Loop body Statements

Here is the program again:

' 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 statements in the loop body have been carefully arranged to do the work properly. This is not an easy thing to get right. Say that the user has just entered 4. Here is how the statements are arranged so that 4 is added to SUM:

• The DO WHILE "gatekeeper" tests NUMBER.
• 4 is not equal to 0, so execution enters the loop body.
• The LET statement adds 4 to SUM.
• The PRINT statement prompts the user for the next value.
• The INPUT statement gets the next value, whatever it is (it could be a 0). The value is stored in NUMBER.
• Execution is sent back to the DO WHILE so that the "gatekeeper" can test the new value in NUMBER.

Now everything is repeated with something new in NUMBER (if the user enters a 0, the DO WHILE will send execution to the PRINT statement after the LOOP.)