9.6. 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)

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

What is printed by the following statements?

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

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

What is printed by the following statements?

s = "python rocks"
print(s[len(s)-5])

• 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.

What is printed by the following statements?

s = "python rocks"
print(s[-3])

• 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.

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