We looked at the basic programming constructs of sequence, selection, and iteration in 1.02 (Pseudocode) and 1.03 (Flowcharts). We will go into more detail here.

Example 1 shows a basic IF statement.

Example 1
passcode ← USERINPUT
IF passcode = 1234 THEN
  OUTPUT "OK"
ELSE
  OUTPUT "Wrong password"
ENDIF

A nested IF statement means an IF statement inside another IF statement.

Example 2 shows a nested IF statement.

Example 2
user ← USERINPUT
passcode ← USERINPUT
IF passcode = 1234 THEN
  IF user = "admin" THEN
    OUTPUT "Full access"
  ELSE
    OUTPUT "Restricted access"
  ENDIF
ELSE
  OUTPUT "Wrong password"
ENDIF

ELSE IF statements can be used to chain together multiple conditions.

Example 3 shows an IF statement with multiple ELSE IF statements.

Example 3
option ← USERINPUT
IF option = 1 THEN
  option_1()
ELSE IF option = 2 THEN
  option_2()
ELSE IF option = 3 THEN
  option_3()
ELSE IF option = 4 THEN
  option_4()
ELSE
  OUTPUT "Invalid"
ENDIF

ELSE IF statements can often be simplified by using CASE statements.

Example 4 shows an algorithm which performs exactly the same task as the one in Example 3.

Example 3
option ← USERINPUT
CASE option OF
  1: option_1()
  2: option_2()
  3: option_3()
  4: option_4()
  DEFAULT: OUTPUT "Invalid"
ENDCASE

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How does including a DEFAULT case in a CASE statement make the code more robust?

It's a good idea to make sure the program knows what to do if it receives an unexpected input. Otherwise, it might crash.

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