Data Types, Assignment, and Tracing

This summary covers the four kinds of values we have met, how to tell them apart by how they are written, what the arrow and = really do, and how to trace a few lines of code.

Four kinds of values

Every value a program keeps is one of a few kinds. Swift calls a kind a type. Here are the four from the grid, and how to recognise each one by how it is written.

Value	Kind	Swift type	How you can tell
7	a whole number	Int	digits only, no decimal point
7.5	a number with a decimal point	Double	it has a decimal point
"7"	text	String	it is in quotation marks
false	a yes-or-no answer	Bool	it is true or false

Quotation marks change everything. 7 is a number, so 7 + 7 is 14. "7" is text, so "7" + "7" is "77": text is joined, not added.

Assignment

A line that stores a value in a name is an assignment statement, or just an assignment. The AP exam writes it with an arrow. Swift writes it with =, the assignment operator. They mean the same thing: work out the value on the right, then store it in the name on the left.

a ← 5

var a: Int = 5

Neither one means “equals” in the math sense. That is why this line makes sense to a computer, even though it is nonsense as an equation:

a ← a + 3

The right side is worked out first, using the value a holds now. The result is then stored back in a, and the old value is gone.

Whole numbers divide into whole numbers

In Swift, 7 / 2 is 3, not 3.5. When both numbers are Int, the answer is an Int, and whatever is after the decimal point is dropped, not rounded. Write 7.0 / 2 to get 3.5.

Copies, not links

Example 1

Trace this code one line at a time, keeping a column for each name.

a ← 5
b ← a
a ← a + 3
DISPLAY(b)
DISPLAY(a)

Line	a	b
a ← 5	5
b ← a	5	5
a ← a + 3	8	5

It displays 5 and then 8.

b ← a copies the value in a into b once. After that, nothing links them. Changing a later does not change b.

The same code in Swift

Example 2

var a: Int = 5
let b: Int = a
a = a + 3
print(b)
print(a)

a changes on the third line, so it must be a var. b is set once and never changes, so it can be a let. Both hold whole numbers, so their type is Int. Where the AP code says DISPLAY, Swift says print: that is how a command-line Swift program shows a value. Swift has a shorter way to write the third line, which the Guessing Game uses: a += 3.