For loops and while loops

Learning objectives

• Trace a loop over range.
• Guard a while loop with a changing condition.

A bounded sequence

total = 0
for number in range(1, 5):
    total += number
print(total)

range(1, 5) yields 1, 2, 3, and 4: the start is included and the stop is excluded. The trace adds each value, so the final output is 10.

While and termination

A while reevaluates its condition before every iteration. Ensure the body changes a value used by that condition, or the loop can be infinite.

Common mistakes and references

• Do not modify a collection while iterating over it unless you understand the consequences.
• Use break for an exceptional early exit, not to hide the normal termination rule.

• Python tutorial: for statements
• Python documentation: range