Sum and Count with a Python for Loop

Add Up Numbers with a Running Total

To add up numbers in a for loop, keep a running total: a variable that starts at 0 and grows by each item with += (chapter 5). This is called the accumulator pattern ("accumulate" means "collect bit by bit").

12345prices = [40, 25, 60]
total = 0
for price in prices:
    total += price
print("Total:", total)

Think of it like a shop's cash register: it starts at 0, and every item the cashier scans is added to the amount on the screen.

The three steps

1. Before the loop: start the total at 0.
2. Inside the loop: add the current item, total += price.
3. After the loop: use the result, here print().

Watch the total grow by printing it inside the loop:

1234total = 0
for price in [40, 25, 60]:
    total += price
    print("so far:", total)

Add up input

With range() and input() from the last topic, you can add numbers the user types:

1234total = 0
for i in range(3):
    total += int(input())
print("Total:", total)

For a list you already have, sum(prices) from chapter 9 gives the same answer in one go. The loop shines when the numbers arrive one at a time, or when you only want to add some of them (next lesson).

Common mistakes

• Forgetting total = 0: Python stops with a NameError, because total += price needs a value to add to.
• Putting total = 0 inside the loop: the total is reset every time and ends up as the last price only.

Start at 0 before the loop, add with += inside it, use the total after it.

Count with an if Inside the Loop

To count how many items match a rule, start a counter at 0 and add 1 each time an if inside the loop is True.

123456marks = [72, 38, 91, 25, 60]
passed = 0
for mark in marks:
    if mark >= 40:
        passed += 1
print("Passed:", passed)

Think of it like a referee with a clicker: they watch every player run past, and click only when a player crosses the line.

The rules

• The counter starts at 0, before the loop.
• count += 1 goes inside the if, so it only runs for matching items.
• Print the count after the loop, at the left edge.

Mind the indentation: the loop's block is 4 spaces in, and the if block is 8 spaces in.

Count and add in one loop

One loop can do several jobs at once. Here it counts the big spenders and adds up their money:

12345678bills = [120, 800, 45, 1500]
big = 0
big_total = 0
for bill in bills:
    if bill > 500:
        big += 1
        big_total += bill
print(big, "big bills, worth", big_total)

Count characters

The same pattern works on a string. Here in from chapter 9 checks whether each letter is in a list of vowels:

1234567word = "education"
vowels = ["a", "e", "i", "o", "u"]
count = 0
for ch in word:
    if ch in vowels:
        count += 1
print("Vowels:", count)

Common beginner doubt: "Isn't len() a counter too?" Yes, but it counts everything. A loop with an if counts only what you choose.

A counter starts at 0 and goes up by 1 inside an if in the loop.

Find the Largest or Smallest Value

To find the largest value with a loop, remember the best one so far. Start with the first item, then compare every item with it and keep the bigger one.

123456scores = [42, 87, 65, 91, 30]
best = scores[0]
for score in scores:
    if score > best:
        best = score
print("Best score:", best)

Think of it like a high jump contest: the judge remembers the best jump so far, and only changes it when someone jumps higher.

The rules

• Start best with the first item, scores[0].
• Inside the loop, if score > best: then best = score.
• For the smallest value, flip the comparison to <.

Why not start at 0?

Starting at 0 looks fine, until every number is negative:

123456temps = [-5, -2, -9]
warmest = 0
for t in temps:
    if t > warmest:
        warmest = t
print(warmest)

The answer should be -2, but no temperature is bigger than 0, so warmest never changes. Starting with a real item from the list is always safe.

With input

max() from chapter 9 does this for a list in one step. But when numbers arrive one by one from input(), the loop is the tool. Read the first number before the loop:

123456best = int(input())
for i in range(3):
    n = int(input())
    if n > best:
        best = n
print("Largest:", best)

Keep the best so far: start with the first item, swap when you find a bigger one.

Build a New List or String in a Loop

A loop can also build something new: start with an empty list [] or an empty string "", and add a piece in every iteration.

Think of it like threading beads on a string: you start with an empty thread and add one bead at a time.

Build a list with append()

append() from chapter 9 adds one item to the end of a list:

123456marks = [72, 38, 91, 25]
passed = []
for mark in marks:
    if mark >= 40:
        passed.append(mark)
print(passed)

The new list passed keeps only the marks you chose. The old list doesn't change.

Build a string with +=

+= joins text to a string (chapter 5), so a loop can build a word piece by piece:

123456word = "banana"
result = ""
for ch in word:
    if ch != "a":
        result += ch
print(result)

Reverse a string

Put each new character in front of the result instead of behind it, and the string comes out backwards:

12345word = "Sunny"
backwards = ""
for ch in word:
    backwards = ch + backwards
print(backwards)

Step by step: "S", then "uS", then "nuS", then "nnuS", then "ynnuS".

The pattern

Every one has the same three steps: start before the loop, grow inside it, use the result after it.

Start empty before the loop, add a piece inside it, use the result after it.

Practice: Sum and count

Solve these to lock in what you learned in this topic. Each one runs in your browser.

1. 85.Python Program to Find the Sum of N NumbersEasySolve
2. 86.Python Program to Count the Vowels in a WordMediumSolve
3. 87.Python Program to Find the Largest of N NumbersMediumSolve