Why 5 + 5 can be 55

A user types 5 and 5 into your calculator. Your program shows 55. It should show 10.

The reason is the data type. A data type is the kind of value. What a user types with input() is always text. Text joins together. It does not add. To add, you must change the text into numbers.

Key words

Word	What it means	Example
Type conversion	Changing a value from one data type to another.	text "5" to number 5
Data type	The kind of value.	integer, string
Integer	A whole number.	5
Float	A number with a decimal point.	2.5
String	Text inside quote marks.	"5"
Boolean	True or False.	True
Function	A named piece of code that does one job.	int()
Concatenation	Joining strings together with +.	"5" + "5" is "55"

The cells students fill in are left blank in this preview.

Try it: text or numbers?

Code: python

print("5" + "5")
print(5 + 5)
print(int("5") + int("5"))

# The first joins text. The second adds numbers.
# The third changes text to numbers first.

Four conversion functions

Python gives you four functions to change type. Each one takes a value and gives back the same value in a new type.

int() makes a whole number. float() makes a number with a decimal point. str() makes text. bool() makes True or False.

Worked example: what each one gives

Code	Result	Type of result
int("7")	7	Integer
int(4.9)	4 (it cuts off the decimal part, it does not round)	Integer
float(3)	3.0	Float
float("2.5")	2.5	Float
str(12)	"12"	String
bool(0)	False	Boolean
bool(5)	True (any number except 0 is True)	Boolean
bool("hello")	True	Boolean
bool("")	False (empty text)	Boolean

The cells students fill in are left blank in this preview.

Try it: use all four

Code: python

age_text = "15"
age = int(age_text)
print(age + 1)

price = float("2.50")
print(price * 3)

score = 7
print("Score: " + str(score))

print(bool(0))
print(bool(5))

# Try int("hello") and see the error.

Not every conversion works

data-callouts.warning.label

int("hello") stops the program with a ValueError. Python cannot turn the word hello into a number. int("2.5") also fails, because 2.5 has a decimal point. Data from a user can be wrong, so check it first.