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6.5. Length

The len function, when applied to a string, returns the number of characters in a string.

fruit = "Banana" print(len(fruit))

To get the last letter of a string, you might be tempted to try something like this:

fruit = "Banana" sz = len(fruit) last = fruit[sz] # ERROR! print(last)

That won’t work. It causes the runtime error IndexError: string index out of range. The reason is that there is no letter at index position 6 in "Banana". Since we started counting at zero, the six indexes are numbered 0 to 5. To get the last character, we have to subtract 1 from the length. Give it a try in the example above.

fruit = "Banana" sz = len(fruit) lastch = fruit[sz-1] print(lastch)

Typically, a Python programmer would combine lines 2 and 3 from the above example into a single line:

lastch = fruit[len(fruit)-1]

Though, from what you just learned about using negative indices, using fruit[-1] would be a more appropriate way to access the last index in a list.

You can still use the len function to access other predictable indices, like the middle character of a string.

fruit = "grape"
midchar = fruit[len(fruit)//2]
# the value of midchar is "a"

As with strings, the function len returns the length of a list (the number of items in the list). However, since lists can have items which are themselves sequences (e.g., strings), it important to note that len only returns the top-most length.

alist = ["hello", 2.0, 5] print(len(alist)) print(len(alist[0]))

Note that alist[0] is the string "hello", which has length 5.

Check your understanding

What is printed by the following statements?

s = "python rocks"
print(len(s))

• 11
• The blank space counts as a character.
• 12
• Yes, there are 12 characters in the string.

What is printed by the following statements?

alist = [3, 67, "cat", 3.14, False]
print(len(alist))

• 4
• len returns the actual number of items in the list, not the maximum index value.
• 5
• Yes, there are 5 items in this list.

Assign the number of elements in lst to the variable output.

lst = ["hi", "morning", "dog", "506", "caterpillar", "balloons", 106, "yo-yo", "python", "moon", "water", "sleepy", "daffy", 45, "donald", "whiteboard", "glasses", "markers", "couches", "butterfly", "100", "magazine", "door", "picture", "window", ["Olympics", "handle"], "chair", "pages", "readings", "burger", "juggle", "craft", ["store", "poster", "board"], "laptop", "computer", "plates", "hotdog", "salad", "backpack", "zipper", "ring", "watch", "finger", "bags", "boxes", "pods", "peas", "apples", "horse", "guinea pig", "bowl", "EECS"] ==== from unittest.gui import TestCaseGui class myTests(TestCaseGui): def testTwo(self): self.assertEqual(output, 52, "Testing that output value is assigned to correct value.") myTests().main()

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