기본 함수
for문
data = ['a','b','c','d']
for문 - range
for i in range(4):
print(i) # 0, 1, 2, 3
print(data[i]) # a, b, c, d
for문 - range(startIdx, endIdx, step)
for i in range(0,4,1):
print(data[i]) # a, b, c, d
for문 - 역행
for i in reversed(data):
print(i) # d, c, b, a