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For & Range

Slide 1 of 57

  1. A game club needs a 5-by-5 multiplication board. Each row must be assembled as one string and printed on its own line:

    1 2 3 4 5
2 4 6 8 10
3 6 9 12 15
4 8 12 16 20
5 10 15 20 25

    How can a program visit every row and every column without a separate instruction for each product?

  2. We will build that payoff in three lessons. First, for visits a known sequence of integers. Then familiar total-and-count patterns use that sequence. Finally, one loop inside another builds tables and triangles.

    • Use for and range() to visit a known sequence of numbers.

    • Choose a start, stop, and step, then use each number in a calculation.

    • Put one loop inside another to build rows and columns.

  4. Lesson 1: Visit a Range

    Reproduce the 5050 sum

    Unit 02 showed this total as a boxed peek. Unit 04 rebuilt it with while. Here is the same running total with for and range.

  5. total = 0

for n in range(1, 101):
    total = total + n

print(f"Sum: {total}")

    Output

    Sum: 5050

    This reproduces Sum: 5050. range(1, 101) supplies the integers 1 through 100, and n receives one value per pass. Do not modify the for variable inside its loop body.

  6. Two forms of range

    range(n) starts at 0. It supplies 0 through n - 1.

  7. for number in range(5):
    print(number)

    Output

    0
1
2
3
4

    The output is 0, 1, 2, 3, 4. The stop value 5 is not included.

    range(a, b) starts at a and stops before b.

  8. for number in range(3, 8):
    print(number)

    Output

    3
4
5
6
7

    range(3, 8) includes 3 but not 8, so the final printed value is 7. Set the stop one beyond the last value you want.

  9. One twist: a third range argument chooses the step size.

  10. for number in range(0, 20, 5):
    print(number)

    Output

    0
5
10
15

    This prints 0, 5, 10, 15. The stop value 20 is excluded even when the step is 5.

  11. One twist: a negative step counts down, so the stop must be below the start.

  12. for number in range(10, 0, -1):
    print(number)

    Output

    10
9
8
7
6
5
4
3
2
1

    The countdown prints 10 through 1; it stops before 0.

  13. Real input: choose the upper bound for a count from 1.

  14. Try it
    n = int(input("How high? "))
for number in range(1, n + 1):
    print(number)

    int() converts the typed number; the range includes it by stopping at n + 1.

  15. Comment the job, not every pass

    A short comment can explain why a loop exists.

  16. # Print the five badge numbers.
for badge_number in range(1, 6):
    print(f"Badge: {badge_number}")

    Output

    Badge: 1
Badge: 2
Badge: 3
Badge: 4
Badge: 5

    The name badge_number explains the changing value, and the comment explains the loop's purpose.

  17. Lesson 2: Calculate across a Range

    Running totals and conditional counts

    The accumulator patterns from Unit 04 work unchanged. Only the source of each number changes.

  18. square_total = 0

for number in range(1, 6):
    square_total = square_total + number * number

print(f"Sum of squares: {square_total}")

    Output

    Sum of squares: 55

    The range supplies 1 through 5, and the accumulator produces Sum of squares: 55.

  19. count = 0

for number in range(1, 31):
    if number % 4 == 0:
        count = count + 1

print(f"Count: {count}")

    Output

    Count: 7

    Every integer from 1 through 30 is checked, but only seven values pass the divisibility test.

  20. FizzBuzz with an elif ladder

    FizzBuzz classifies each integer. Multiples of both 3 and 5 must be checked first because they also match the separate tests.

  21. for number in range(1, 16):
    if number % 15 == 0:
        line = "FizzBuzz"
    elif number % 3 == 0:
        line = "Fizz"
    elif number % 5 == 0:
        line = "Buzz"
    else:
        line = f"{number}"
    print(line)

    Output

    1
2
Fizz
4
Buzz
Fizz
7
8
Fizz
Buzz
11
Fizz
13
14
FizzBuzz

    Each pass assigns exactly one string to line. Use a print(line) at the loop-body level (same indent as the if), after the ladder. The order lets 15 become FizzBuzz.

  22. One twist: continue skips the rest of one for pass and moves to the next value.

  23. for n in range(1, 11):
    if n % 3 == 0:
        continue
    print(n)

    Output

    1
2
4
5
7
8
10

    3, 6, and 9 are skipped; every other number from 1 through 10 prints.

  24. Real input: choose the last FizzBuzz number. The familiar decision ladder now runs over the typed range.

  25. Try it
    n = int(input("How high? "))
for number in range(1, n + 1):
    if number % 15 == 0:
        line = "FizzBuzz"
    elif number % 3 == 0:
        line = "Fizz"
    elif number % 5 == 0:
        line = "Buzz"
    else:
        line = f"{number}"
    print(line)

    The input changes only the range endpoint; each number still gets one label.

  26. Find a divisor, then test for prime

    A divisor divides a number with no remainder. First, stop as soon as one divisor of 91 is found.

    Minimal: print the first divisor of 91, then leave the search loop.

  27. for d in range(2, 91):
    if 91 % d == 0:
        print(d)
        break

    Output

    7

    7 is the first divisor found; break ends this for loop immediately.

  28. One twist: keep a boolean flag so the search also reports whether a number is prime.

  29. n = 29
is_prime = True

for divisor in range(2, n):
    if n % divisor == 0:
        is_prime = False
        break

print(f"{n} is prime: {is_prime}")

    Output

    29 is prime: True

    break stops this loop when a divisor proves the flag false. For n == 2, range(2, 2) is empty, so is_prime stays True.

  30. Lesson 3: Put One Loop inside Another

    Build rows in stages

    First, build one row string by adding one star per pass.

  31. row_text = ""
for col in range(1, 5):
    row_text = row_text + "*"
print(row_text)

    Output

    ****

    One loop builds **** as a single string; printing happens after the row is complete.

  32. One twist: repeat that row-building loop inside an outer loop to make a rectangle.

  33. for row in range(1, 4):
    row_text = ""
    for col in range(1, 5):
        row_text = row_text + "*"
    print(row_text)

    Output

    ****
****
****

    The inner body has 8 spaces and adds four stars. print(row_text) has 4 spaces, so it runs once per completed row. A break in the inner loop would leave only that loop.

  34. A growing triangle

    The row number can control how many columns the inner loop visits.

  35. height = 5

for row in range(1, height + 1):
    row_text = ""
    for col in range(1, row + 1):
        row_text = row_text + "*"
    print(row_text)

    Output

    *
**
***
****
*****

    Row 1 gets one inner pass, while row 5 gets five. The first accumulated row is *; the last is *****.

  36. One twist: string repetition can make each triangle row without an inner loop.

  37. height = 5
for r in range(1, height + 1):
    print("*" * r)

    Output

    *
**
***
****
*****

    "*" * r makes exactly as many stars as the row number. The earlier loop shows how to build the same row one star at a time.

  38. One twist: print(piece, end="") keeps the next piece on the same line; a plain print() finishes it.

  39. for col in range(1, 5):
    print("*", end="")
print()

    Output

    ****

    The four pieces print as **** on one line. The final print() moves to the next line.

  40. One number row

    Before building a full table, make a single row with numbers separated by one space.

  41. row_text = ""
for col in range(1, 6):
    if col == 1:
        row_text = f"{col}"
    else:
        row_text = row_text + f" {col}"
print(row_text)

    Output

    1 2 3 4 5

    The result is 1 2 3 4 5 with no trailing space; the first number needs no separator.

  42. A multiplication table

    Build each table row as a string. A small condition inserts spaces between products without leaving a hidden trailing space.

  43. table_size = 3

for row in range(1, table_size + 1):
    row_text = ""
    for col in range(1, table_size + 1):
        product = row * col
        if col == 1:
            row_text = f"{product}"
        else:
            row_text = row_text + f" {product}"
    print(row_text)

    Output

    1 2 3
2 4 6
3 6 9

    The first row is 1 2 3 and the last is 3 6 9. The inner loop builds one complete row_text; the outer loop prints three rows. No special print options are needed.

  44. One twist: a width inside an f-string, f"{number:3}", pads the number with spaces in front until it is 3 characters wide.

  45. row_text = ""
for col in range(1, 6):
    row_text = row_text + f"{col * 4:3}"
print(row_text)

    Output

      4  8 12 16 20

    The row prints as 4 8 12 16 20. Every product fills exactly 3 characters — one-digit numbers get two spaces in front, two-digit numbers get one — so the columns of a table line up. The padding is on the left, so no row ends with a trailing space.

  46. ASCII pattern ladder

    Start with the triangle you built above. These next shapes change one row rule at a time.

    One twist: visit row lengths in reverse to make an inverted triangle.

  47. height = 4
for r in range(height, 0, -1):
    print("*" * r)

    Output

    ****
***
**
*

    The negative step makes rows of 4, 3, 2, then 1 star.

  48. One twist: add leading spaces so the triangle lines up on the right.

  49. height = 4
for r in range(1, height + 1):
    print(" " * (height - r) + "*" * r)

    Output

       *
  **
 ***
****

    The spaces shrink as the stars grow. No row ends with trailing spaces.

  50. One twist: widen each star row to odd widths to center a pyramid.

  51. height = 4
for r in range(1, height + 1):
    print(" " * (height - r) + "*" * (2 * r - 1))

    Output

       *
  ***
 *****
*******

    The star widths are 1, 3, 5, and 7, so the rows form a centered pyramid.

  52. Real input: choose the height of the pyramid.

  53. Try it
    height = int(input("Height: "))
for r in range(1, height + 1):
    print(" " * (height - r) + "*" * (2 * r - 1))

    The typed height controls both the leading spaces and each row's star width.

  54. Final build

    Return to the 5-by-5 multiplication board. The outer loop selects rows 1 through 5, the inner loop selects columns 1 through 5, and string accumulation makes one assertable row at a time.

  55. table_size = 5

for row in range(1, table_size + 1):
    row_text = ""
    for col in range(1, table_size + 1):
        product = row * col
        if col == 1:
            row_text = f"{product}"
        else:
            row_text = row_text + f" {product}"
    print(row_text)

    Output

    1 2 3 4 5
2 4 6 8 10
3 6 9 12 15
4 8 12 16 20
5 10 15 20 25

  56. The finished board matches the opening exactly. Known ranges choose the positions, nested loops visit every row-column pair, and each row is printed only after its string is complete.

    • for takes each number from range() in turn.

    • range(n) begins at 0 and stops before n; a start and step change the visit.

    • Running totals and conditional counts work inside a for loop too.

    • In nested loops, the inner loop finishes for each outer-loop pass.

    You can now build a multiplication board one row at a time.