Sequence Selection Iteration

3. Selection: Comparisons

Other than for the most basic tasks, most programs involve having to make choices. Maybe you want to do one thing if a condition is true and another if it is not. The process of determining which choice to take is called 'selection'.

Comparison operators

Selection always involves a comparison operation, where you check whether certain conditions are met. The comparison operators are

The last one (==) is often confused with the equal sign =. They are not the same thing.

If you use =, you are setting (aka assigning) one value to be equal to another.

If you use ==, you are comparing one value to another to see if they are equal.

For example if you say "chalk = cheese", then you are telling the computer that chalk is cheese. If you say "chalk == cheese", you are asking the computer to check whether chalk is cheese.

Comparison - Boolean operators

This is a logical comparion, the result is either logical TRUE or logical FALSE.

The comparison is laid out for the AND operator as

Expression 1 AND Expression 2

The OR operator is similar with

Expression 1 OR Expression 2

The NOT operator only uses one expression

NOT (Expression)

The boolean operators are

For example (Expression 1 AND Expression 2) the comparison leads to a TRUE result only if both expressions are also TRUE, otherwise the comparison leads to FALSE

And for the OR operator, the statement (Expression 1 OR Expression 2) is TRUE if either one or both is TRUE. If both expressions are FALSE then the comparison is also FALSE

Combine comparisons

You can combine size operators with a boolean operator to make two comparisons in the same statement, like this

                     (a < 2) AND (b > 4)

The boolean AND operator means that both expressions need to be true for the evaluation to be true.