Foundations

Conditionals

Make Decisions

You're building a login system. If the password matches, show the dashboard. Otherwise, show "Invalid password". Programs need to take different paths based on conditions - that's what conditionals do.

Simple if

Execute code only when a condition is true.

simple.py 
x = 

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

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

x = 

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

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

x = 

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

if x > 20:
    print("x > 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 
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]

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

Find the minimum in a list

Same idea, but track the smallest.

min.py 
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]

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.

fibonacci_n.py 
n = 
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 = 
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 = 
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))

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.

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

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 = 

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 = 

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

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

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