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Recap
Iterating over a list
Modifying lists
6. Updating list elements – part 2
Working with multiple lists
Congratulations

## Instruction

So, how can we actually modify list elements? We need to use yet another type of for loop. Look closely at the example below:

sleep_durations = [368, 690, 426, 494, 445, 690, 423, 534, 606, 390]
for i in range(len(sleep_durations)):
sleep_durations[i] -= 20
print(sleep_durations)


Result:

[348, 670, 406, 474, 425, 670, 403, 514, 586, 370]

This time, we don't iterate over the list. Instead, we iterate over the indexes of the list using a temporary variable named i, whose values are generated by the range(...) function. The range(len(sleep_durations)) expression returns all integer values from 0 (inclusive) to len(sleep_durations) (exclusive). That is, it returns all valid indexes of the sleep_durations list.

Inside the loop, we use square brackets to access the i-th element of the list, and we change that element's value. Mark's sleep duration values have now been updated correctly.

## Exercise

Martha forgot to include phone bills in her monthly spending totals. Add 20.50 to each element in the monthly_spendings list and print the list to the output.

### Stuck? Here's a hint!

Create a for loop in the following way:

for i in range(len(monthly_spendings)):