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Variables and operators

Variables

A variable is a container that stores a value. You create one by giving it a name and assigning a value with =:

power = 30

You can read the value later, change it, or use it in a calculation. The name on the left always receives the value on the right.

power = 30
power = power + 10   # power is now 40

Data types

A data type describes what kind of value a variable holds. You need four of them:

int_variable = 1        # whole numbers, can be negative
float_variable = 1.0    # decimal numbers, can be negative
string_variable = "1"   # text, written in quotes
bool_variable = True    # True or False only
Type Name in Python Example
Integer int 1, 0, -45
Float float 1.0, 0.5, -12.75
String str "M1", "hello"
Boolean bool True, False

1 and "1" are not the same

1 is a number you can do maths with. "1" is text. Quotes make the difference.

In MARLIN code you meet all four: port names like "M1" are strings, thruster duty is a number, and robot.running is a Boolean.

Operators

Operators are symbols that perform an operation on values. There are four groups you need.

Arithmetic operators

These do maths and give back a number.

a = 5
b = 7

print(a + b)    # Addition -> 12
print(a - b)    # Subtraction -> -2
print(a * b)    # Multiplication -> 35
print(a / b)    # Division -> 0.7142857142857143
print(a // b)   # Floor division, drops the remainder -> 0
print(a % b)    # Modulus, the remainder only -> 5
print(a ** b)   # Power, a to the power of b -> 78125

The results above are passed straight to print(). They are not stored, so a and b do not change.

You can combine int and float values freely. You cannot combine numbers with strings or Booleans directly.

Print things

print() is your main debugging tool. When you are not sure what a value is, print it and look.

Assignment operators

These change the value of a variable. They are shortcuts.

a = 5
b = 7

a += b   # same as a = a + b
a -= b   # same as a = a - b
a *= b   # same as a = a * b
a /= b   # same as a = a / b
a %= b   # same as a = a % b
a **= b  # same as a = a ** b

Only a changes. b is left alone.

Comparison operators

These compare two values and return a Boolean.

a = 5
b = 7

print(a == b)   # Equal -> False
print(a != b)   # Not equal -> True
print(a > b)    # Greater than -> False
print(a < b)    # Less than -> True
print(a >= b)   # Greater than or equal -> False
print(a <= b)   # Less than or equal -> True

= is not ==

= assigns a value. == asks whether two values are the same. Mixing them up is one of the most common beginner bugs.

Logical operators

These combine conditions and return a Boolean. Use them when you have more than one condition to check.

Operator Result
and True only if both conditions are true
or True if one or both conditions are true
not Inverts the condition 
a = 5
b = 7

print(a > b and a < b)   # False
print(a > b or a < b)    # True
print(not a > b)         # True

You can chain them, and brackets control the order:

print((a > b and a < b) or a == b)   # False

This matters on a robot, where you often need two things to be true at once:

if reading.ok and robot.running:
    ...

Full documentation: https://docs.python.org/3/library/stdtypes.html

Data structures: how to store many values in one variable.