9.3. Traversing a list¶
The most common way to traverse the elements of a list is with a
for loop. The syntax is the same as for strings:
This works well if you only need to read the elements of the list. But
if you want to write or update the elements, you need the indices. A
common way to do that is to combine the functions range and
len:
for i in range(len(numbers)):
numbers[i] = numbers[i] * 2
- 8
- Iteration by item will process once for each item in the sequence, even the empty list.
- 9
- Yes, there are nine elements in the list so the for loop will iterate nine times.
- 15
- Iteration by item will process once for each item in the sequence. Each string is viewed as a single item, even if you are able to iterate over a string itself.
- Error, the for statement needs to use the range function.
- The for statement can iterate over a sequence item by item.
Q-2: How many times will the for loop iterate in the following statements?
p = [3, 4, "Me", 3, [], "Why", 0, "Tell", 9.3]
for ch in p:
print(ch)
This loop traverses the list and updates each element. len
returns the number of elements in the list. range returns a
list of indices from 0 to n-1, where n is the length of the list.
Each time through the loop, i gets the index of the next
element. The assignment statement in the body uses i to
read the old value of the element and to assign the new value.
A for loop over an empty list never executes the body:
- The loop will run once.
- The loop will not run because the initial conditions are not met. You cannot traverse over nothing.
- Nothing will happen.
- Nothing will happen when traversing through an empty loop because there are no elements to iterate through.
- It will cause an error.
- It is legal to call this, but nothing will happen. It will not call an error.
- The list will add items to traverse.
- Python will not add items to the list so it is no longer empty, empty lists are okay.
Q-4: What will happen if you attempt to traverse an empty list?
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Although a list can contain another list, the nested list still counts as a single element. Check out the length of this list:
- 3
- Remember that the length of a list is only the elements in the outside list.
- 1
- There is technically only one element in this list, but that element has its own items.
- 2
- Remember that the length of a list is only the elements in the outside list.
Q-6: How many items are in nestedList?
nestedList = [["First", 2, ["Third"]]]