4. The Slice Operator¶

A substring of a string is called a slice. Selecting a slice is similar to selecting a character:

singers = "Peter, Paul, and Mary" print(singers[0:5]) print(singers[7:11]) print(singers[17:21])

The slice operator [n:m] returns the part of the string from the n’th character to the m’th character, including the first but excluding the last. In other words, start with the character at index n and go up to the character at index m - 1. This behavior may seem counter-intuitive but if you recall the range function, it did not include its end point either.

If you omit the first index (before the colon), the slice starts at the beginning of the string. If you omit the second index, the slice goes to the end of the string.

fruit = "banana" print(fruit[:3]) print(fruit[3:])

What do you think fruit[:] means?

Check your understanding

What is printed by the following statements?

s = "python rocks"
print(s[3:8])

• python
• That would be s[0:6].
• rocks
• That would be s[7:].
• hon r
• Yes, start with the character at index 3 and go up to but not include the character at index 8.
• Error, you cannot have two numbers inside the [ ].
• This is called slicing, not indexing. It requires a start and an end.

What is printed by the following statements?

s = "python rocks"
print(s[7:11] * 3)

• rockrockrock
• Yes, rock starts at 7 and goes through 10. Repeat it 3 times.
• rock rock rock
• Repetition does not add a space.
• rocksrocksrocks
• Slicing will not include the character at index 11. Just up to it (10 in this case).
• Error, you cannot use repetition with slicing.
• The slice will happen first, then the repetition. So it is ok.