Sentinel Values

The program can be used to add up a list of numbers. But the user first has to count up how many numbers are to be added up. This could be annoying. It would be nice to have a program that keeps adding up numbers until you tell it to stop. Here is a program that does that. The user will enter numbers, one by one, and the program will add each number to SUM. The user tells the program to stop by entering a zero. The zero is how the user "signals" that the program should stop looping.

A sentinel value is a special value that is tested in a DO WHILE statement to determine if the loop should end.

The program is not quite finished, yet.

' Add up numbers that the user enters.
' When the user enters 0, print the sum and end the program.
'
LET SUM = 0
'
PRINT "Enter a number"       
INPUT NUMBER                 
'
DO WHILE NUMBER ______ 0
  LET SUM = SUM + NUMBER   
  PRINT "Enter a number. (Type 0 to stop)"       
  INPUT NUMBER                 
LOOP
'
PRINT "The sum is", SUM
END

The DO WHILE statement acts like a gatekeeper. It allows execution to (re-)enter the loop body only if the condition is true. Say that the program has just started. The user wants to add up a list of numbers. The first number is 4, so the user enters it:

Enter a number
? 4

Now the program should execute the first statement in the loop body.