Selection is the process of making a decision based upon the available data. All decisions in a programming language are based on a condition, or multiple conditions. It is important to remember to write our conditions so they can resolve to either true or false, depending on the data.
Conditions, or conditional statements, use the Relational Operators to compare values (see link to refresh your memory of relational operators)
The simplest form of selection is the IF statement.
if (condition is true) then do something
In Turing, the IF statement has a specific form. In this case, we are only interested in the true outcome for the selection, so we have one possible outcome.
if (age >= 16) then
put "You are old enough to drive"
end if
For situations where we are interested in both the true and the false outcome, we add the else statement, and now have two possible outcomes.
if (age >= 16) then
put "You are old enough to drive"
else
put "Sorry, you're not ready to drive yet."
end if
For more than two outcomes, it is possible to use nested-if statements to handle the logic. Fortunately, Turing provides a simpler form called the "else if", which Turing writes as "elsif". This allows a condition for each outcome. It is important to note that the conditions will be evaluated in order, and the first true condition is the only one that gets processed! Compare the following fragments of code:
| Example 1 - works correctly | Example 2 - will not catch older drivers |
|
if (age >= 70) then put "You can drive after an eye exam" elsif (age >= 16) put "You are old enough to drive" else put "Sorry, you're not ready to drive yet." end if |
if (age >= 16) then put "You are old enough to drive" elsif (age >= 70) put "You can drive after an eye exam" else put "Sorry, you're not ready to drive yet." end if |
It is possible to create more complicated selection statements by using multiple conditions. Conditions are combined through the use of the Logical Operators (and, or).
if ( age >= 18 ) and ( citizenship = "Canadian" ) and ( validIdentification >= 2 ) then
put "Allow this person to vote here."
else
put "This person must fill out a special form in order to vote."
end if