Loop Patterns - Python Programming

A countdown timer needs to go 10, 9, 8... 1, 0. A report processes every 5th record for sampling. A game loop runs until the player quits. Each scenario uses a different loop pattern.

Count up (1 to 10)

The most basic loop pattern - counting from start to end.

                            example
                            start = 1, end = 10 (default)start = 0start = 5end = 5end = 20
                        

                            count_up.py
                        
Replay: real traced execution (multi-file project)

                            start = 1
end = 10

print(f"Counting from {start} to {end}:")
for i in range(start, end + 1):
    print(i)

# Practical: sum numbers in range
total = sum(range(1, 101))
print(f"Sum of 1-100: {total}")

# Or with a loop
total = 0
for i in range(1, 101):
    total += i
print(f"Sum (loop): {total}")

start = 0
end = 10

print(f"Counting from {start} to {end}:")
for i in range(start, end + 1):
    print(i)

# Practical: sum numbers in range
total = sum(range(1, 101))
print(f"Sum of 1-100: {total}")

# Or with a loop
total = 0
for i in range(1, 101):
    total += i
print(f"Sum (loop): {total}")

start = 5
end = 10

print(f"Counting from {start} to {end}:")
for i in range(start, end + 1):
    print(i)

# Practical: sum numbers in range
total = sum(range(1, 101))
print(f"Sum of 1-100: {total}")

# Or with a loop
total = 0
for i in range(1, 101):
    total += i
print(f"Sum (loop): {total}")

start = 1
end = 5

print(f"Counting from {start} to {end}:")
for i in range(start, end + 1):
    print(i)

# Practical: sum numbers in range
total = sum(range(1, 101))
print(f"Sum of 1-100: {total}")

# Or with a loop
total = 0
for i in range(1, 101):
    total += i
print(f"Sum (loop): {total}")

start = 1
end = 20

print(f"Counting from {start} to {end}:")
for i in range(start, end + 1):
    print(i)

# Practical: sum numbers in range
total = sum(range(1, 101))
print(f"Sum of 1-100: {total}")

# Or with a loop
total = 0
for i in range(1, 101):
    total += i
print(f"Sum (loop): {total}")


                        
1start ← 1, end ← 101start→ 1 = 1   #@start=5, 02end→ 10 = 10    #@end=5, 2034print(f"Counting from {start1} to {end10}:")5for i in range(start, end + 1):  #?range_inclusive⋯outputCounting from 1 to 10:
2for i in range(start, end + 1):  #?range_inclusivepass 1 of 10⋯4print(f"Counting from {start} to {end}:")5for i1 in range(start1, end10 + 1):  #?range_inclusive6    print(i1)⋯output1All 10 passes — pass 1 is the card abovepassi
11
22
33
44
55
66
77
88
99
1010
3total ← 5050⋯8# Practical: sum numbers in range9total→ 5050 = sum(range(1, 101))10print(f"Sum of 1-100: {total5050}")1112# Or with a loop13total→ 0 = 014for i in range(1, 101):⋯outputSum of 1-100: 5050
4total ← 1pass 1 of 100⋯13total = 014for i1 in range(1, 101):15    total→ 1 += i116print(f"Sum (loop): {total}")⋯100 passes — pass 1 is the card abovepassitotal
110 → 1
221 → 3
333 → 6
446 → 10
5510 → 15
6615 → 21
7721 → 28
8828 → 36
9936 → 45
⋯ 89 more passes ⋯
99994851 → 4950
1001004950 → 5050
5print(f"Sum (loop): {total}")⋯15    total += i16print(f"Sum (loop): {total5050}")⋯outputSum (loop): 5050
1start ← 0, end ← 101start→ 0 = 02end→ 10 = 1034print(f"Counting from {start0} to {end10}:")5for i in range(start, end + 1):⋯outputCounting from 0 to 10:
2for i in range(start, end + 1):pass 1 of 11⋯4print(f"Counting from {start} to {end}:")5for i0 in range(start0, end10 + 1):6    print(i0)⋯output0All 11 passes — pass 1 is the card abovepassi
10
21
32
43
54
65
76
87
98
109
1110
3total ← 5050⋯8# Practical: sum numbers in range9total→ 5050 = sum(range(1, 101))10print(f"Sum of 1-100: {total5050}")1112# Or with a loop13total→ 0 = 014for i in range(1, 101):⋯outputSum of 1-100: 5050
4total ← 1pass 1 of 100⋯13total = 014for i1 in range(1, 101):15    total→ 1 += i116print(f"Sum (loop): {total}")⋯100 passes — pass 1 is the card abovepassitotal
110 → 1
221 → 3
333 → 6
446 → 10
5510 → 15
6615 → 21
7721 → 28
8828 → 36
9936 → 45
⋯ 89 more passes ⋯
99994851 → 4950
1001004950 → 5050
5print(f"Sum (loop): {total}")⋯15    total += i16print(f"Sum (loop): {total5050}")⋯outputSum (loop): 5050
1start ← 5, end ← 101start→ 5 = 52end→ 10 = 1034print(f"Counting from {start5} to {end10}:")5for i in range(start, end + 1):⋯outputCounting from 5 to 10:
2for i in range(start, end + 1):pass 1 of 6⋯4print(f"Counting from {start} to {end}:")5for i5 in range(start5, end10 + 1):6    print(i5)⋯output5All 6 passes — pass 1 is the card abovepassi
15
26
37
48
59
610
3total ← 5050⋯8# Practical: sum numbers in range9total→ 5050 = sum(range(1, 101))10print(f"Sum of 1-100: {total5050}")1112# Or with a loop13total→ 0 = 014for i in range(1, 101):⋯outputSum of 1-100: 5050
4total ← 1pass 1 of 100⋯13total = 014for i1 in range(1, 101):15    total→ 1 += i116print(f"Sum (loop): {total}")⋯100 passes — pass 1 is the card abovepassitotal
110 → 1
221 → 3
333 → 6
446 → 10
5510 → 15
6615 → 21
7721 → 28
8828 → 36
9936 → 45
⋯ 89 more passes ⋯
99994851 → 4950
1001004950 → 5050
5print(f"Sum (loop): {total}")⋯15    total += i16print(f"Sum (loop): {total5050}")⋯outputSum (loop): 5050
1start ← 1, end ← 51start→ 1 = 12end→ 5 = 534print(f"Counting from {start1} to {end5}:")5for i in range(start, end + 1):⋯outputCounting from 1 to 5:
2for i in range(start, end + 1):pass 1 of 5⋯4print(f"Counting from {start} to {end}:")5for i1 in range(start1, end5 + 1):6    print(i1)⋯output1All 5 passes — pass 1 is the card abovepassi
11
22
33
44
55
3total ← 5050⋯8# Practical: sum numbers in range9total→ 5050 = sum(range(1, 101))10print(f"Sum of 1-100: {total5050}")1112# Or with a loop13total→ 0 = 014for i in range(1, 101):⋯outputSum of 1-100: 5050
4total ← 1pass 1 of 100⋯13total = 014for i1 in range(1, 101):15    total→ 1 += i116print(f"Sum (loop): {total}")⋯100 passes — pass 1 is the card abovepassitotal
110 → 1
221 → 3
333 → 6
446 → 10
5510 → 15
6615 → 21
7721 → 28
8828 → 36
9936 → 45
⋯ 89 more passes ⋯
99994851 → 4950
1001004950 → 5050
5print(f"Sum (loop): {total}")⋯15    total += i16print(f"Sum (loop): {total5050}")⋯outputSum (loop): 5050
1start ← 1, end ← 201start→ 1 = 12end→ 20 = 2034print(f"Counting from {start1} to {end20}:")5for i in range(start, end + 1):⋯outputCounting from 1 to 20:
2for i in range(start, end + 1):pass 1 of 20⋯4print(f"Counting from {start} to {end}:")5for i1 in range(start1, end20 + 1):6    print(i1)⋯output120 passes — pass 1 is the card abovepassi
11
22
33
44
55
66
77
88
99
⋯ 9 more passes ⋯
1919
2020
3total ← 5050⋯8# Practical: sum numbers in range9total→ 5050 = sum(range(1, 101))10print(f"Sum of 1-100: {total5050}")1112# Or with a loop13total→ 0 = 014for i in range(1, 101):⋯outputSum of 1-100: 5050
4total ← 1pass 1 of 100⋯13total = 014for i1 in range(1, 101):15    total→ 1 += i116print(f"Sum (loop): {total}")⋯100 passes — pass 1 is the card abovepassitotal
110 → 1
221 → 3
333 → 6
446 → 10
5510 → 15
6615 → 21
7721 → 28
8828 → 36
9936 → 45
⋯ 89 more passes ⋯
99994851 → 4950
1001004950 → 5050
5print(f"Sum (loop): {total}")⋯15    total += i16print(f"Sum (loop): {total5050}")⋯outputSum (loop): 5050

All 10 passes — pass 1 is the card above
pass	`i`
1	1
2	2
3	3
4	4
5	5
6	6
7	7
8	8
9	9
10	10

100 passes — pass 1 is the card above
pass	`i`	`total`
1	1	0 → 1
2	2	1 → 3
3	3	3 → 6
4	4	6 → 10
5	5	10 → 15
6	6	15 → 21
7	7	21 → 28
8	8	28 → 36
9	9	36 → 45
⋯ 89 more passes ⋯
99	99	4851 → 4950
100	100	4950 → 5050

All 11 passes — pass 1 is the card above
pass	`i`
1	0
2	1
3	2
4	3
5	4
6	5
7	6
8	7
9	8
10	9
11	10

100 passes — pass 1 is the card above
pass	`i`	`total`
1	1	0 → 1
2	2	1 → 3
3	3	3 → 6
4	4	6 → 10
5	5	10 → 15
6	6	15 → 21
7	7	21 → 28
8	8	28 → 36
9	9	36 → 45
⋯ 89 more passes ⋯
99	99	4851 → 4950
100	100	4950 → 5050

All 6 passes — pass 1 is the card above
pass	`i`
1	5
2	6
3	7
4	8
5	9
6	10

100 passes — pass 1 is the card above
pass	`i`	`total`
1	1	0 → 1
2	2	1 → 3
3	3	3 → 6
4	4	6 → 10
5	5	10 → 15
6	6	15 → 21
7	7	21 → 28
8	8	28 → 36
9	9	36 → 45
⋯ 89 more passes ⋯
99	99	4851 → 4950
100	100	4950 → 5050

All 5 passes — pass 1 is the card above
pass	`i`
1	1
2	2
3	3
4	4
5	5

100 passes — pass 1 is the card above
pass	`i`	`total`
1	1	0 → 1
2	2	1 → 3
3	3	3 → 6
4	4	6 → 10
5	5	10 → 15
6	6	15 → 21
7	7	21 → 28
8	8	28 → 36
9	9	36 → 45
⋯ 89 more passes ⋯
99	99	4851 → 4950
100	100	4950 → 5050

20 passes — pass 1 is the card above
pass	`i`
1	1
2	2
3	3
4	4
5	5
6	6
7	7
8	8
9	9
⋯ 9 more passes ⋯
19	19
20	20

100 passes — pass 1 is the card above
pass	`i`	`total`
1	1	0 → 1
2	2	1 → 3
3	3	3 → 6
4	4	6 → 10
5	5	10 → 15
6	6	15 → 21
7	7	21 → 28
8	8	28 → 36
9	9	36 → 45
⋯ 89 more passes ⋯
99	99	4851 → 4950
100	100	4950 → 5050

Python's range() makes counting loops clean and readable.

range `range(stop)`, `range(start, stop)`, `range(start, stop, step)`

Count down (countdown timer)

Looping in reverse order - start high, go low.

                            start
                            start = 10 (default)start = 3start = 5
                        

                            count_down.py
                        
Replay: real traced execution (multi-file project)

                            start = 10

print(f"Countdown from {start}:")
for i in range(start, 0, -1):
    print(i)
print("Liftoff!")

# Practical: reverse list traversal
arr = [10, 20, 30, 40, 50]
print("\nArray in reverse:")
for i in range(len(arr) - 1, -1, -1):
    print(f"Index {i}: {arr[i]}")

# Pythonic way: reversed()
print("\nUsing reversed():")
for item in reversed(arr):
    print(item)

# Or slice
print("\nUsing slice:")
for item in arr[::-1]:
    print(item)

start = 3

print(f"Countdown from {start}:")
for i in range(start, 0, -1):
    print(i)
print("Liftoff!")

# Practical: reverse list traversal
arr = [10, 20, 30, 40, 50]
print("\nArray in reverse:")
for i in range(len(arr) - 1, -1, -1):
    print(f"Index {i}: {arr[i]}")

# Pythonic way: reversed()
print("\nUsing reversed():")
for item in reversed(arr):
    print(item)

# Or slice
print("\nUsing slice:")
for item in arr[::-1]:
    print(item)

start = 5

print(f"Countdown from {start}:")
for i in range(start, 0, -1):
    print(i)
print("Liftoff!")

# Practical: reverse list traversal
arr = [10, 20, 30, 40, 50]
print("\nArray in reverse:")
for i in range(len(arr) - 1, -1, -1):
    print(f"Index {i}: {arr[i]}")

# Pythonic way: reversed()
print("\nUsing reversed():")
for item in reversed(arr):
    print(item)

# Or slice
print("\nUsing slice:")
for item in arr[::-1]:
    print(item)


                        
1start ← 101start→ 10 = 10  #@start=5, 323print(f"Countdown from {start10}:")4for i in range(start, 0, -1):  #?negative_step⋯outputCountdown from 10:
2for i in range(start, 0, -1):  #?negative_steppass 1 of 10⋯3print(f"Countdown from {start}:")4for i10 in range(start10, 0, -1):  #?negative_step5    print(i10)6print("Liftoff!")⋯output10All 10 passes — pass 1 is the card abovepassi
110
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38
47
56
65
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83
92
101
3arr ← [10, 20, 30, 40, 50]⋯5    print(i)6print("Liftoff!")78# Practical: reverse list traversal9arr→ [10, 20, 30, 40, 50] = [10, 20, 30, 40, 50]10print("\nArray in reverse:")11for i in range(len(arr) - 1, -1, -1):⋯outputLiftoff!

Array in reverse:
4for i in range(len(arr) - 1, -1, -1):pass 1 of 5⋯10print("\nArray in reverse:")11for i4 in range(len(arr[10, 20, 30, 40, 50]) - 1, -1, -1):12    print(f"Index {i4}: {arr[i]50}")⋯outputIndex 4: 50All 5 passes — pass 1 is the card abovepassiarr[i]
1450
2340
3230
4120
5010
5print(" Using reversed():")⋯14# Pythonic way: reversed()15print("\nUsing reversed():")16for item in reversed(arr):⋯output
Using reversed():
6for item in reversed(arr):pass 1 of 5⋯15print("\nUsing reversed():")16for item50 in reversed(arr[10, 20, 30, 40, 50]):17    print(item50)⋯output50All 5 passes — pass 1 is the card abovepassitem
150
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330
420
510
7print(" Using slice:")⋯19# Or slice20print("\nUsing slice:")21for item in arr[::-1]:⋯output
Using slice:
8for item in arr[::-1]:pass 1 of 5⋯20print("\nUsing slice:")21for item50 in arr[::-1][50, 40, 30, 20, 10]:22    print(item50)⋯output50All 5 passes — pass 1 is the card abovepassitem
150
240
330
420
510
1start ← 31start→ 3 = 323print(f"Countdown from {start3}:")4for i in range(start, 0, -1):⋯outputCountdown from 3:
2for i in range(start, 0, -1):pass 1 of 3⋯3print(f"Countdown from {start}:")4for i3 in range(start3, 0, -1):5    print(i3)6print("Liftoff!")⋯output3All 3 passes — pass 1 is the card abovepassi
13
22
31
3arr ← [10, 20, 30, 40, 50]⋯5    print(i)6print("Liftoff!")78# Practical: reverse list traversal9arr→ [10, 20, 30, 40, 50] = [10, 20, 30, 40, 50]10print("\nArray in reverse:")11for i in range(len(arr) - 1, -1, -1):⋯outputLiftoff!

Array in reverse:
4for i in range(len(arr) - 1, -1, -1):pass 1 of 5⋯10print("\nArray in reverse:")11for i4 in range(len(arr[10, 20, 30, 40, 50]) - 1, -1, -1):12    print(f"Index {i4}: {arr[i]50}")⋯outputIndex 4: 50All 5 passes — pass 1 is the card abovepassiarr[i]
1450
2340
3230
4120
5010
5print(" Using reversed():")⋯14# Pythonic way: reversed()15print("\nUsing reversed():")16for item in reversed(arr):⋯output
Using reversed():
6for item in reversed(arr):pass 1 of 5⋯15print("\nUsing reversed():")16for item50 in reversed(arr[10, 20, 30, 40, 50]):17    print(item50)⋯output50All 5 passes — pass 1 is the card abovepassitem
150
240
330
420
510
7print(" Using slice:")⋯19# Or slice20print("\nUsing slice:")21for item in arr[::-1]:⋯output
Using slice:
8for item in arr[::-1]:pass 1 of 5⋯20print("\nUsing slice:")21for item50 in arr[::-1][50, 40, 30, 20, 10]:22    print(item50)⋯output50All 5 passes — pass 1 is the card abovepassitem
150
240
330
420
510
1start ← 51start→ 5 = 523print(f"Countdown from {start5}:")4for i in range(start, 0, -1):⋯outputCountdown from 5:
2for i in range(start, 0, -1):pass 1 of 5⋯3print(f"Countdown from {start}:")4for i5 in range(start5, 0, -1):5    print(i5)6print("Liftoff!")⋯output5All 5 passes — pass 1 is the card abovepassi
15
24
33
42
51
3arr ← [10, 20, 30, 40, 50]⋯5    print(i)6print("Liftoff!")78# Practical: reverse list traversal9arr→ [10, 20, 30, 40, 50] = [10, 20, 30, 40, 50]10print("\nArray in reverse:")11for i in range(len(arr) - 1, -1, -1):⋯outputLiftoff!

Array in reverse:
4for i in range(len(arr) - 1, -1, -1):pass 1 of 5⋯10print("\nArray in reverse:")11for i4 in range(len(arr[10, 20, 30, 40, 50]) - 1, -1, -1):12    print(f"Index {i4}: {arr[i]50}")⋯outputIndex 4: 50All 5 passes — pass 1 is the card abovepassiarr[i]
1450
2340
3230
4120
5010
5print(" Using reversed():")⋯14# Pythonic way: reversed()15print("\nUsing reversed():")16for item in reversed(arr):⋯output
Using reversed():
6for item in reversed(arr):pass 1 of 5⋯15print("\nUsing reversed():")16for item50 in reversed(arr[10, 20, 30, 40, 50]):17    print(item50)⋯output50All 5 passes — pass 1 is the card abovepassitem
150
240
330
420
510
7print(" Using slice:")⋯19# Or slice20print("\nUsing slice:")21for item in arr[::-1]:⋯output
Using slice:
8for item in arr[::-1]:pass 1 of 5⋯20print("\nUsing slice:")21for item50 in arr[::-1][50, 40, 30, 20, 10]:22    print(item50)⋯output50All 5 passes — pass 1 is the card abovepassitem
150
240
330
420
510

All 10 passes — pass 1 is the card above
pass	`i`
1	10
2	9
3	8
4	7
5	6
6	5
7	4
8	3
9	2
10	1

All 5 passes — pass 1 is the card above
pass	`i`	`arr[i]`
1	4	50
2	3	40
3	2	30
4	1	20
5	0	10

All 5 passes — pass 1 is the card above
pass	`item`
1	50
2	40
3	30
4	20
5	10

All 5 passes — pass 1 is the card above
pass	`item`
1	50
2	40
3	30
4	20
5	10

All 3 passes — pass 1 is the card above
pass	`i`
1	3
2	2
3	1

All 5 passes — pass 1 is the card above
pass	`i`	`arr[i]`
1	4	50
2	3	40
3	2	30
4	1	20
5	0	10

All 5 passes — pass 1 is the card above
pass	`item`
1	50
2	40
3	30
4	20
5	10

All 5 passes — pass 1 is the card above
pass	`item`
1	50
2	40
3	30
4	20
5	10

All 5 passes — pass 1 is the card above
pass	`i`
1	5
2	4
3	3
4	2
5	1

All 5 passes — pass 1 is the card above
pass	`i`	`arr[i]`
1	4	50
2	3	40
3	2	30
4	1	20
5	0	10

All 5 passes — pass 1 is the card above
pass	`item`
1	50
2	40
3	30
4	20
5	10

All 5 passes — pass 1 is the card above
pass	`item`
1	50
2	40
3	30
4	20
5	10

range(start, end, step) with negative step counts backwards.

Step by N (every nth element)

Skip elements by using a step value.

                            step
                            step = 5 (default)step = 2step = 10
                        

                            step_by_n.py
                        
Replay: real traced execution (multi-file project)

                            step = 5

# Count by step
print(f"Counting by {step}:")
for i in range(0, 51, step):
    print(i)

# Practical: process every other element
names = ["Alice", "Bob", "Charlie", "Diana", "Eve", "Frank"]
print("\nEvery other name:")
for i in range(0, len(names), 2):
    print(names[i])

# Pythonic: slice with step
print("\nUsing slice:")
for name in names[::2]:
    print(name)

# Odd numbers only
print("\nOdd numbers 1-20:")
odds = list(range(1, 21, 2))
print(odds)
step = 2

# Count by step
print(f"Counting by {step}:")
for i in range(0, 51, step):
    print(i)

# Practical: process every other element
names = ["Alice", "Bob", "Charlie", "Diana", "Eve", "Frank"]
print("\nEvery other name:")
for i in range(0, len(names), 2):
    print(names[i])

# Pythonic: slice with step
print("\nUsing slice:")
for name in names[::2]:
    print(name)

# Odd numbers only
print("\nOdd numbers 1-20:")
odds = list(range(1, 21, 2))
print(odds)
step = 10

# Count by step
print(f"Counting by {step}:")
for i in range(0, 51, step):
    print(i)

# Practical: process every other element
names = ["Alice", "Bob", "Charlie", "Diana", "Eve", "Frank"]
print("\nEvery other name:")
for i in range(0, len(names), 2):
    print(names[i])

# Pythonic: slice with step
print("\nUsing slice:")
for name in names[::2]:
    print(name)

# Odd numbers only
print("\nOdd numbers 1-20:")
odds = list(range(1, 21, 2))
print(odds)

                        
1step ← 51step→ 5 = 5  #@step=2, 1023# Count by step4print(f"Counting by {step5}:")5for i in range(0, 51, step):⋯outputCounting by 5:
2for i in range(0, 51, step):pass 1 of 11⋯4print(f"Counting by {step}:")5for i0 in range(0, 51, step5):6    print(i0)⋯output0All 11 passes — pass 1 is the card abovepassi
10
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520
625
730
835
940
1045
1150
3names ← ['Alice', 'Bob', 'Charlie', 'Diana', 'Eve', 'Frank']⋯8# Practical: process every other element9names→ ['Alice', 'Bob', 'Charlie', 'Diana', 'Eve', 'Frank'] = ["Alice", "Bob", "Charlie", "Diana", "Eve", "Frank"]10print("\nEvery other name:")11for i in range(0, len(names), 2):⋯output
Every other name:
4for i in range(0, len(names), 2):pass 1 of 3⋯10print("\nEvery other name:")11for i0 in range(0, len(names['Alice', 'Bob', 'Charlie', 'Diana', 'Eve', 'Frank']), 2):12    print(names[i]Alice)⋯outputAliceAll 3 passes — pass 1 is the card abovepassinames[i]
10Alice
22Charlie
34Eve
5print(" Using slice:")⋯14# Pythonic: slice with step15print("\nUsing slice:")16for name in names[::2]:⋯output
Using slice:
6for name in names[::2]:pass 1 of 3⋯15print("\nUsing slice:")16for nameAlice in names[::2]['Alice', 'Charlie', 'Eve']:17    print(nameAlice)⋯outputAliceAll 3 passes — pass 1 is the card abovepassname
1Alice
2Charlie
3Eve
7odds ← [1, 3, 5, 7, 9, 11, 13, 15, 17, 19]⋯19# Odd numbers only20print("\nOdd numbers 1-20:")21odds→ [1, 3, 5, 7, 9, 11, 13, 15, 17, 19] = list(range(1, 21, 2))22print(odds[1, 3, 5, 7, 9, 11, 13, 15, 17, 19])⋯output
Odd numbers 1-20:
[1, 3, 5, 7, 9, 11, 13, 15, 17, 19]
1step ← 21step→ 2 = 223# Count by step4print(f"Counting by {step2}:")5for i in range(0, 51, step):⋯outputCounting by 2:
2for i in range(0, 51, step):pass 1 of 26⋯4print(f"Counting by {step}:")5for i0 in range(0, 51, step2):6    print(i0)⋯output026 passes — pass 1 is the card abovepassi
10
22
34
46
58
610
712
814
916
⋯ 15 more passes ⋯
2548
2650
3names ← ['Alice', 'Bob', 'Charlie', 'Diana', 'Eve', 'Frank']⋯8# Practical: process every other element9names→ ['Alice', 'Bob', 'Charlie', 'Diana', 'Eve', 'Frank'] = ["Alice", "Bob", "Charlie", "Diana", "Eve", "Frank"]10print("\nEvery other name:")11for i in range(0, len(names), 2):⋯output
Every other name:
4for i in range(0, len(names), 2):pass 1 of 3⋯10print("\nEvery other name:")11for i0 in range(0, len(names['Alice', 'Bob', 'Charlie', 'Diana', 'Eve', 'Frank']), 2):12    print(names[i]Alice)⋯outputAliceAll 3 passes — pass 1 is the card abovepassinames[i]
10Alice
22Charlie
34Eve
5print(" Using slice:")⋯14# Pythonic: slice with step15print("\nUsing slice:")16for name in names[::2]:⋯output
Using slice:
6for name in names[::2]:pass 1 of 3⋯15print("\nUsing slice:")16for nameAlice in names[::2]['Alice', 'Charlie', 'Eve']:17    print(nameAlice)⋯outputAliceAll 3 passes — pass 1 is the card abovepassname
1Alice
2Charlie
3Eve
7odds ← [1, 3, 5, 7, 9, 11, 13, 15, 17, 19]⋯19# Odd numbers only20print("\nOdd numbers 1-20:")21odds→ [1, 3, 5, 7, 9, 11, 13, 15, 17, 19] = list(range(1, 21, 2))22print(odds[1, 3, 5, 7, 9, 11, 13, 15, 17, 19])⋯output
Odd numbers 1-20:
[1, 3, 5, 7, 9, 11, 13, 15, 17, 19]
1step ← 101step→ 10 = 1023# Count by step4print(f"Counting by {step10}:")5for i in range(0, 51, step):⋯outputCounting by 10:
2for i in range(0, 51, step):pass 1 of 6⋯4print(f"Counting by {step}:")5for i0 in range(0, 51, step10):6    print(i0)⋯output0All 6 passes — pass 1 is the card abovepassi
10
210
320
430
540
650
3names ← ['Alice', 'Bob', 'Charlie', 'Diana', 'Eve', 'Frank']⋯8# Practical: process every other element9names→ ['Alice', 'Bob', 'Charlie', 'Diana', 'Eve', 'Frank'] = ["Alice", "Bob", "Charlie", "Diana", "Eve", "Frank"]10print("\nEvery other name:")11for i in range(0, len(names), 2):⋯output
Every other name:
4for i in range(0, len(names), 2):pass 1 of 3⋯10print("\nEvery other name:")11for i0 in range(0, len(names['Alice', 'Bob', 'Charlie', 'Diana', 'Eve', 'Frank']), 2):12    print(names[i]Alice)⋯outputAliceAll 3 passes — pass 1 is the card abovepassinames[i]
10Alice
22Charlie
34Eve
5print(" Using slice:")⋯14# Pythonic: slice with step15print("\nUsing slice:")16for name in names[::2]:⋯output
Using slice:
6for name in names[::2]:pass 1 of 3⋯15print("\nUsing slice:")16for nameAlice in names[::2]['Alice', 'Charlie', 'Eve']:17    print(nameAlice)⋯outputAliceAll 3 passes — pass 1 is the card abovepassname
1Alice
2Charlie
3Eve
7odds ← [1, 3, 5, 7, 9, 11, 13, 15, 17, 19]⋯19# Odd numbers only20print("\nOdd numbers 1-20:")21odds→ [1, 3, 5, 7, 9, 11, 13, 15, 17, 19] = list(range(1, 21, 2))22print(odds[1, 3, 5, 7, 9, 11, 13, 15, 17, 19])⋯output
Odd numbers 1-20:
[1, 3, 5, 7, 9, 11, 13, 15, 17, 19]

All 11 passes — pass 1 is the card above
pass	`i`
1	0
2	5
3	10
4	15
5	20
6	25
7	30
8	35
9	40
10	45
11	50

All 3 passes — pass 1 is the card above
pass	`i`	`names[i]`
1	0	Alice
2	2	Charlie
3	4	Eve

All 3 passes — pass 1 is the card above
pass	`name`
1	Alice
2	Charlie
3	Eve

26 passes — pass 1 is the card above
pass	`i`
1	0
2	2
3	4
4	6
5	8
6	10
7	12
8	14
9	16
⋯ 15 more passes ⋯
25	48
26	50

All 3 passes — pass 1 is the card above
pass	`i`	`names[i]`
1	0	Alice
2	2	Charlie
3	4	Eve

All 3 passes — pass 1 is the card above
pass	`name`
1	Alice
2	Charlie
3	Eve

All 6 passes — pass 1 is the card above
pass	`i`
1	0
2	10
3	20
4	30
5	40
6	50

All 3 passes — pass 1 is the card above
pass	`i`	`names[i]`
1	0	Alice
2	2	Charlie
3	4	Eve

All 3 passes — pass 1 is the card above
pass	`name`
1	Alice
2	Charlie
3	Eve

The third argument to range() is the step size.

While with condition

Loop until some condition becomes false.

                            secret
                            secret = 7 (default)secret = 3secret = 9
                        

                            while_condition.py
                        
Replay: real traced execution (multi-file project)

                            # Guessing game simulation
secret = 7
guess = 1
attempts = 0

while guess != secret:
    print(f"Trying {guess}... wrong!")
    guess += 1
    attempts += 1

print(f"Found {secret} in {attempts} attempts!")

# Practical: process until sentinel value
data = [10, 20, 30, -1, 40, 50]  # -1 is sentinel
i = 0
total = 0

print("\nSum until -1:")
while i < len(data) and data[i] != -1:
    total += data[i]
    i += 1

print(f"Sum: {total}")

# Pythonic: itertools.takewhile
from itertools import takewhile
data2 = [10, 20, 30, -1, 40, 50]
total2 = sum(takewhile(lambda x: x != -1, data2))
print(f"Sum (takewhile): {total2}")

# Guessing game simulation
secret = 3
guess = 1
attempts = 0

while guess != secret:
    print(f"Trying {guess}... wrong!")
    guess += 1
    attempts += 1

print(f"Found {secret} in {attempts} attempts!")

# Practical: process until sentinel value
data = [10, 20, 30, -1, 40, 50]  # -1 is sentinel
i = 0
total = 0

print("\nSum until -1:")
while i < len(data) and data[i] != -1:
    total += data[i]
    i += 1

print(f"Sum: {total}")

# Pythonic: itertools.takewhile
from itertools import takewhile
data2 = [10, 20, 30, -1, 40, 50]
total2 = sum(takewhile(lambda x: x != -1, data2))
print(f"Sum (takewhile): {total2}")

# Guessing game simulation
secret = 9
guess = 1
attempts = 0

while guess != secret:
    print(f"Trying {guess}... wrong!")
    guess += 1
    attempts += 1

print(f"Found {secret} in {attempts} attempts!")

# Practical: process until sentinel value
data = [10, 20, 30, -1, 40, 50]  # -1 is sentinel
i = 0
total = 0

print("\nSum until -1:")
while i < len(data) and data[i] != -1:
    total += data[i]
    i += 1

print(f"Sum: {total}")

# Pythonic: itertools.takewhile
from itertools import takewhile
data2 = [10, 20, 30, -1, 40, 50]
total2 = sum(takewhile(lambda x: x != -1, data2))
print(f"Sum (takewhile): {total2}")


                        
1secret ← 7, guess ← 1, attempts ← 01# Guessing game simulation2secret→ 7 = 7    #@secret=3, 93guess→ 1 = 14attempts→ 0 = 0⋯
2guess ← 2, attempts ← 1pass 1 of 6⋯6while guess1 != secret7:  #?while_condition7    print(f"Trying {guess1}... wrong!")8    guess→ 2 += 19    attempts→ 1 += 1⋯outputTrying 1... wrong!All 6 passes — pass 1 is the card abovepassguessattempts
11 → 20 → 1
22 → 31 → 2
33 → 42 → 3
44 → 53 → 4
55 → 64 → 5
66 → 75 → 6
3data ← [10, 20, 30, -1, 40, 50], i ← 0, total ← 0⋯11print(f"Found {secret7} in {attempts6} attempts!")1213# Practical: process until sentinel value14data→ [10, 20, 30, -1, 40, 50] = [10, 20, 30, -1, 40, 50]  # -1 is sentinel15i→ 0 = 016total→ 0 = 01718print("\nSum until -1:")19while i < len(data) and data[i] != -1:⋯outputFound 7 in 6 attempts!

Sum until -1:
4total ← 10, i ← 1pass 1 of 3⋯18print("\nSum until -1:")19while i0 < len(data[10, 20, 30, -1, 40, 50]) and data[i]10 != -1:20    total→ 10 += data[i]1021    i→ 1 += 1⋯All 3 passes — pass 1 is the card abovepassdata[i]totali
1100 → 100 → 1
22010 → 301 → 2
33030 → 602 → 3
5data2 ← [10, 20, 30, -1, 40, 50], total2 ← 60⋯23print(f"Sum: {total60}")2425# Pythonic: itertools.takewhile26from itertools import takewhile27data2→ [10, 20, 30, -1, 40, 50] = [10, 20, 30, -1, 40, 50]28total2→ 60 = sum(takewhile(lambda x: x != -1, data2[10, 20, 30, -1, 40, 50]))29print(f"Sum (takewhile): {total260}")⋯outputSum: 60
Sum (takewhile): 60
1secret ← 3, guess ← 1, attempts ← 01# Guessing game simulation2secret→ 3 = 33guess→ 1 = 14attempts→ 0 = 0⋯
2guess ← 2, attempts ← 1pass 1 of 2⋯6while guess1 != secret3:7    print(f"Trying {guess1}... wrong!")8    guess→ 2 += 19    attempts→ 1 += 1⋯outputTrying 1... wrong!
3guess ← 3, attempts ← 2pass 2 of 2⋯6while guess2 != secret3:7    print(f"Trying {guess2}... wrong!")8    guess→ 3 += 19    attempts→ 2 += 1⋯outputTrying 2... wrong!
4data ← [10, 20, 30, -1, 40, 50], i ← 0, total ← 0⋯11print(f"Found {secret3} in {attempts2} attempts!")1213# Practical: process until sentinel value14data→ [10, 20, 30, -1, 40, 50] = [10, 20, 30, -1, 40, 50]  # -1 is sentinel15i→ 0 = 016total→ 0 = 01718print("\nSum until -1:")19while i < len(data) and data[i] != -1:⋯outputFound 3 in 2 attempts!

Sum until -1:
5total ← 10, i ← 1pass 1 of 3⋯18print("\nSum until -1:")19while i0 < len(data[10, 20, 30, -1, 40, 50]) and data[i]10 != -1:20    total→ 10 += data[i]1021    i→ 1 += 1⋯All 3 passes — pass 1 is the card abovepassdata[i]totali
1100 → 100 → 1
22010 → 301 → 2
33030 → 602 → 3
6data2 ← [10, 20, 30, -1, 40, 50], total2 ← 60⋯23print(f"Sum: {total60}")2425# Pythonic: itertools.takewhile26from itertools import takewhile27data2→ [10, 20, 30, -1, 40, 50] = [10, 20, 30, -1, 40, 50]28total2→ 60 = sum(takewhile(lambda x: x != -1, data2[10, 20, 30, -1, 40, 50]))29print(f"Sum (takewhile): {total260}")⋯outputSum: 60
Sum (takewhile): 60
1secret ← 9, guess ← 1, attempts ← 01# Guessing game simulation2secret→ 9 = 93guess→ 1 = 14attempts→ 0 = 0⋯
2guess ← 2, attempts ← 1pass 1 of 8⋯6while guess1 != secret9:7    print(f"Trying {guess1}... wrong!")8    guess→ 2 += 19    attempts→ 1 += 1⋯outputTrying 1... wrong!All 8 passes — pass 1 is the card abovepassguessattempts
11 → 20 → 1
22 → 31 → 2
33 → 42 → 3
44 → 53 → 4
55 → 64 → 5
66 → 75 → 6
77 → 86 → 7
88 → 97 → 8
3data ← [10, 20, 30, -1, 40, 50], i ← 0, total ← 0⋯11print(f"Found {secret9} in {attempts8} attempts!")1213# Practical: process until sentinel value14data→ [10, 20, 30, -1, 40, 50] = [10, 20, 30, -1, 40, 50]  # -1 is sentinel15i→ 0 = 016total→ 0 = 01718print("\nSum until -1:")19while i < len(data) and data[i] != -1:⋯outputFound 9 in 8 attempts!

Sum until -1:
4total ← 10, i ← 1pass 1 of 3⋯18print("\nSum until -1:")19while i0 < len(data[10, 20, 30, -1, 40, 50]) and data[i]10 != -1:20    total→ 10 += data[i]1021    i→ 1 += 1⋯All 3 passes — pass 1 is the card abovepassdata[i]totali
1100 → 100 → 1
22010 → 301 → 2
33030 → 602 → 3
5data2 ← [10, 20, 30, -1, 40, 50], total2 ← 60⋯23print(f"Sum: {total60}")2425# Pythonic: itertools.takewhile26from itertools import takewhile27data2→ [10, 20, 30, -1, 40, 50] = [10, 20, 30, -1, 40, 50]28total2→ 60 = sum(takewhile(lambda x: x != -1, data2[10, 20, 30, -1, 40, 50]))29print(f"Sum (takewhile): {total260}")⋯outputSum: 60
Sum (takewhile): 60

All 6 passes — pass 1 is the card above
pass	`guess`	`attempts`
1	1 → 2	0 → 1
2	2 → 3	1 → 2
3	3 → 4	2 → 3
4	4 → 5	3 → 4
5	5 → 6	4 → 5
6	6 → 7	5 → 6

All 3 passes — pass 1 is the card above
pass	`data[i]`	`total`	`i`
1	10	0 → 10	0 → 1
2	20	10 → 30	1 → 2
3	30	30 → 60	2 → 3

All 3 passes — pass 1 is the card above
pass	`data[i]`	`total`	`i`
1	10	0 → 10	0 → 1
2	20	10 → 30	1 → 2
3	30	30 → 60	2 → 3

All 8 passes — pass 1 is the card above
pass	`guess`	`attempts`
1	1 → 2	0 → 1
2	2 → 3	1 → 2
3	3 → 4	2 → 3
4	4 → 5	3 → 4
5	5 → 6	4 → 5
6	6 → 7	5 → 6
7	7 → 8	6 → 7
8	8 → 9	7 → 8

All 3 passes — pass 1 is the card above
pass	`data[i]`	`total`	`i`
1	10	0 → 10	0 → 1
2	20	10 → 30	1 → 2
3	30	30 → 60	2 → 3

Use while when you don't know how many iterations ahead of time.

while Condition-controlled loop: unknown iterations, check before each.

Emulating do-while

Python doesn't have do-while, but you can emulate it.

                            do_while_emulate.py
                        
Replay: real traced execution (multi-file project)

                            # Python doesn't have do-while, but here are alternatives:

# Pattern 1: while True with break at end
print("=== Pattern 1: while True + break ===")
choice = 0

while True:
    print("=== Menu ===")
    print("1. Play")
    print("2. Settings")
    print("3. Exit")
    print(f"Choice: {choice}")

    # Simulate getting input
    choice = 3

    if choice == 3:
        break

print("Goodbye!")

# Pattern 2: Loop-and-a-half
print("\n=== Pattern 2: Loop-and-a-half ===")
attempts = 0

while True:
    attempts += 1
    print(f"Attempt {attempts}")

    # Simulate: succeed on 3rd try
    success = (attempts == 3)

    if success:
        print("Success!")
        break

    print("Retrying...")

# Pattern 3: Assignment expression (walrus operator)
print("\n=== Pattern 3: Walrus operator ===")
numbers = iter([1, 2, 3, 0, 4, 5])

while (n := next(numbers, None)) is not None and n != 0:
    print(f"Processing: {n}")

print("Stopped at 0 or end")


                        
1choice ← 0⋯3# Pattern 1: while True with break at end4print("=== Pattern 1: while True + break ===")5choice→ 0 = 0⋯output=== Pattern 1: while True + break ===
2choice ← 3⋯7while True:8    print("=== Menu ===")9    print("1. Play")10    print("2. Settings")11    print("3. Exit")12    print(f"Choice: {choice0}")13    14    # Simulate getting input15    choice→ 3 = 3  #?simulate_input⋯output=== Menu ===
1. Play
2. Settings
3. Exit
Choice: 0
3if choice == 3:⋯17if choice3 == 3:18    break⋯
4attempts ← 0⋯20print("Goodbye!")2122# Pattern 2: Loop-and-a-half23print("\n=== Pattern 2: Loop-and-a-half ===")24attempts→ 0 = 0⋯outputGoodbye!

=== Pattern 2: Loop-and-a-half ===
5attempts ← 1, success ← Falsepass 1 of 3⋯26while True:27    attempts→ 1 += 128    print(f"Attempt {attempts1}")29    30    # Simulate: succeed on 3rd try31    success→ False = (attempts1 == 3)32    33    if success:34        print("Success!")35        break36    37    print("Retrying...")⋯outputAttempt 1
Retrying...All 3 passes — pass 1 is the card abovepassattemptssuccess
10 → 1False
21 → 2False
32 → 3True
6if success:⋯33if successTrue:34    print("Success!")35    break⋯outputSuccess!
7numbers ← ⟨list_iterator A⟩⋯39# Pattern 3: Assignment expression (walrus operator)40print("\n=== Pattern 3: Walrus operator ===")41numbers→ ⟨list_iterator A⟩ = iter([1, 2, 3, 0, 4, 5])⋯output
=== Pattern 3: Walrus operator ===
8while (n := next(numbers, None)) is not None and n != 0:pass 1 of 3⋯43while (n1 := next(numbers⟨list_iterator A⟩, None)) is not None and n != 0:44    print(f"Processing: {n1}")⋯outputProcessing: 1All 3 passes — pass 1 is the card abovepassn
11
22
33
9print("Stopped at 0 or end")⋯46print("Stopped at 0 or end")⋯outputStopped at 0 or end

All 3 passes — pass 1 is the card above
pass	`attempts`	`success`
1	0 → 1	False
2	1 → 2	False
3	2 → 3	True

All 3 passes — pass 1 is the card above
pass	`n`
1	1
2	2
3	3

Use while True with break at the end, or loop-and-a-half pattern.

enumerate `for i, item in enumerate(list)` - get index and value together.

zip `for a, b in zip(list1, list2)` - iterate multiple lists in parallel.

Exercise: infinite_patterns.py

Explore intentional infinite loops and how to use them safely