Loops

Loops in Python

If you want to repeat a task multiple times, Python has a way to do that easily using loops. There are two types of basic loops in python: for loops and while loops.

For loops

For loops iterate over a sequence. Let’s say you wanted to repeat a simple task 5 times. Here is how you would do it.

The following for loop iterates through each of the numbers in the list [1,2,3,4,5], and assigns the value x to it. Then, it executes the code within the for loop.

for x in [1,2,3,4,5]:    
    print("We're on loop number",x)

What if you wanted to repeat a task 10 times instead? 100 times? 1000 times??

It would be difficult to write a list with 1000 numbers [1,2,3,4,5,6,……….,998,999,1000]. There is a function that can create this list for you.

Here’s an example of how to create the list using the range function:

The below code creates a list starting at 1 and ending at 10.

range(1,11)

Now try replacing the list [1,2,3,4,5] in the above example with this range function!

The list in the for loop doesn’t just have to be numbers, it can be anything! Here is another example of what you can do with it:

fruits = ["apple","banana","orange"]

for x in fruits:
    print(x)

While loops

While loops are similar to for loops, but instead of iterating over a sequence, they repeat for as long as a certain condition is met. For example, let’s create a while loop that repeates 5 times.

count = 0

while count < 5:
    print("The current count is", count)
    count += 1  #This is the same thing as count = count + 1

As you can see, all of the code below the while loop gets executed as long as the variable count is less than 5.

Be careful! Make sure that the condition becomes False at some point, or the loop will continue infinitely.

Break statements

You learned earlier that for loops and while loops keep iterating over a sequence until it reaches the end or until a condition is met.

There is another way to stop running these loops before the end of the list or before a condition is met.

You can do this with the break statement. This lets you break out of the loop in either a for loop or while loop.

Here’s an example with a while loop:

count = 0

while True:  #This will make the while loop run forever! unless there is a break statement somewhere in the loop.
    print("count is", count)
    
    if (count == 10): #This says that i the count is equal to 10, then exit the while loop
        print("Breaking out of the loop!")
        break
    
    count += 1  # this is equal to count = count + 1

Challenges

1. Given a list of foods, write a program that writes out “I like [name of food]” unless it is the user’s least favorite, which you ask them for at the beginning.

foodList = ["apples", "ice cream", "pizza", "bananas", "carrots", "squash", "cucumbers", "sandwich", "oatmeal", "cookies", "cake", "spinach", "kale"]

2. Given a large list such as the one below, write a program that can find a particular number from this list.

numList = [69, 430, 363, 80, 166, 476, 250, 38, 84, 405, 41, 302, 480, 57, 155, 17, 350, 87, 313, 36, 191, 367, 163, 167, 185, 93, 131, 314, 444, 452, 47, 457, 28, 21, 405, 350, 241, 121, 411, 152, 244, 207, 199, 371, 193, 4, 286, 482, 496, 283, 23, 466, 8, 84, 47, 231, 112, 329, 328, 85, 208, 161, 42, 251, 277, 207, 496, 3, 137, 138, 436, 447, 442, 313, 479, 464, 191, 19, 78, 333, 480, 36, 378, 232, 438, 451, 139, 444, 392, 75, 245, 151, 225, 50, 369, 49, 73, 325, 430, 335]

For example, ask the user to enter a number in the console, then write to console if it was found. Hint: You will need to convert the information entered by the user into an integer by using int(x).

Question

How would you change this program if the list is very large? If you’ve found a number, do you need to continue searching the rest of the list?