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Loops

First PublishedByAtif Alam

for over a list — When you need to do something with each item in a sequence (list, tuple, etc.).

for fruit in ['apple', 'banana', 'cherry']:
    print(fruit)

With index using enumerate:

fruits = ['apple', 'banana', 'cherry']
for index, fruit in enumerate(fruits):
    print(index, fruit)  # 0 apple, 1 banana, 2 cherry

for with range — When you need a loop that runs N times or over numeric indices (0 to n-1, or 1 to n-1).

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


for i in range(1, n):     # 1 .. n-1 (e.g. split between i-1 and i)
    ...

for over a string — When you need to process or check each character in a string.

for char in "hello":
    print(char)  # h, e, l, l, o

for over a dictionary — When you need to loop over keys, values, or key–value pairs of a dict.

person = {'name': 'Alice', 'age': 30}


# Keys only
for key in person:
    print(f"Key: {key}")


# Keys and values
for key, value in person.items():
    print(f"{key} → {value}")


# Keys in sorted order
for key in sorted(person):
    print(key)

Parallel iteration with zip — When you have two (or more) lists of the same length and want to loop over them together, one pair at a time.

names = ['Alice', 'Bob']
scores = [85, 92]


for name, score in zip(names, scores):
    print(f"{name} scored {score}")

Reading a file line by line — When you need to process a text file one line at a time without loading the whole file into memory.

with open(filename, 'r') as file:
    for line in file:
        print(line.rstrip())  # or process line

while loop — When you need to repeat until a condition becomes false (e.g. “keep going while count < 3” or “until user quits”).

count = 0
while count < 3:
    print("Count:", count)
    count += 1