What is a double?

There are 3 ways to store numbers: int, double and float.

int - only whole numbers

55
409834
45425346

float - up to 7 decimal places (32-bit)

0.1234567
1234.7654321
22.45

double - up to 15 decimal places (64 bit)

0.123456789123456
22.45
55.0

you do not have to use all the decimal places, it is just imporant to know what type of number will be used and then pick the type with the least amount of decimal places that will hold that number. Example: if you need 9 decimal places, pick double. If you need 6 decimal places, pick float. If you don't need any decimal places then use int.

Unlike integers (type: Int) which are whole numbers without fractional values, i.e.

1
250
55234125134124

Doubles can be numbers with fractional values, such as

6.234253
8.8
83765438762.357

You can tell when a number is a Double when the number includes a decimal point (also known as a floating point).

Whole numbers can be represented as a Double by adding ".0" at the end:

1.0
250.0
55234125134124.0

Bonus fact: Doubles in Swift are numbers with at least 15 decimal place precision.

double is just basically a floating point or basically decimals.

From Apple's documentation:

"Floating-point numbers are numbers with a fractional component, such as 3.14159, 0.1, and -273.15."

A Float is 32-bit floating point number and a Double is a 64-bit floating point number.

Because Float has some accuracy issues, it's always considered best practice to use Double when dealing with currency.