Integers & Floats

整数与浮点数

≈ 15 min · python-types · Open in WolfCode

Python's two numeric types — int for whole numbers, float for fractions — look similar but behave differently. Knowing when each one shows up will save you a lot of confusing bugs.

Two kinds of numbers

In Python, 3 and 3.0 are not the same value. The first is an int (integer); the second is a float (floating-point number). You can ask Python which is which:

Python · runnable

print(type(3))
print(type(3.0))
print(type(3 + 4))
print(type(3 + 4.0))    # mixing int and float → result is float

Loading Python runtime (~5 MB, one-time)…

The rule for arithmetic: if all operands are int, the result is int. As soon as one float sneaks in, the whole expression "promotes" to float.

Operators you'll use every week

+ addition
- subtraction
* multiplication
/ true division — always returns a float
// floor division — drops the remainder, returns the smaller type
% modulo — the remainder
** exponent — 2 ** 10 is 1024

The / vs // distinction trips up almost everyone

print(10 / 4)     # → 2.5      (true division, always float)
print(10 // 4)    # → 2        (floor division, drops the .5)
print(10 % 4)     # → 2        (the remainder)

# // and % together = "how many full groups, and how many left over?"
minutes = 137
hours = minutes // 60
left  = minutes % 60
print(hours, "hours", left, "minutes")

Loading Python runtime (~5 MB, one-time)…

Converting between types

The functions int(), float(), and str() take a value and return a new value of the requested type. They do not change the original.

Python · runnable

x = 5.9
print(int(x))        # → 5   (truncates toward zero, doesn't round!)
print(round(x))      # → 6   (this one rounds)

n = 7
print(float(n))      # → 7.0
print(str(n))        # → '7'  (now it's a string — useful for printing)

# What int() does to negatives surprises some people:
print(int(-5.9))     # → -5, not -6

Loading Python runtime (~5 MB, one-time)…

Check yourself

What does this print? x = 7 / 2 → print(type(x))

Check yourself

`x = int(3.9)` — what is x?

The exercise in WolfCode

Open 02-types-int-float.py. Four variables are set to None — replace each with the correct expression. The tests check both the value and the type.