Conditionals (College Board AP® Computer Science Principles): Revision Note

Conditional execution & IF statements

What is conditional execution?

• Conditional execution is a form of selection: a section of code runs only if a specified condition is true

• Selection is one of the three core building blocks of all algorithms, alongside sequencing and iteration

• Without selection, a program follows the same path regardless of input; conditionals allow programs to respond differently to different situations

IF statements

• An IF statement evaluates a Boolean condition and executes the statements inside its block only if that condition is true

• If the condition is false, the block is skipped and flow control passes to the next statement after the IF block

score ← 72

IF (score ≥ 50)
{
   DISPLAY("Pass")
}
 

• If score is 72, the condition 72 ≥ 50 is true, so "Pass" is displayed

• If score were 40, the condition would be false and nothing would be displayed

Branching & nested conditionals

What is branching logic?

• An IF-ELSE statement creates two branches: one that executes when the condition is true, and one that executes when the condition is false

• Exactly one branch always executes; unlike a plain IF statement, an IF-ELSE guarantees an outcome in both cases

IF (score ≥ 50)
{
   DISPLAY("Pass")
}
ELSE
{
   DISPLAY("Fail")
}
 

Nested conditionals

• A nested conditional is an IF or IF-ELSE statement placed inside another IF or IF-ELSE block

• Nested evaluation works from the outside in: the outer condition is checked first; the inner condition is only evaluated if the outer condition is true

• Nested conditionals allow programs to test multiple conditions in sequence

IF (score ≥ 50)
{
   IF (score ≥ 90)
   {
      DISPLAY("Honors pass")
   }
   ELSE
   {
      DISPLAY("Pass")
   }
}
ELSE
{
   DISPLAY("Fail")
}
 

• The inner conditions are only reached if the outer condition (score ≥ 50) is already true

• A score of 95 displays "Honors pass", a score of 72 displays "Pass", and a score of 40 displays "Fail"