Class python strings

Python Strings

A string is a sequence of characters: 'spam' or "spam"

1. Characters in strings

Use a number in square brackets after the string name to talk about a

character in the string.

Numbering starts at 0. Think of the number as the distance from the

beginning of the string.

Examples:

word = "cabbage"

print(word[0])

print(word[5])

# prints c

# prints g

Odd but useful: The numbering wraps around, so the last character is -1,

the next-to-last is -2, etc.

print(word[-1])

print(word[-2])

# prints e

# prints g

2. A string function: len

len(s) is the number of characters in string s.

len("apple")

len("How are you?")

5

12

3. String operators

in

word1 in word2 is true iff word1 occurs inside word2

Examples:

"a" in "apple"

"a" in "banana"

"a" in "berry"

"berry" in "strawberry"

True

True

False

True

not in

word1 not in word2 is true iff word1 does not occur in word2

Examples:

"a" not in "apple"

"a" not in "berry"

Relational operators: <

>

=

==

!=

The order used is alphabetical order, but all capital letters come before

all lower case letters: "A" < "a", and "Z" < "a".

Examples:

"apple" < "banana"

"apple" < "applesauce"

"Banana" < "apple"

Concatenation: +

Example: "abc" + "def"

abcdef

Concatenating multiple copies: *

Example: 2 * "abc"

abcabc

4. String slices

Assume word is a string. You can take slices (sequences of characters) out of

word by specifying where the slice starts and stops.

Remember that string numbers start at 0.

As with ranges, the stopping value is not included in the list of values.

word = "pineapple"

print(word[0:4]) # pine (characters 0 through 3)

print(word[1:3]) # in (characters 1 through 2)

If you leave off the end of the range, the slice extends to the end of the string:

print(word[4:])

# apple

If you leave off the beginning of the range, the slice begins at the beginning

of the string:

print(word[:4])

# pine (characters 0 through 3)

String slices can be stored in other variables:

fruit = word[4:]

print(fruit)

print(fruit + "sauce")

# apple

# apple

# applesauce

If you want to, you can specify a step size in the slice:

newstring = word[1:5:2]

newstring = word[1:7:2]

newstring = word[1::2]

# ie

# iep

# iepl

5. How string assignment works (string constants are immutable)

You can change what string is stored in a variable:

word = "apple"

word = "banana"

but you can't change the string constant itself:

word = "apple"

word[0] = "A"

# Can't do this

If you want to form a new string, you can make a brand new string from

pieces of the old, using slices, concatenation, etc.

new_word = "A" + word[1:]

or even

word =

"A" + word[1:]

6. String methods

If word , word1, and word2 are strings, here are some methods that you

can use on them:

word.upper()

word.lower()

word1.find(word2)

returns the upper-case version of word

returns the lower-case version of word

returns the index of the beginning of the

first occurrence of word2 in word1

(-1 if not found)

word1.find(word2, start)

same as above, but start the search at start

word1.find(word2, start, stop) same as above, but stop at stop

word.split(delimiter)

separate word into pieces using the specified

delimiter. Returns a list of strings.

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