For, Range, and Iteration Patterns

What a for loop does

A for loop takes values from an iterable one at a time. On each iteration, Python assigns the next value to the loop variable and runs the indented block.

for ch in 'PCEP':
    print(ch)

The loop variable ch becomes 'P', then 'C', then 'E', then 'P'. The loop does not need a manual index unless the code actually needs positions.

The range function

range() is the most common PCEP loop source. It creates a sequence-like object that produces integers.

The stop value is never included. This is the source of many off-by-one exam mistakes.

Negative steps and empty ranges

A negative step moves downward. The start must be greater than the stop for the range to produce values.

print(list(range(5, 0, -2)))  # [5, 3, 1]
print(list(range(0, 5, -1)))  # []

The second range is empty because it starts at 0 and tries to move down while the stop is 5. It is already past the stopping boundary for a negative step. Also, range(1, 5, 0) is an error because the step cannot be zero.

Iterating over data

For loops can iterate directly over strings and lists:

letters = ['a', 'b', 'c']
word = ''

for letter in letters:
    word += letter.upper()

print(word)  # ABC

When indexes are needed, use range(len(data)):

items = [10, 20, 30]
for i in range(len(items)):
    print(i, items[i])

The values of i are 0, 1, and 2. Do not confuse the index with the item. In for item in items, item is 10, then 20, then 30. In for i in range(len(items)), i is the position.

Trace-table method

Before solving a for-loop question, write the iteration list first:

Then trace variables changed in the body:

Loop variable after the loop

If a for loop runs at least once, the loop variable still exists after the loop and holds the last value assigned. After for n in range(3):, n is 2. If the loop never runs and the variable was not already defined, reading it afterward raises NameError. PCEP may use this to test whether you noticed an empty range.

Exam traps

Do not assume a loop runs because it is written. An empty range runs zero times. Do not include the stop value. Do not assume the loop variable resets automatically after the loop; if the loop ran, it still refers to the last assigned value. Finally, read whether the code iterates over values or indexes before deciding what gets printed.