Data types and casting

Choosing a data type

A data type tells you what kind of value a variable stores. Choose one that suits the value:

Type	Use it for	ERL	Python
Integer	Whole numbers, such as a score or count	`INTEGER`	`int`
Real	Measurements or other numbers with a fractional part	`REAL`	`float`
Boolean	A yes/no state or a condition	`BOOLEAN`	`bool`
Character	One letter, digit or symbol	`CHARACTER`	A one-character `str`
String	Text with zero or more characters	`STRING`	`str`

Python infers types from assigned values. Both characters and strings use str in Python; use a character when exactly one symbol is intended. For example, count = 12 is an integer and height = 1.65 is a real/float.

ERL

count = 12
height = 1.65
ready = count > 0
initial = "A"
name = "Ada"

Python

count = 12
height = 1.65
ready = count > 0
initial = "A"
name = "Ada"

Key terms

Casting input to a number

When a program reads a number from the keyboard, convert the input text before doing arithmetic. Use int() when the answer must be a whole number and float() when it may contain a decimal point:

ERL

INPUT age
INPUT height
OUTPUT age + 1
OUTPUT height

Python

age = int(input("Age: "))
height = float(input("Height: "))
print(age + 1)
print(height)

With inputs 15 and 1.6, the program outputs 16 and 1.6. In Python, int("15") works, but int("1.6") raises ValueError; use float("1.6") for a decimal. If unconverted input is added to a number, Python reports a type error.

Key terms

Casting between types

Casting changes a value from one data type to another:

Cast	Example	Result
To a string	`str(42)`	`"42"`
To an integer	`int("15")`	`15`
To a real	`float("2.5")`, or `real("2.5")` in ERL	`2.5`
To a Boolean	`bool("True")` in ERL	`true`

Cast to a string to join a number onto text, as in "Score: " + str(score). Cast to a number before doing arithmetic or comparing values as numbers.

In Python, bool() of any non-empty string is True, even bool("False"), so compare the text instead: answer == "True".

Why the type matters

Strings are compared character by character, not as numbers. So "10" < "9" is true: the first characters are compared, and "1" comes before "9". As numbers, 10 < 9 is false.

This matters whenever a program sorts or compares values typed in by a user. Cast the inputs to numbers first, and the comparison works as expected.

Common mistake

A phone number is stored as a string, not an integer: it's never used in arithmetic, and an integer would lose its leading 0.

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