💻Intro to Programming in R Unit 7 – Control Structures: Conditionals in R

What Are Control Structures?

• Control structures determine the flow of a program by specifying the order in which statements are executed
• Allow for decision-making and repetition in code execution based on certain conditions
• Enable programs to respond differently to various inputs and situations
• Fundamental building blocks in programming languages, including R
• Types of control structures include conditionals, loops, and functions
  • Conditionals test conditions and execute different code blocks based on the results
  • Loops repeat a block of code multiple times until a condition is met
  • Functions encapsulate reusable code that can be called with different arguments

Intro to Conditionals in R

• Conditionals evaluate a condition and execute a specific block of code if the condition is true
• Allow programs to make decisions and perform different actions based on the input or state of variables
• In R, the most common conditional statements are

  if

  ,

  else

  , and

  else if

• Conditionals use comparison operators to evaluate conditions, such as

  ==

  (equal to),

  !=

  (not equal to),

  >

  (greater than), and

  <

  (less than)
• Logical operators, such as

  &

  (and),

  |

  (or), and

  !

  (not), can be used to combine multiple conditions

If Statements: The Basics

• The

  if

  statement is the simplest form of a conditional in R
• It tests a condition and executes a block of code if the condition is true
• Syntax:

  if (condition) { code to execute if condition is true }

• The condition is placed inside parentheses

  ()

  , and the code block is enclosed in curly braces

  {}

• If the condition evaluates to

  TRUE

  , the code block is executed; otherwise, it is skipped
• Example:

  x <- 5
  if (x > 0) {
    print("x is positive")
  }
  

Else and Else If: Adding More Options

• The

  else

  statement is used in conjunction with an

  if

  statement to specify an alternative code block to execute when the

  if

  condition is false
• Syntax:

  if (condition) { code for true condition } else { code for false condition }

• The

  else if

  statement allows for testing multiple conditions in sequence
• Syntax:

  if (condition1) { code for condition1 } else if (condition2) { code for condition2 } else { code for all false conditions }

• R evaluates the conditions in order and executes the code block corresponding to the first true condition
• Example:

  x <- 0
  if (x > 0) {
    print("x is positive")
  } else if (x < 0) {
    print("x is negative")
  } else {
    print("x is zero")
  }
  

Nested Conditionals: Going Deeper

• Nested conditionals involve placing one conditional statement inside another
• Allow for more complex decision-making based on multiple conditions
• Each nested conditional is evaluated independently, and the code block is executed if its condition is true
• Indentation is important for readability and to clearly distinguish the levels of nesting
• Example:

  x <- 10
  if (x > 0) {
    if (x %% 2 == 0) {
      print("x is a positive even number")
    } else {
      print("x is a positive odd number")
    }
  } else {
    print("x is not positive")
  }
  

Logical Operators in Conditionals

• Logical operators allow combining multiple conditions in a single conditional statement
• The

  &

  (and) operator returns

  TRUE

  if all conditions are true
  • Syntax:

    if (condition1 & condition2) { code }

• The

  |

  (or) operator returns

  TRUE

  if at least one condition is true
  • Syntax:

    if (condition1 | condition2) { code }

• The

  !

  (not) operator negates a condition, returning

  TRUE

  if the condition is false and vice versa
  • Syntax:

    if (!condition) { code }

• Logical operators can be combined to create more complex conditions
• Example:

  x <- 5
  if (x > 0 & x %% 2 == 1) {
    print("x is a positive odd number")
  }
  

Common Mistakes and How to Avoid Them

• Forgetting to use the comparison operator

  ==

  for equality instead of the assignment operator

  =

  • Double-check the usage of

    ==

    in conditional statements
• Misplacing or omitting curly braces

  {}

  for code blocks
  • Ensure that code blocks are properly enclosed in curly braces
• Incorrect indentation, making the code difficult to read and understand
  • Consistently indent code blocks for clarity and readability
• Overlooking the order of evaluation in

  else if

  statements
  • Arrange conditions in the correct order, from most specific to least specific
• Confusing logical operators, such as using

  &

  instead of

  &&

  or

  |

  instead of

  ||

  • Use

    &

    and

    |

    for element-wise operations, and

    &&

    and

    ||

    for short-circuiting logical operations

Practical Applications and Examples

• Input validation: Checking if user input meets specific criteria before processing
  • Example: Verifying that a user-entered age is a positive integer
• Grading systems: Assigning grades based on numeric scores or percentages
  • Example: Converting a test score to a letter grade (A, B, C, etc.)
• Decision-making in games or simulations: Determining outcomes based on player choices or game states
  • Example: Checking if a player's inventory contains a specific item before allowing an action
• Data analysis: Filtering or categorizing data based on certain conditions
  • Example: Classifying data points as outliers if they fall outside a specified range
• Error handling: Checking for potential errors and providing appropriate feedback or fallback actions
  • Example: Displaying an error message if a required input is missing or invalid