Conditionals - Python Programming

Simple if

Execute code only when a condition is true.

                            x
                            x = 10 (default)x = 3x = 25
                        

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

                            x = 10

if x > 5:
    print("x > 5")

if x > 20:
    print("x > 20")
x = 3

if x > 5:
    print("x > 5")

if x > 20:
    print("x > 20")
x = 25

if x > 5:
    print("x > 5")

if x > 20:
    print("x > 20")

                        
1x ← 101x→ 10 = 10  #@x=3, 25⋯
2if x > 5:⋯3if x10 > 5:4    print("x > 5")⋯outputx > 5
1x ← 31x→ 3 = 3⋯
1x ← 251x→ 25 = 25⋯
2if x > 5:⋯3if x25 > 5:4    print("x > 5")⋯outputx > 5
3if x > 20:⋯6if x25 > 20:7    print("x > 20")⋯outputx > 20

The code inside if only runs when the condition is true.

if `if condition:` - executes the block only when condition is true.

comparison `>` greater, `<` less, `>=` greater or equal, `<=` less or equal, `==` equal.

Find the maximum in a list

Scan through all elements, keep track of the largest one found.

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

                            nums = [23, 45, 12, 67, 34, 89, 41]

print("nums=" + str(nums))
max = nums[0]

for i in range(1, len(nums)):

    if nums[i] > max:
        max = nums[i]


                        
1nums ← [23, 45, 12, 67, 34, 89, 41], max ← 231nums→ [23, 45, 12, 67, 34, 89, 41] = [23, 45, 12, 67, 34, 89, 41]23print("nums=" + str(nums[23, 45, 12, 67, 34, 89, 41]))4max→ 23 = nums[0]23⋯outputnums=[23, 45, 12, 67, 34, 89, 41]
2for i in range(1, len(nums)):     #?update_maxpass 1 of 6⋯6for i1 in range(1, len(nums[23, 45, 12, 67, 34, 89, 41])):7    #?update_max8    if nums[i] > max:9        max = nums[i]⋯All 6 passes — pass 1 is the card abovepassi
11
22
33
44
55
66
3max ← 45pass 1 of 3⋯7#?update_max8if nums[i]45 > max:9    max→ 45 = nums[i]45⋯All 3 passes — pass 1 is the card abovepassnums[i]max
14545
26767
38989

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

All 3 passes — pass 1 is the card above
pass	`nums[i]`	`max`
1	45	45
2	67	67
3	89	89

Compare each element to our current max. If bigger, update max. This is the find maximum pattern.

See the Branches

The max scan makes the branch visible: some values replace max, and others leave it alone. The diagrams pin the exact list from max.py.

For each number, the condition `nums[i] > max` chooses one of two paths: update `max`, or keep the current value.

Only 45, 67, and 89 are bigger than the best value so far. The final maximum is 89.

Find the minimum in a list

Same idea, but track the smallest.

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

                            nums = [23, 45, 12, 67, 34, 89, 41]

print("nums=" + str(nums))
min = nums[0]

for i in range(1, len(nums)):
    if nums[i] < min:
        min = nums[i]

                        
1nums ← [23, 45, 12, 67, 34, 89, 41], min ← 231nums→ [23, 45, 12, 67, 34, 89, 41] = [23, 45, 12, 67, 34, 89, 41]23print("nums=" + str(nums[23, 45, 12, 67, 34, 89, 41]))4min→ 23 = nums[0]23⋯outputnums=[23, 45, 12, 67, 34, 89, 41]
2for i in range(1, len(nums)):pass 1 of 6⋯6for i1 in range(1, len(nums[23, 45, 12, 67, 34, 89, 41])):7    if nums[i] < min:8        min = nums[i]⋯All 6 passes — pass 1 is the card abovepassinums[i]min
11——
221212
33——
44——
55——
66——
3min ← 12⋯6for i in range(1, len(nums)):7    if nums[i]12 < min:8        min→ 12 = nums[i]12⋯

All 6 passes — pass 1 is the card above
pass	`i`	`nums[i]`	`min`
1	1	—	—
2	2	12	12
3	3	—	—
4	4	—	—
5	5	—	—
6	6	—	—

Same pattern, opposite comparison: update when you find something smaller.

else `else:` - executes when the if condition is false.

First N Fibonacci numbers

Generate a variable-length Fibonacci sequence.

                            n
                            n = 12 (default)n = 5n = 20
                        

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

                            n = 12
fib = [0] * n

fib[0] = 0
fib[1] = 1


for i in range(2, n):
    fib[i] = fib[i - 1] + fib[i - 2]

print("fib=" + str(fib))

n = 5
fib = [0] * n

fib[0] = 0
fib[1] = 1


for i in range(2, n):
    fib[i] = fib[i - 1] + fib[i - 2]

print("fib=" + str(fib))

n = 20
fib = [0] * n

fib[0] = 0
fib[1] = 1


for i in range(2, n):
    fib[i] = fib[i - 1] + fib[i - 2]

print("fib=" + str(fib))


                        
1n ← 12, fib ← [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], fib[0] ← 01n→ 12 = 12  #@n=5, 202fib→ [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] = [0] * n1234fib[0]→ 0 = 05fib[1]→ 1 = 1⋯
2fib[i] ← 1pass 1 of 10⋯7#?range_start8for i2 in range(2, n12):9    fib[i]→ 1 = fib[i - 1]1 + fib[i - 2]0⋯All 10 passes — pass 1 is the card abovepassifib[i - 1]fib[i - 2]fib[i]
12101
23112
34213
45325
56538
678513
7813821
89211334
910342155
1011553489
3print("fib=" + str(fib))⋯11print("fib=" + str(fib[0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]))⋯outputfib=[0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]
1n ← 5, fib ← [0, 0, 0, 0, 0], fib[0] ← 0, fib[1] ← 11n→ 5 = 52fib→ [0, 0, 0, 0, 0] = [0] * n534fib[0]→ 0 = 05fib[1]→ 1 = 1⋯
2fib[i] ← 1pass 1 of 3⋯8for i2 in range(2, n5):9    fib[i]→ 1 = fib[i - 1]1 + fib[i - 2]0⋯All 3 passes — pass 1 is the card abovepassifib[i - 1]fib[i - 2]fib[i]
12101
23112
34213
3print("fib=" + str(fib))⋯11print("fib=" + str(fib[0, 1, 1, 2, 3]))⋯outputfib=[0, 1, 1, 2, 3]
1n ← 20, fib ← [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]1n→ 20 = 202fib→ [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] = [0] * n2034fib[0]→ 0 = 05fib[1]→ 1 = 1⋯
2fib[i] ← 1pass 1 of 18⋯8for i2 in range(2, n20):9    fib[i]→ 1 = fib[i - 1]1 + fib[i - 2]0⋯18 passes — pass 1 is the card abovepassifib[i - 1]fib[i - 2]fib[i]
12101
23112
34213
45325
56538
678513
7813821
89211334
910342155
⋯ 7 more passes ⋯
171815979872584
1819258415974181
3print("fib=" + str(fib))⋯11print("fib=" + str(fib[0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181]))⋯outputfib=[0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181]

All 10 passes — pass 1 is the card above
pass	`i`	`fib[i - 1]`	`fib[i - 2]`	`fib[i]`
1	2	1	0	1
2	3	1	1	2
3	4	2	1	3
4	5	3	2	5
5	6	5	3	8
6	7	8	5	13
7	8	13	8	21
8	9	21	13	34
9	10	34	21	55
10	11	55	34	89

All 3 passes — pass 1 is the card above
pass	`i`	`fib[i - 1]`	`fib[i - 2]`	`fib[i]`
1	2	1	0	1
2	3	1	1	2
3	4	2	1	3

18 passes — pass 1 is the card above
pass	`i`	`fib[i - 1]`	`fib[i - 2]`	`fib[i]`
1	2	1	0	1
2	3	1	1	2
3	4	2	1	3
4	5	3	2	5
5	6	5	3	8
6	7	8	5	13
7	8	13	8	21
8	9	21	13	34
9	10	34	21	55
⋯ 7 more passes ⋯
17	18	1597	987	2584
18	19	2584	1597	4181

The user specifies how many Fibonacci numbers they want. The condition i < n controls how many iterations run.

Search for a value in list

Check if a specific number exists.

                            target
                            target = 67 (default)target = 12target = 99
                        

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

                            nums = [23, 45, 12, 67, 34, 89, 41]
target = 67

print("nums=" + str(nums))
found = 0
position = -1

for i in range(len(nums)):
    if nums[i] == target:
        found = 1
        position = i
nums = [23, 45, 12, 67, 34, 89, 41]
target = 12

print("nums=" + str(nums))
found = 0
position = -1

for i in range(len(nums)):
    if nums[i] == target:
        found = 1
        position = i
nums = [23, 45, 12, 67, 34, 89, 41]
target = 99

print("nums=" + str(nums))
found = 0
position = -1

for i in range(len(nums)):
    if nums[i] == target:
        found = 1
        position = i

                        
1nums ← [23, 45, 12, 67, 34, 89, 41], target ← 67, found ← 0, position ← -11nums→ [23, 45, 12, 67, 34, 89, 41] = [23, 45, 12, 67, 34, 89, 41]2target→ 67 = 67  #@target=12, 9934print("nums=" + str(nums[23, 45, 12, 67, 34, 89, 41]))5found→ 0 = 06position→ -1 = -1⋯outputnums=[23, 45, 12, 67, 34, 89, 41]
2for i in range(len(nums)):pass 1 of 7⋯8for i0 in range(len(nums[23, 45, 12, 67, 34, 89, 41])):9    if nums[i] == target:10        found = 1⋯All 7 passes — pass 1 is the card abovepassinums[i]targetfoundposition
10————
21————
32————
43676713
54————
65————
76————
3found ← 1, position ← 3⋯8for i in range(len(nums)):9    if nums[i]67 == target67:10        found→ 1 = 111        position→ 3 = i3⋯
1nums ← [23, 45, 12, 67, 34, 89, 41], target ← 12, found ← 0, position ← -11nums→ [23, 45, 12, 67, 34, 89, 41] = [23, 45, 12, 67, 34, 89, 41]2target→ 12 = 1234print("nums=" + str(nums[23, 45, 12, 67, 34, 89, 41]))5found→ 0 = 06position→ -1 = -1⋯outputnums=[23, 45, 12, 67, 34, 89, 41]
2for i in range(len(nums)):pass 1 of 7⋯8for i0 in range(len(nums[23, 45, 12, 67, 34, 89, 41])):9    if nums[i] == target:10        found = 1⋯All 7 passes — pass 1 is the card abovepassinums[i]targetfoundposition
10————
21————
32121212
43————
54————
65————
76————
3found ← 1, position ← 2⋯8for i in range(len(nums)):9    if nums[i]12 == target12:10        found→ 1 = 111        position→ 2 = i2⋯
1nums ← [23, 45, 12, 67, 34, 89, 41], target ← 99, found ← 0, position ← -11nums→ [23, 45, 12, 67, 34, 89, 41] = [23, 45, 12, 67, 34, 89, 41]2target→ 99 = 9934print("nums=" + str(nums[23, 45, 12, 67, 34, 89, 41]))5found→ 0 = 06position→ -1 = -1⋯outputnums=[23, 45, 12, 67, 34, 89, 41]
2for i in range(len(nums)):pass 1 of 7⋯8for i0 in range(len(nums[23, 45, 12, 67, 34, 89, 41])):9    if nums[i] == target:10        found = 1⋯All 7 passes — pass 1 is the card abovepassi
10
21
32
43
54
65
76

All 7 passes — pass 1 is the card above
pass	`i`	`nums[i]`	`target`	`found`	`position`
1	0	—	—	—	—
2	1	—	—	—	—
3	2	—	—	—	—
4	3	67	67	1	3
5	4	—	—	—	—
6	5	—	—	—	—
7	6	—	—	—	—

All 7 passes — pass 1 is the card above
pass	`i`	`nums[i]`	`target`	`found`	`position`
1	0	—	—	—	—
2	1	—	—	—	—
3	2	12	12	1	2
4	3	—	—	—	—
5	4	—	—	—	—
6	5	—	—	—	—
7	6	—	—	—	—

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

Loop through, check each element against the target. When found, we can stop early or record the position.

pass	`i`	`fib[i - 1]`	`fib[i - 2]`	`fib[i]`
1	2	1	0	1
2	3	1	1	2
3	4	2	1	3
4	5	3	2	5
5	6	5	3	8
6	7	8	5	13
7	8	13	8	21
8	9	21	13	34
9	10	34	21	55
10	11	55	34	89

pass	`i`	`nums[i]`	`target`	`found`	`position`
1	0	—	—	—	—
2	1	—	—	—	—
3	2	—	—	—	—
4	3	67	67	1	3
5	4	—	—	—	—
6	5	—	—	—	—
7	6	—	—	—	—

pass	`i`	`nums[i]`	`target`	`found`	`position`
1	0	—	—	—	—
2	1	—	—	—	—
3	2	12	12	1	2
4	3	—	—	—	—
5	4	—	—	—	—
6	5	—	—	—	—
7	6	—	—	—	—

pass	`i`	`fib[i - 1]`	`fib[i - 2]`	`fib[i]`
1	2	1	0	1
2	3	1	1	2
3	4	2	1	3
4	5	3	2	5
5	6	5	3	8
6	7	8	5	13
7	8	13	8	21
8	9	21	13	34
9	10	34	21	55
10	11	55	34	89

pass	`i`	`nums[i]`	`target`	`found`	`position`
1	0	—	—	—	—
2	1	—	—	—	—
3	2	—	—	—	—
4	3	67	67	1	3
5	4	—	—	—	—
6	5	—	—	—	—
7	6	—	—	—	—

pass	`i`	`nums[i]`	`target`	`found`	`position`
1	0	—	—	—	—
2	1	—	—	—	—
3	2	12	12	1	2
4	3	—	—	—	—
5	4	—	—	—	—
6	5	—	—	—	—
7	6	—	—	—	—

pass	`i`	`fib[i - 1]`	`fib[i - 2]`	`fib[i]`
1	2	1	0	1
2	3	1	1	2
3	4	2	1	3
4	5	3	2	5
5	6	5	3	8
6	7	8	5	13
7	8	13	8	21
8	9	21	13	34
9	10	34	21	55
10	11	55	34	89

pass	`i`	`nums[i]`	`target`	`found`	`position`
1	0	—	—	—	—
2	1	—	—	—	—
3	2	—	—	—	—
4	3	67	67	1	3
5	4	—	—	—	—
6	5	—	—	—	—
7	6	—	—	—	—

pass	`i`	`nums[i]`	`target`	`found`	`position`
1	0	—	—	—	—
2	1	—	—	—	—
3	2	12	12	1	2
4	3	—	—	—	—
5	4	—	—	—	—
6	5	—	—	—	—
7	6	—	—	—	—