Loops & reusable functions

Understand the concept

A for loop visits each item in an iterable such as a list. Python uses indentation to group the loop body. range(3) yields 0, 1, and 2; the stop value is excluded. An if statement lets you choose which values to process.

A function groups work under a name. Parameters receive inputs; return sends a result back to the caller. print displays a value but does not replace return. A function without an explicit return returns None.

Keep functions focused. An accumulator starts with an initial value and updates during iteration. Return after the loop when you need the result from all items; returning inside it often ends the function too early. Think about empty inputs and boundary values.

def total_positive(values):
    total = 0
    for value in values:
        if value > 0:
            total += value
    return total

print(total_positive([4, -2, 6, 0]))
print(total_positive([]))

Output / what to expect

10
0

Try it yourself

Write count_even(values) that returns how many integers are even. Check [2, 3, 4, 0] and an empty list.

def count_even(values):
    count = 0
    for value in values:
        if value % 2 == 0:
            count += 1
    return count

print(count_even([2, 3, 4, 0]))  # 3
print(count_even([]))  # 0

Concept samjho

for loop list jaise iterable ka har item ek-ek karke leta hai. Indentation batata hai kaunsa code loop ke andar hai. range(3) se 0, 1, 2 milte hain; stop value include nahi hoti. if condition se decide karo kaunsi values process karni hain.

Function repeated kaam ko ek naam deta hai. Parameters inputs lete hain; return result caller ko wapas deta hai. print sirf screen par dikhata hai, return ka replacement nahi hai. Explicit return nahi ho toh function None return karta hai.

Function ko ek clear task do. Accumulator initial value se shuru hota hai aur loop mein update hota hai. Sab items ka result chahiye toh return loop ke baad rakho. Loop ke andar return karne se function jaldi khatam ho sakta hai. Empty list aur boundary values bhi socho.

def total_positive(values):
    total = 0
    for value in values:
        if value > 0:
            total += value
    return total

print(total_positive([4, -2, 6, 0]))
print(total_positive([]))

Output / kya expect karein

10
0

Ab khud try karo

count_even(values) banao jo even integers ki ginti return kare. [2, 3, 4, 0] aur empty list dono check karo.

def count_even(values):
    count = 0
    for value in values:
        if value % 2 == 0:
            count += 1
    return count

print(count_even([2, 3, 4, 0]))  # 3
print(count_even([]))  # 0