Conditionals and Loops

Conditional Statements (if statements)

Basic if statement

age = 20

if age >= 18:
    print("성인입니다")

# 출력: 성인입니다

if-else statement

age = 15

if age >= 18:
    print("성인입니다")
else:
    print("미성년자입니다")

# 출력: 미성년자입니다

if-elif-else statement

score = 85

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
elif score >= 60:
    grade = "D"
else:
    grade = "F"

print(f"학점: {grade}")  # 학점: B

Nested if statement

age = 20
has_license = True

if age >= 18:
    if has_license:
        print("운전 가능합니다")
    else:
        print("면허를 취득하세요")
else:
    print("성인이 되어야 합니다")

Conditional Expression (Ternary Operator)

# One-line if-else
age = 20
status = "성인" if age >= 18 else "미성년자"
print(status)  # 성인

# Nested conditional expressions
score = 85
grade = "A" if score >= 90 else "B" if score >= 80 else "C"
print(grade)  # B

# Practical example
numbers = [1, -2, 3, -4, 5]
positive = [n if n > 0 else 0 for n in numbers]
print(positive)  # [1, 0, 3, 0, 5]

Combining multiple conditions

age = 25
income = 3000

# and operator
if age >= 18 and income >= 2000:
    print("대출 가능")

# or operator
if age < 18 or age >= 65:
    print("할인 대상")

# not operator
is_weekend = False
if not is_weekend:
    print("평일입니다")

# Complex conditions
score = 85
attendance = 0.9

if (score >= 80 and attendance >= 0.8) or score >= 95:
    print("합격")

Loops (for loop)

Basic for loop

# Iterate over list
fruits = ["사과", "바나나", "체리"]
for fruit in fruits:
    print(fruit)

# Iterate over string
for char in "Python":
    print(char)

# Using range
for i in range(5):
    print(i)  # 0, 1, 2, 3, 4

for i in range(1, 6):
    print(i)  # 1, 2, 3, 4, 5

for i in range(0, 10, 2):
    print(i)  # 0, 2, 4, 6, 8

Index and value with enumerate

fruits = ["사과", "바나나", "체리"]

for index, fruit in enumerate(fruits):
    print(f"{index}: {fruit}")
# 0: 사과
# 1: 바나나
# 2: 체리

# Specify starting index
for index, fruit in enumerate(fruits, 1):
    print(f"{index}. {fruit}")
# 1. 사과
# 2. 바나나
# 3. 체리

Iterate multiple sequences simultaneously with zip

names = ["홍길동", "김철수", "이영희"]
scores = [85, 92, 78]

for name, score in zip(names, scores):
    print(f"{name}: {score}점")
# 홍길동: 85점
# 김철수: 92점
# 이영희: 78점

# Different lengths (stops at the shortest)
numbers = [1, 2, 3, 4, 5]
letters = ['a', 'b', 'c']

for num, letter in zip(numbers, letters):
    print(num, letter)
# 1 a
# 2 b
# 3 c

Iterate over dictionary

person = {"name": "홍길동", "age": 25, "city": "서울"}

# Iterate over keys
for key in person:
    print(key)

# Iterate over values
for value in person.values():
    print(value)

# Iterate over key-value pairs
for key, value in person.items():
    print(f"{key}: {value}")

Nested loops

# Multiplication table
for i in range(2, 10):
    for j in range(1, 10):
        print(f"{i} x {j} = {i*j}")
    print()  # 단 사이 빈 줄

# 2D list
matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

for row in matrix:
    for value in row:
        print(value, end=" ")
    print()  # 줄바꿈

Loops (while loop)

Basic while loop

# Countdown
count = 5
while count > 0:
    print(count)
    count -= 1
print("발사!")

# Until condition is met
number = 1
while number < 100:
    number *= 2
print(number)  # 128

Infinite loop

# Repeat user input
while True:
    answer = input("계속하시겠습니까? (y/n): ")
    if answer.lower() == 'n':
        break
    print("계속합니다...")

print("종료")

while-else

# else executes when condition becomes False (normal termination)
count = 0
while count < 3:
    print(count)
    count += 1
else:
    print("정상 종료")

# else doesn't execute when terminated with break
count = 0
while count < 10:
    if count == 3:
        break
    count += 1
else:
    print("이 부분은 실행 안 됨")

Loop Control

break - Exit loop

# Exit when desired value is found
numbers = [1, 3, 5, 7, 9, 2, 4, 6]

for num in numbers:
    if num % 2 == 0:
        print(f"첫 번째 짝수: {num}")
        break

# break in nested loops
found = False
for i in range(10):
    for j in range(10):
        if i * j > 50:
            found = True
            break
    if found:
        break

continue - Skip to next iteration

# Print only odd numbers
for i in range(1, 11):
    if i % 2 == 0:
        continue
    print(i)  # 1, 3, 5, 7, 9

# Skip specific values
words = ["apple", "banana", "", "cherry", ""]
for word in words:
    if not word:
        continue
    print(word.upper())

pass - Do nothing

# Code to be implemented later
for i in range(5):
    if i == 2:
        pass  # TODO: 나중에 구현
    else:
        print(i)

# Empty class or function
class MyClass:
    pass  # 일단 뼈대만

def my_function():
    pass  # 나중에 구현

Practical Examples

Prime number check

def is_prime(n):
    """소수 판별"""
    if n < 2:
        return False

    for i in range(2, int(n ** 0.5) + 1):
        if n % i == 0:
            return False
    return True

# 100 이하 소수 출력
print("100 이하 소수:")
for num in range(2, 101):
    if is_prime(num):
        print(num, end=" ")

Greatest Common Divisor (GCD)

def gcd(a, b):
    """유클리드 호제법"""
    while b != 0:
        a, b = b, a % b
    return a

print(gcd(48, 18))  # 6
print(gcd(100, 75))  # 25

Fibonacci sequence

def fibonacci(n):
    """n번째 피보나치 수"""
    if n <= 1:
        return n

    a, b = 0, 1
    for _ in range(2, n + 1):
        a, b = b, a + b
    return b

# 처음 10개 출력
print("피보나치 수열:")
for i in range(10):
    print(fibonacci(i), end=" ")

Number guessing game

import random

def guessing_game():
    """숫자 맞추기 게임"""
    answer = random.randint(1, 100)
    attempts = 0
    max_attempts = 10

    print("1부터 100 사이의 숫자를 맞춰보세요!")
    print(f"기회는 {max_attempts}번입니다.")

    while attempts < max_attempts:
        try:
            guess = int(input(f"\n남은 기회 {max_attempts - attempts}번 > "))

            attempts += 1

            if guess < answer:
                print("UP! 더 큰 숫자입니다.")
            elif guess > answer:
                print("DOWN! 더 작은 숫자입니다.")
            else:
                print(f"정답! {attempts}번 만에 맞췄습니다!")
                return True

        except ValueError:
            print("숫자를 입력하세요!")
            continue

    print(f"\n실패! 정답은 {answer}였습니다.")
    return False

# 게임 실행
guessing_game()

Print calendar

def print_calendar(year, month):
    """간단한 달력 출력"""
    import calendar

    # 해당 월의 달력
    cal = calendar.monthcalendar(year, month)

    # 헤더
    print(f"\n{'':^20}")
    print(f"{year}년 {month}월".center(20))
    print(f"{'':^20}")
    print("월  화  수  목  금  토  일")
    print("-" * 22)

    # 날짜 출력
    for week in cal:
        for day in week:
            if day == 0:
                print("   ", end=" ")
            else:
                print(f"{day:2d} ", end=" ")
        print()

print_calendar(2024, 12)

Simple calculator

def calculator():
    """사칙연산 계산기"""
    print("=== 계산기 ===")
    print("종료하려면 'q'를 입력하세요")

    while True:
        # 수식 입력
        expression = input("\n수식 입력 (예: 2 + 3): ")

        if expression.lower() == 'q':
            print("종료합니다")
            break

        try:
            # 수식 파싱
            parts = expression.split()
            if len(parts) != 3:
                print("형식: 숫자 연산자 숫자")
                continue

            num1 = float(parts[0])
            operator = parts[1]
            num2 = float(parts[2])

            # 계산
            if operator == '+':
                result = num1 + num2
            elif operator == '-':
                result = num1 - num2
            elif operator == '*':
                result = num1 * num2
            elif operator == '/':
                if num2 == 0:
                    print("0으로 나눌 수 없습니다")
                    continue
                result = num1 / num2
            else:
                print("지원하지 않는 연산자입니다")
                continue

            print(f"결과: {result}")

        except ValueError:
            print("올바른 숫자를 입력하세요")
        except Exception as e:
            print(f"오류: {e}")

# calculator()  # 주석 해제하여 실행

Star patterns

def print_patterns():
    """다양한 별 패턴"""
    n = 5

    # 패턴 1: 삼각형
    print("패턴 1:")
    for i in range(1, n + 1):
        print("*" * i)

    print()

    # 패턴 2: 역삼각형
    print("패턴 2:")
    for i in range(n, 0, -1):
        print("*" * i)

    print()

    # 패턴 3: 피라미드
    print("패턴 3:")
    for i in range(1, n + 1):
        spaces = " " * (n - i)
        stars = "*" * (2 * i - 1)
        print(spaces + stars)

    print()

    # 패턴 4: 다이아몬드
    print("패턴 4:")
    # 상단
    for i in range(1, n + 1):
        print(" " * (n - i) + "*" * (2 * i - 1))
    # 하단
    for i in range(n - 1, 0, -1):
        print(" " * (n - i) + "*" * (2 * i - 1))

print_patterns()

Grade processing

def process_grades():
    """성적 입력 및 통계"""
    scores = []

    print("=== 성적 입력 ===")
    print("종료하려면 -1을 입력하세요")

    while True:
        try:
            score = int(input("점수 입력: "))

            if score == -1:
                break

            if score < 0 or score > 100:
                print("0~100 사이의 점수를 입력하세요")
                continue

            scores.append(score)

        except ValueError:
            print("숫자를 입력하세요")

    if not scores:
        print("입력된 점수가 없습니다")
        return

    # 통계 계산
    total = sum(scores)
    average = total / len(scores)
    highest = max(scores)
    lowest = min(scores)

    # 학점 분포
    grade_count = {"A": 0, "B": 0, "C": 0, "D": 0, "F": 0}
    for score in scores:
        if score >= 90:
            grade_count["A"] += 1
        elif score >= 80:
            grade_count["B"] += 1
        elif score >= 70:
            grade_count["C"] += 1
        elif score >= 60:
            grade_count["D"] += 1
        else:
            grade_count["F"] += 1

    # 결과 출력
    print("\n=== 성적 통계 ===")
    print(f"인원: {len(scores)}명")
    print(f"총점: {total}점")
    print(f"평균: {average:.1f}점")
    print(f"최고점: {highest}점")
    print(f"최저점: {lowest}점")

    print("\n=== 학점 분포 ===")
    for grade, count in grade_count.items():
        if count > 0:
            print(f"{grade}: {count}명")

# process_grades()  # 주석 해제하여 실행

Frequently Asked Questions

Q1. Should I use for or while?

A: Choose based on the situation

# for: When you know the number of iterations
for i in range(10):
    print(i)

# while: Until a condition is met
while user_input != "quit":
    user_input = input("명령: ")

# Practical choice
# - Iterate collections: for
# - Condition-based iteration: while
# - Infinite loop: while True

Q2. range vs enumerate, which to use?

A: Use enumerate if you need both index and value

fruits = ["사과", "바나나", "체리"]

# ❌ Not recommended
for i in range(len(fruits)):
    print(f"{i}: {fruits[i]}")

# ✅ Recommended
for i, fruit in enumerate(fruits):
    print(f"{i}: {fruit}")

Q3. What's the difference between break and return?

A: break only exits the loop, return exits the function

def find_number(numbers, target):
    for num in numbers:
        if num == target:
            return num  # 함수 종료
    return None

def process_data(data):
    for item in data:
        if item < 0:
            break  # 반복문만 종료
        print(item)
    print("함수는 계속 실행")  # 이 부분은 실행됨

Q4. How do I prevent infinite loops?

A: Make conditions and exit conditions clear

# ❌ Dangerous infinite loop
count = 0
while count < 10:
    print(count)
    # count를 증가시키지 않음!

# ✅ Safe loop
count = 0
max_iterations = 1000  # 안전장치
while count < 10 and max_iterations > 0:
    print(count)
    count += 1
    max_iterations -= 1

# ✅ Clear exit condition
while True:
    user_input = input("입력 (q=종료): ")
    if user_input == 'q':
        break
    process(user_input)

Next Steps

You've mastered program flow control!

Key takeaways:
✅ Conditional branching with if-elif-else
✅ Iterate collections with for
✅ Condition-based iteration with while
✅ Control with break, continue, pass
✅ Practical examples (games, calculator, etc.)

Next step: Learn to make code reusable in Functions!