────┬────────┘
                 ▼
         Continue program

Basic Example

temperature <- 38

if (temperature > 37.5) {
  cat("You have a fever. Please rest.\n")
}

Output:

You have a fever. Please rest.

If temperature were 36, nothing would print — the block would be skipped entirely.

The Condition Must Be Logical

x <- 10

# Valid conditions (return TRUE or FALSE)
if (x > 5)  { cat("greater\n") }
if (x == 10) { cat("ten\n") }
if (is.numeric(x)) { cat("numeric\n") }

# What happens with non-logical values
if (1) { cat("runs") }    # 1 is treated as TRUE
if (0) { cat("skips") }   # 0 is treated as FALSE

Single Line if (No Braces)

When the code block has only one statement, braces are optional — but keeping them is recommended for clarity.

score <- 75
if (score >= 50) cat("Pass\n")          # works, no braces
if (score >= 50) { cat("Pass\n") }      # preferred style

Nested If Statements

balance <- 5000
transaction <- 3000

if (transaction > 0) {
  if (balance >= transaction) {
    cat("Transaction approved\n")
  }
}

Diagram:
  transaction > 0?
       │ YES
       ▼
  balance >= transaction?
       │ YES
       ▼
  "Transaction approved"

Practical Example: Age Verification

age <- 20

if (age >= 18) {
  cat("Access granted. Welcome!\n")
}

if (age < 13) {
  cat("Parental guidance required.\n")
}

Output (age = 20):

Access granted. Welcome!

Using if With Variables

rainfall_mm <- 85

if (rainfall_mm > 75) {
  alert <- "Heavy rain warning issued"
  send_alert <- TRUE
}

if (exists("alert")) {
  cat(alert, "\n")
}

Common Mistakes

# Mistake 1: Using = instead of ==
if (x = 5)  { }    # ERROR: use == for comparison
if (x == 5) { }    # CORRECT

# Mistake 2: Condition outside parentheses
if x > 5 { }       # ERROR: condition must be in ()
if (x > 5) { }     # CORRECT

# Mistake 3: Comparing NA with ==
if (x == NA) { }        # WRONG: always returns NA
if (is.na(x)) { }       # CORRECT

The if statement is the simplest form of control flow in R. Every program that responds to data conditions — validation, alerting, filtering — starts with an if statement. Master this before moving to if-else and more complex control structures.