1. Conditional or Decision Logic
INT213
Conditional or Decision Logic

2. Simple IF statement IF true THEN run_this
true is a resolved logical expression
run_this is a single command
IF mark >= 55 THEN grade = "PASS"
When the logical expression resolves to false, THEN is not done.

3. Simple IF statement IF true THEN run_this run_that END IF
runs multiple commands
easily readable even for single commands IF mark >= 55 THEN grade = "PASS" semester = semster + 1 END IF ' mark >= 55

4. Either / Or Logic
IF true THEN run_this ELSE ' above IF is false run_that END IF IF mark >= 55 THEN grade = "PASS" semester = semster + 1 ELSE ' mark is less than a pass grade = "FAIL" END IF ' mark >= 55 (or not)

5. Multiple Exclusive Conditions IF ... ELSEIF ... ELSEIF ... ELSE ... END IF
IF mark >= 80 THEN grade = 'A' gpa = ELSEIF mark >= 70 THEN grade = 'B' gpa = ELSEIF mark >= 60 THEN grade = 'C' gpa = 2.0
ELSEIF mark >= 55 THEN grade = 'D' gpa = ELSE grade = 'F' gpa = END IF ' marks, grades

6. SELECT CASE instead of IF...ELSEIF for a list of equality tests
SELECT CASE grade ' match Grade to GPA
CASE "A+", "A"
gpa = 4.0
CASE "B"
gpa = 3.0
CASE "C"
gpa = CASE "D"
gpa = CASE ELSE ' all other grades: F, DNC, ***, gpa = 0.0 END SELECT

7. Boolean Logic AND / OR used to evaluate multiple logical conditions
AND means everything joined by AND must be true – a logical conjunction
OR is a new set of logical condition(s) – a logical disjunction
IF school="ICT" AND program="CNS" _ OR school="ICT" AND program="CTY" THEN

8. Boolean Logic use (parenthesis) to change evaluation order
IF school="ICT" AND _ (program="CNS" OR program="CTY") THEN
evaluates as
IF school="ICT" AND true THEN
IF true AND true THEN
IF true THEN

9. Boolean Logic – AND vs OR
check for one of a set of allowed values
program = "CNS" OR program = "CTY" OR …
check if NOT in a set of allowed values
program<>"CNS" AND program<>"CTY" AND …
this will NEVER be true:
program = "CNS" AND program = "CTY"
this will ALWAYS be true:
program <> "CNS" OR program <> "CTY"

10. Nested IFs IF school = "ICT" THEN required = "APC100,EAC150,ICA002"
IF program = "CTY" then
isINT400required = TRUE
ELSE ' programs other than CTY
isINT400required = FALSE
END IF ' program is CNS | CTY
END IF ' school = "ICT"

11. Gotchas ELSEIF vs ELSE IF watch for message: Expected 'End'
do not add END IFs to make it stop complaining
IF – ELSEIF – ELSEIF – END IF
cascades from one to the other
only one will happen
IF x = y THEN ELSE IF a = b THEN ' evaluated only when x <> y END IF END IF
nested IFs. More than one or none may happen.

12. Gotchas ALWAYS use a logical expression.
Say what you mean. Do not use "trick" of assuming Zero means False and non-Zero means True. VBscript does not do this well.
IF number THEN
0 is FALSE as expected
NOT 0 is TRUE -- as expected
1 is TRUE as expected
NOT 1 is TRUE ** not as expected