Repeated Iterations with Conditions

item = 25 

if item > 20:
    magnitude = 'greater than 20'
elif item > 10:
    magnitude = 'between 10 and 20'
else:
    magnitude = '10 or less'
 
magnitude

'greater than 20'

We know that we can have loops contained in another loop, so it should be no surprise that we can combine loops with conditional arguments that we learned at the beginning of this module.

Similarly to how we can have a loop nested in an existing loop, we can have conditional statements within a for loop.

When we learned about conditions, we had the following code.

We checked the object item with a value of 25 to see if it met the if condition, the elif condition, and if it didn’t meet either of those, then it would undergo the code in the else statement body.

item = 13 

if item > 20:
    magnitude = 'greater than 20'
elif item > 10:
    magnitude = 'between 10 and 20'
else:
    magnitude = '10 or less'
 
magnitude

'between 10 and 20'

Then we had to repeat and rewrite the same conditional statements code to check item with a new value of 13, which violated the DRY principle since we repeated and copied the code again.

item_list = [25, 13]

for item in item_list:
    if item > 20:
        magnitude = 'greater than 20'
    elif item > 10:
        magnitude = 'between 10 and 20'
    else:
        magnitude = '10 or less'
    print(magnitude)

greater than 20
between 10 and 20

Instead, we could put the values 13, and 25 in a list and used a for loop containing the conditional statements.

This small change helped us adhere to the DRY principle and avoided repeating redundant code, and now we iterate over each element in the list and assigns the object magnitude a different value depending on which condition was met.

Break

item_list = [25, 13, 21, 8, 17, 11, 4]

for item in item_list:
    if item > 20:
        magnitude = 'greater than 20'
    elif item > 10:
        magnitude = 'between 10 and 20'
    else:
        magnitude = '10 or less'
    print(magnitude)

greater than 20
between 10 and 20
greater than 20
10 or less
between 10 and 20
between 10 and 20
10 or less

There are occasions where we may want to stop iterating over an entire sequence once a condition is met.

Perhaps we only need to find one number that is 10 or less in a list.

We can see that even after we located a number that is 10 or less, the loop continues until it reaches the last element. If we have a very large list, it’s inefficient to continue searching for something we have already found.

item_list = [25, 13, 21, 8, 17, 11, 4]

for item in item_list:
    if item > 20:
        magnitude = 'greater than 20'
    elif item > 10:
        magnitude = 'between 10 and 20'
    else:
        magnitude = '10 or less'
        break
        
    print(magnitude)

greater than 20
between 10 and 20
greater than 20

Instead, we can use something called a break. A break will stop the loop from continuing.

Now we can see that as soon as we found a value with magnitude equal to 10 or less, all subsequent iterations came to a halt, and Python exits the loop.

Not even the last print(magnitude) in the loop is returned.

A stylizing side point

item_list = [25, 13, 21, 8, 17, 11, 4]

number_over_20 = 0

for item in item_list:
    if item > 20:
        number_over_20 = number_over_20 + item
        
number_over_20

46

Instead of writing this line:

number_over_20 = number_over_20 + item

We can write:

number_over_20 +=  item

You’ve seen that we often add to an existing object when we use loops.

For instance, when we count the number of items that are over 20 in the list item_list.

We keep the object name number_over_20 and just add to it while iterating through the loop.

Instead of writing the line:

number_over_20 = number_over_20 + item

We can avoid writing the object name twice and write a more stylized approach:

number_over_20 +=  item

This simply means that we are adding item to the number_over_20 object.

item_list = [25, 13, 21, 8, 17, 11, 4]
number_over_20 = 0

for item in item_list:
    if item > 20:
        number_over_20 += item

number_over_20

46

If we replace this code into the loop from before, it works in exactly the same way.

Let’s apply what we learned!