Conditional Statements
Keith Burgess-Jackson
1. You’d better be in your smallest gear at the foot of the climb, or you won’t make it to the top.
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S or not-M (abbreviate)
S unless not-M (replace “or” with “unless”)
S if not not-M (replace “unless” with “if not”)
If not not-M, then S (invert)
If M, then S (double negate)
If not-S, then not-M (contrapose)
Unless S, not-M (replace “if not” with “unless”)
M only if S
S if M
Not-M if not-S
Not-M unless S (replace “if not” with “unless”)
Only if S, M
M is a sufficient condition for S
S is a necessary condition for M
2. You can’t drink alcohol in Texas unless you’re at least 21 years old.
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Not-D unless T (abbreviate)
Not-D if not-T (replace “unless” with “if not”)
If not-T, then not-D (invert)
If D, then T (contrapose)
Unless T, not-D
D only if T
T if D
Only if T, D
D is a sufficient condition for T
T is a necessary condition for D
Not-D or T (replace “unless” with “or”)