introducing while loops

introducing_loops.py

# Python 2.7

# example code from Crash Course Python by Eric Matthes

# Recall for loops
# The for loop takes a collection of items and executes a block of code once
# for each item in the collection.

# PYTHON WHILE LOOPS
# In contrast, the while loop runs as longs as (or while) a certain condition is true

# For example, you can use a while loop to count up through a series of numbers.
# Let's say, we need a while loop that counts from 1 to 5:

current_number = 1 # we start counting from 1
while current_number <= 5: 
    print(current_number) 
    current_number += 1

# This loop is set to keep running as long as the value of current_number is
# less than or equal to 5.
# The code inside the loop prints the value of current_number and
# then adds 1 to that value with current_number += 1.

# (The += operator is shorthand for current_number = current_number + 1.)

# Python runs the loop as long as the condition ccurrent_number<= 5 is true.
# Because 1 is less than 5, Python prints 1 and then adds 1,
# making the current number 2, and so on.

# The loop stops when the value of current_number is greater than 5 and our program ends.

# While loops are used widely!
# The programs you use every day most likely contain while loops.

# For example, a game needs a while loop to keep running as long as you
# want to keep playing, so it can stop running when you want to quit.

# While loops are super useful!

####
####

# end_loop.py

# HOW TO STOP A LOOP

# Remember to write clear prompts!

prompt = ("\nTell me your name, and I will repeat it back to you:")
prompt += "\nEnter 'quit' to end the program. "

message = ""
while message != 'quit':
    message = raw_input(prompt)
    print(message)

# This program works well, except that it prints the word 'quit' as if it
# were an actual message.

# How can we fix this?



message = ""
while message != 'quit':
    message = raw_input(prompt)

    if message != 'quit':
        print(message)

####
####

# again, consider a imple parrot program...
##prompt = ("\nTell me something, and I will repeat it back to you:")
##prompt += "\nEnter 'quit' to end the program. "
##
##message = ""
##while message != 'quit':
##    message = raw_input(prompt)

##    if message != 'quit':
##        print(message)

# This program performs its task while a given condition is true.
# But, what about a more compplicated programs in which many different events
# could cause the program to stop running?

# Think of a game.
# Usually, several differentt events can end the game.
# i.e. when time runs out, when the player dies, when an enemy reaches a certain point, etc.
# If these examples occur, it can make the program end.

# If many possible events might occur to stop the program, just trying to test
# all these conditions in one while statement can become complicated and difficult!
###################
# Enter... FLAG
# For a program that should run only as long as many conditions are true, you
# can define one variable that determiens whether or not the entire the program is active.
# This variable, called a flag, acts as a signal to the program.

# We can run our programs so they run while the flag is set to True
# and stop running when any of several events sets the value of the flag to False

# so... SO, our overall while statement needs to check only one condition:
# whether or not the flag is currently True.
# This way, all our other tests can be neatly organized in the rest of the program.

# Alright, enough talking, let's add a flag to the parrot program!!!

prompt = ("\nTell me something, and I will repeat it back to you:")
prompt += "\nEnter 'quit' to end the program. "

active = True # starting our flag
while active:
    message = raw_input(prompt)

    if message != 'quit':
        active = False # checking flag validity
    else:
        print(message)
        
# Python 2.7

# Example from Python Crash Course by Eric Matthes
# Using break to Exit a loop

# So far, we've looked at stopping a while loop by using a flag variable
# You can also use a BREAK statement...
# To exit a while loop immediately without running any remaining code in the loop,
# regardless of the results of any conditional test, use the break statement.

# The break statement directs the flow of your program
# You can use it to control which lines of code are executed and which aren't,
# so the program only executes code that you want it to, when you want it to
# Think of the term abstraction?

# Let's look at an example
# Consider a program that asks the user about places they've visited.
# To stop the while loop in this program by calling break as soon
# as the user enters the 'quit' value:

prompt = ("\nPlease enter the name of a city you have visited:")
prompt += "\n(Enter 'quit' when you are finished.) "

while True:
    city = raw_input(prompt)

    if city == 'quit':
        break
    else:
        print("I'd love to go " + city.title() + "!")

# this loop that starts with a "while True" will run continuously
# until it reaches a break statement

# You can use the break statement in any Python loops.