string opperation in python

String operations

Words and sentences are collections of characters, and so are Python strings.

You can access any character in a Python string using its index, either counting from the

beginning or the end of the string. When you start from the beginning, you count from 0.

In Python integer indices are zero based.

>>> name = “My Name is Mike”

>>> name[0]

‘M’

>>> name[1]

‘y’

>>> name[9]

‘s’

>>> name[-1]

‘e’

>>> name[-15]

‘M’

>>> name[7]

‘ ‘s

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Slicing

You can take shorter substrings from inside a longer string.

This operation is called ‘slicing’.

>>> name[11:14] # from 11th to 14th, 14th one is excluded

‘Mik’

>>> name[11:15] # from 11th to 15th, 15th one is excluded

‘Mike’

If you want to check if a string contains a particular substring, you can use the ‘in’

operator.

>>> “B” in “Foo”

False

>>> “B” not in “Foo”

True

>>> “G” in “Gohl”

True

>>> “James” in “James is tall”

True

The len function calculates the length of a string.

>>> len(name)

15

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Concatenation

In Python it’s possible to combine two strings into one:

The operation is called concatenation.

>>> boo = “cone” + “cat” + “ten” + “ate”

>>> print(boo)

conecattenate

>>> foo = “one”

>>> bar = “two”

>>> full = foo + “ “ + bar

>>> print(full)

one two

If you want a string containing N substrings, you use the * operator.

>>> foo = “foo “ * 3

>>> print(foo)

foo foo foo

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Quiz

PROBLEM:

1. In Python it is possible to combine multiple strings into one.

What is the name of this operation?

2. There is a string value = “foobar”. You access a substring the following way:

other_value = value[1:3].

What is the name of this operation? How long is other_value?

3. What is the index of the first character in a string?

ANSWER:

1. Concatenation

2. Slicing. The length is 2. The last character defined by slicing operation is not

included.

3. 0

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Exercises

PROBLEM: There are two strings: one = “John is strong” two = “Tea is not warm”.

Write Python code to produce a string result = “John is not strong” out of one and two:

ANSWER:

result = one[0:8] + two[7:11] + one[8:14]

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Exercises

PROBLEM: Find which of these strings doesn’t contain the letter “B”: “Foo”, “Bar”, “Zoo”,

“Loo”.

ANSWER:

>>> “B” in “Foo”

False

>>> “B” in “Bar”

True

>>> “B” in “Zoo”

False

>>> “B” in “Loo”

False

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Exercises

PROBLEM: Use the Python interpreter to find the length of the string “John is strong”

ANSWER:

>>> len(“John is strong”)

14

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