3.10 Modifying Lists Through Loops

Consider the following code block:

years = [1346, 1665, 1855]

for i in years:
  i = i - 5

print(years)

## [1346, 1665, 1855]

Notice that running this code block does not change the underlying list. When you run this for loop, the following happens:

i is set equal to the value of the corresponding value from years
We subtract 5 from i and update the value of i
We do not do anything with this updated i - the original list, years, is not modified

What if we want to loop through years and update each vallue in the process. There are two ways of doing this:

3.10.1 Appending to an empty list

One way is to loop through your list, update each value, and then add it to the end of a new list. Look at the code below.

years = [1346, 1665, 1855]
updatedYears = [] # Initialize a new list to store updated values

for i in years:
  i = i - 5
  updatedYears.append(i) # Add updated value to the end of your new list

print(updatedYears)

## [1341, 1660, 1850]

Here is how the for loop works:

i is set equal to the value of the corresponding value from years
We subtract 5 from i and update the value of i
We save this new value of i by adding it in to a new list

At the end of this loop, we have two lists:

years, which contains original values
updatedYears, which contains updated values (original values - 5)

3.10.2 Looping through indices

Another way to update these values is to assign them back into the original list using the following syntax:

years[<index>] = <new value>

For this, we need to iterate through the indices of years.

years = [1346, 1665, 1855]

for i in range(len(years)): # Loop through the positions of years 
  newValue = years[i] - 5 # Extract the appropriate value from years and subtract 5 from it, storing it as a new variable
  years[i] = newValue # Add updated value back into years
  
print(years)

## [1341, 1660, 1850]

Breaking this down:

We loop through a range from 0 to the length of years. I.e. here, we would loop through the values 0, 1, 2
We extract the value from years at our current position using years[i]
We update this value by subtracting 5 from it. Updated value is saved as newValue
We add this new value back into years at the original position

At the end of this loop, we have one list, years, which no longer contains its initial values. years now contains our updated values.