The for Loop and range()

Python's for is not a counted loop. It is a for-each: it walks through the items of a sequence.

for name in ["Anjali", "Rahul", "Meera"]:
    print(name)

There is no counter, no condition, no increment. To count, you iterate over a sequence of numbers — and range() produces one.

range()

Two rules that account for most mistakes:

1. The start is included, the stop is excluded. range(1, 6) gives 1 to 5. To count 1 to n you write range(1, n + 1).

2. Counting down needs a negative step. range(10, 0, -1) stops at 1, not 0, because the stop is still excluded.

How many iterations?

For range(a, b) the count is $b - a$ — no +1, unlike the pseudocode FOR i ← a TO b in Module 2, which included both ends. Mixing the two conventions is the single commonest off-by-one in this course.

Pseudocode:  FOR i ← 1 TO 5     runs 5 times   (1,2,3,4,5)
Python:      range(1, 5)        runs 4 times   (1,2,3,4)
             range(1, 6)        runs 5 times   (1,2,3,4,5)

Iterating a sequence directly

If you don't need the index, don't create one:

total = 0
for mark in marks:          # preferred
    total += mark

for i in range(len(marks)): # avoid when i is unused
    total += marks[i]

When you need both index and value, enumerate gives you both:

for i, mark in enumerate(marks):
    print(f"Student {i + 1}: {mark}")

Accumulator pattern

Same as Module 2 — initialise before the loop, update inside:

total = 0                 # 0 for sums
for x in numbers:
    total += x

product = 1               # 1 for products
for x in numbers:
    product *= x

Loop variable after the loop

The loop variable survives, holding the last value used:

for i in range(3):
    pass
print(i)        # 2

If the range was empty the variable never existed, and referring to it raises NameError. Don't rely on it.

break, continue, else

for x in numbers:
    if x < 0:
        print("Found a negative")
        break            # leave the loop entirely
    if x == 0:
        continue         # skip to the next item
    print(x)
else:
    print("No negative found")

The for … else is unusual: the else runs only if the loop finished without break. It is genuinely useful for searches, and it is examinable because it surprises people.