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04 Control-Flow - 04 Loops
1. ‘for’ Loop: Iterating Over a Collection
The for loop is used to iterate over a sequence (like a list, tuple, or string) and execute a block of code for each item in the sequence.
Basic Example 1: Iterating Over a List:
for i in [0, 1, 2]:
print("Meow")Basic Example 2: Using range() to Repeat a Fixed Number of Times:
for i in range(3): # Iterates over a range of 3 numbers (0, 1, 2)
print("Meow")Example 3: Using range() to Iterate Over a Sequence of Numbers:
for i in range(0, n): # Sequence from 0 to n-1
# Code to execute2. ‘while’ Loop: Loops Based on Condition
The while loop is useful when you don’t know how many times the loop should run in advance. It keeps executing as long as the condition evaluates to True.
Decrementing Example:
i = 3
while i != 0:
print("Meow")
i = i - 1Incrementing Example:
i = 0
while i != 3: # Alternatively: while i < 3
print("Meow")
i = i + 1 # or i += 1Using while to Write Clear Prompts:
prompt = "Type this message\n"
prompt += "Add 'quit' to exit\n:"
message = ""
while message != "quit":
message = input(prompt)
print(message)Example of a counter with while loop:
number = 1
while number <= 5:
print(number)
number += 1 # Can also be number *= 2 to double the value3. Using a Flag to Exit a Loop
A flag is a variable that controls whether the loop continues or stops. It’s particularly useful when there are multiple conditions to consider.
When many possible events might occur to stop the program, testing all these conditions in a single while statement can become complicated. Instead, we can use a flag to simplify this.
The flag acts as a signal to the program. The loop will run as long as the flag is set to True, and it will stop when any event sets the flag to False. This way, our while statement only needs to check one condition: whether the flag is True.
Other conditions that could modify the flag can be handled separately in the rest of the program.
prompt = "\nType to have it repeated"
prompt += "\n: "
active = True
while active: # Loop continues while 'active' is True
message = input(prompt)
if message == "quit":
active = False # When "quit" is entered, the flag is set to False, ending the loop
else:
print(message)This approach keeps the program flow simple, only needing to check the flag condition in the while loop.
4. Using ‘break’ to Exit a Loop
The break statement immediately exits the loop, regardless of the condition.
prompt = "\nEnter the city you have visited:"
prompt += "\nUse 'quit' to exit. "
while True: # This loop will run indefinitely until 'break' is encountered
city = input(prompt)
if city == "quit":
break # Exits the loop when 'quit' is entered
else:
print(f"\n{city.title()} is lovely")5. Using ‘continue’ in a Loop
The continue statement skips the current iteration of the loop and moves on to the next one.
Example:
number = 0
while number < 10:
number += 1
if number % 2 == 0: # Skips even numbers
continue
else:
print(number) # Prints only odd numbers6. Modifying Lists Inside Loops
You can use a while loop to modify lists as you work through them. For example, you can move items from one list to another or remove specific values.
Example 1: Moving Items Between Lists:
unconfirmed_users = ['alice', 'brian', 'candace']
confirmed_users = []
while unconfirmed_users: # Continues as long as the list is not empty
user = unconfirmed_users.pop()
print(f"Verifying User: {user.title()}")
confirmed_users.append(user)
print("\nThe following users have been verified:")
for user in confirmed_users:
print(f"{user.title()}")Example 2: Removing Specific Values from a List:
pets = ['dog', 'cat', 'dog', 'goldfish', 'cat', 'rabbit', 'cat']
print(pets)
while "cat" in pets:
pets.remove("cat") # Removes the first instance of "cat"
print(pets)7. Filling a Dictionary with User Input
A while loop can be used to collect multiple user inputs and store them in a dictionary.
responses = {}
polling = True
while polling:
name = input("\nWhat's your name: ")
response = input("What mountain would you like to climb someday? ")
responses[name] = response # Add user response to dictionary
repeat = input("\nWould you like another to take this poll? (y/n): ")
if repeat == "n":
polling = False # Ends the loop when 'n' is entered
print("\n-- Poll Results --")
for name, response in responses.items():
print(f"{name.title()} wants to climb {response.title()}.")8. Functions with Loops
You can define functions that use loops to perform repetitive tasks.
def main():
number = get_number()
meow(number)
def get_number():
while True:
n = int(input("What's n? "))
if n > 0:
break # Ensures that n is positive
return n
def meow(n):
for _ in range(n): # Repeats n times
print("Meow")
main()9. Simulating a ‘for’ Loop with a ‘while’ Loop
Sometimes you may want to simulate a for loop using a while loop.
Example 1: Simulating for with a Range:
n = i
while n < j:
statement
n += 1Example 2: Simulating Iterating Over a List:
i = 0
while i < len(l):
n = l[i]
statement
i += 1