Operators and Expressions

Expressions and operators

An expression is any piece of code that Python can work out to a single value, such as 2 + 3 or mass / volume. Expressions are built from values, variables and operators: symbols that perform an operation, like + for addition.

Arithmetic operators

// and % are a pair. // tells you how many whole times one number fits into another, and % tells you what is left over. For example, 130 minutes is 2 whole hours (130 // 60) and 10 minutes (130 % 60).

total_minutes = 130hours = total_minutes // 60minutes = total_minutes % 60print(hours, "h", minutes, "min")

Order of operations

Python follows the same order of operations as mathematics. Powers are worked out first, then multiplication and division, then addition and subtraction. Operators with the same priority are worked out from left to right. Brackets override all of this: anything inside brackets is calculated first.

print(2 + 3 * 4)     # multiplication firstprint((2 + 3) * 4)   # brackets first

When you turn a scientific formula into code, add brackets wherever they make the meaning clearer, even if they aren't strictly needed. A common mistake is forgetting brackets around a denominator: a / b + c is not the same as a / (b + c).

atoms_per_cell = 4molar_mass = 63.55               # g/molcell_volume = (3.615e-8) ** 3    # cm³ (3.615 Å = 3.615e-8 cm)avogadro = 6.022e23density = (atoms_per_cell * molar_mass) / (cell_volume * avogadro)print("Density:", round(density, 2), "g/cm³")

Assignment operators

Updating a variable based on its own value is so common that Python has shortcuts for it. total += 5 means exactly the same as total = total + 5.

total_mass = 0.0total_mass += 12.5   # add the first sampletotal_mass += 8.3    # add the second sampleprint(total_mass)

Comparison operators

A comparison checks how two values relate and gives a Boolean result: True or False. Comparisons are how programs make decisions, which you will use in the Control Flow module.

melting_point = 1085   # °C, copperprint(melting_point > 1000)print(melting_point == 1000)

Combining conditions

The logical operators and, or and not combine Boolean values. and is True only if both sides are true; or is True if at least one side is true; not flips True to False and back.

density = 2.70          # g/cm³melting_point = 660     # °Cis_light = density < 5is_high_temperature = melting_point > 1000print(is_light and is_high_temperature)print(is_light or is_high_temperature)print(not is_light)

Built-in functions and the math module

Python has some numeric functions built in. abs() gives the absolute value, round() rounds, and min() and max() return the smallest and largest of several values.

print(abs(-3.2))print(round(2.71828, 3))print(min(660, 1085, 1538))print(max(660, 1085, 1538))

For square roots, logarithms, trigonometry and constants such as π, use the math module. A module is a file of ready-made code. import math makes it available, and you then use its contents with a dot: math.sqrt(), math.pi.

import mathradius = 1.28e-8   # cm, atomic radius of copperatom_volume = 4 / 3 * math.pi * radius ** 3print(math.sqrt(16))print(round(math.pi, 4))print(atom_volume)