Calculations and comparisons

Now that we can store values, let’s do something with them. If the robot travels three metres in two seconds, how fast is it moving? Python can calculate that for us. The symbols used for calculations and comparisons are called operators.

Calculations, comparisons, and units¶

For numbers, +, -, *, and / mean addition, subtraction, multiplication, and division. ** raises to a power. // performs floor division and % gives the remainder. For example, 7 // 3 is 2 and 7 % 3 is 1. Use parentheses to make grouping clear: 2 + 3 * 4 is 14, while (2 + 3) * 4 is 20.

Comparisons return a boolean. == means equal, != not equal, < less than, > greater than, and <=/>= include equality. Remember that = assigns; == asks a question. Combine boolean questions with and, or, and not: and needs both conditions, or needs at least one, and not reverses a boolean.

Units are part of the meaning even when Python doesn’t enforce them. Dividing metres by seconds gives metres per second. Use names or comments that make units clear; dividing centimetres by seconds would produce a different numeric scale.

Read it before running it¶

Put this worked example in student-work/foundations/operators.py. Read it once and write down what you think it prints. Then save and run python student-work/foundations/operators.py using your environment’s Python.

distance_m = 3.0
elapsed_s = 2.0
speed_m_per_s = distance_m / elapsed_s
enabled = True
under_limit = speed_m_per_s <= 2.0
print("Speed:", speed_m_per_s)
print("Allowed:", enabled and under_limit)

This prints speed 1.5 and allowed True. Change the distance to 6 metres: the speed becomes 3.0 and the combined question becomes false. Then make enabled false and use a small distance. Why does the result remain false?

Try to calculate speed with zero elapsed time and you’ll get ZeroDivisionError. There isn’t a meaningful speed calculation from those inputs, so read that error as a clue to check your measurements.

One detail we’ll return to later: computers store many decimal numbers approximately. When checking real measurements, we’ll allow a small range around the target instead of expecting every decimal to match exactly. You don’t need that extra machinery for this exercise.

Try it yourself¶

distance_m = 4.0
elapsed_s = 2.0
speed = distance_m / elapsed_s
print(speed, speed <= 2.0)
distance_m = 5.0
speed = distance_m / elapsed_s
print(speed, speed <= 2.0)

Before moving on¶

Explain the difference between = and == without looking. Predict the result for exactly 0 metres in 2 seconds, then check it.