9.4. Length¶
The len function, when applied to a string, returns the number of
characters in a string.
To get the last letter of a string, you might be tempted to try something like this:
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.
Alternatively in Python, we can use negative indices, which count
backward from the end of the string. The expression fruit[-1]
yields the last letter, fruit[-2] yields the second to last, and
so on. Try it! Most other languages do not allow the negative
indices, but they are a handy feature of Python!
Check your understanding
- 11
- The blank counts as a character.
- 12
- Yes, there are 12 characters in the string.
strings-6-1: What is printed by the following statements?
s: str
s = "python rocks"
print(str(len(s)))
- o
- Take a look at the index calculation again, len(s)-5.
- r
- Yes, len(s) is 12 and 12-5 is 7. Use 7 as index and remember to start counting with 0.
- s
- s is at index 11
- Error, len(s) is 12 and there is no index 12.
- You subtract 5 before using the index operator so it will work.
strings-6-2: What is printed by the following statements?
s: str
s = "python rocks"
print(s[len(s) - 5])
- c
- Yes, 3 characters from the end.
- k
- Count backward 3 characters.
- s
- When expressed with a negative index the last character s is at index -1.
- Error, negative indices are illegal.
- Python does use negative indices to count backward from the end.
strings-6-3: What is printed by the following statements?
s: str
s = "python rocks"
print(s[-3])