Logical operator NOT

Computer Science · Logical expressions

Same card, two questions

Ask a yes/no question about the card. Then ask not, which turns that answer round.

Let A be the question suit == "hearts". Step through two cards. Before each Next, predict: what will not A be?

Computer Science · Operators

Compare, or combine?

Here are parts of three expressions: age > 12 and age < 60, value > 7 or suit == "spades", and not (suit == "hearts"). Is each highlighted piece a relational operator or a logical operator?

6 of 6 still to sort.

not lives in a family of operators. Which job does each piece do?

Predict, then check

Comparison first. Flip second.

A card has the value 3. What does not (value > 7) give?

Computer Science · Selection

Which way does the program go?

The program checks: if not (suit == "hearts"), run the if branch, otherwise run the else branch. Pick a card and follow the route.

Card drawn

On if not (suit == "hearts"). 3 branches to choose from.

A selection statement follows exactly one route. The condition decides which.

What you need to know

• A logical expression always ends up as True or False, and the three basic logical operators are and, or and not.
• Relational operators compare values. Logical operators then combine or reverse those True/False results into a single value.
• You can build bigger expressions, like age > 12 and age < 60, and the whole thing still returns just one True or False.
• The odd one out is not: it acts on a single expression, whereas and and or each combine two.
• Have a goThree logical operators: and, or, not. Which one is the odd one out, and what makes it odd?

  not. It acts on a single expression.

  and and or each need two expressions to combine, so not is the only one of the three with a single input.

• What not does is reverse the result. If the comparison is True, not gives False, and if it is False, not gives True.
• So the truth table for not A has two rows: not True gives False, and not False gives True.
• Have a goYour friend Sam says, "I know this one: not always gives False." Use the truth table to catch Sam out. What is not False?

  True

  not reverses its input, so a False input gives True. Sam only has half of the truth table.

• Work from the inside out: evaluate the comparison first, then apply not to the True or False it gave you.
• Have a goA card has the value 9 and the test is not (value > 7). Comparison first. What is the final answer?

  False

  9 > 7 is True, and not reverses True to False. Stopping at the comparison would give True, which skips the flip.

• Card check: for the ten of spades, suit != "hearts" returns True, because the suit is spades, not hearts.
• In a selection statement, a True condition runs the if branch and a False condition runs the else branch.

The big picture

The not operator is a logical operator that acts on a single expression and reverses its True or False result: not True is False and not False is True. Work out the comparison first, then flip it. That one flip can send a selection statement down the other branch.

⚠ Watch out

Applying not before you have an answer to flip. not works on the True or False that the comparison gives, so work that out first. Skip it and there is nothing for not to reverse.

Memory hook

Compare first, flip second. not never picks an answer; it only turns the answer round.

Check yourself

Your friend says not (suit == "hearts") is False for the ten of spades. Which step did they skip, and what is the real answer?