QBasic Two-way Decision

The "windshield wiper" decision is a two-way decision (sometimes called a "true-false" decision.) It seems small, but in programming very complicated decisions can be made up of many small decisions.

Here is a QBasic program that includes a two-way decision:

PRINT "Enter a Number"
INPUT NUMBER
'
IF NUMBER >= 0 THEN
  PRINT "The number is zero or positive"    ' true branch
ELSE
  PRINT "The number is negative"    ' false branch
END IF
'
PRINT "Bye"     ' this statement is always done
END

The words IF, THEN, ELSE, and END IF are brackets that divide the part of the program into two branches. The ELSE is like a dividing line between the "true branch" and the "false branch".

• The IF statement always asks a question (usually about the number in a variable).
  • The answer will be TRUE or FALSE.
• If the answer is TRUE only the true-branch is executed.
• If the answer is FALSE only the false-branch is executed.
• No matter which branch was chosen, execution continues with the statement after the END IF.

A two-way decision is like picking which of two roads to take to the same destination. The fork in the road is the IF statement, and the two roads come together just after the END IF statement.