1.3_strings.ipynb

Kernel: Python 3

Lesson 1.3: Strings

During this lesson, you will learn the following:

• String Basics (indexing, slicing, membership, iterating)

• String Formatting (for print statements)

Working with String (text) data

String Operators

• For strings, + is concatenation.

• For strings, * is repitition.

'hello' + 'world'

'ma-na ' * 2

'hello' + 3  # fails due to incorrect data type

'hello' + str(3)  # succeeds due to matching data types

Strings as Ordered Sequences

A string is a sequence of characters:

• Each character in the string has a position, called the index.

• We can access each character in the string by its index position, using square brackets .

• In Python, index positions start their counting with 0 (zero), not 1.

Strings are immutable (they cannot be changed)

fruit = 'banana'  # create a string
print(fruit)

type(fruit)

Forward indexing starts at position 0

Python support reverse indexing (starting at position -1)

fruit[0]          # use the string index - it starts at 0

fruit[5]          # use the string index

fruit[6]         # what happens here?

You can "count down" from the end of a string using the index as follows:

fruit[-1]        # get the last character in the string

fruit[-2]        # get the second to last character in the string

Slicing Strings

Python supports a special operation called a "slice" (using the operator "😊 to select a subset of a sequence, in this case here, a subset of characters in a string. The general format of a slice is as follows:

some_sequence[n:m]

This yields the sub-sequence:

• starting with the $n^{th}$ character

• up to, but not including, the $m^{th}$ character

fruit[2:5]  # starting at index 2, going up to index 5 - note grabs index=2 (the third letter)

fruit[2:]   # starting at index 2, going to the end

fruit[:5]   # starting at the beginning, going up to index 5

fruit[:]    # what will this do?

# you can use negative index values (from the end) in your slices
fruit[:-1]  # everything up to, but not including, the last character

Iterating Over Strings with a While Loop

• To iterate over a string with a while loop, you leverage the string index

• The len() function is useful building for while loops over strings:

  • It returns the length of a sequence (not just strings!)

  • It can be used to control loops over the string

  • Some care is needed when using the length b/c counting is zero-based (remember OBOE!)

len(fruit)

i = 0
while i < len(fruit):
    print(fruit[i])
    i += 1

Iterating Over Strings with For Loop

• A for loop iterates over each element in the sequence from start to end (the use of the index is implicit).

• You use a variable (in this case "char") to represent each element in the list during loop execution:

# here is a for loop over the string "fruit" - note the use of the descriptive variable "char" to represent the index
for char in fruit: 
    print(char)

The in Operator for Strings

• The word in is a boolean operator that takes two strings and returns True if the first appears as a substring in the second

• This provides a very convenient way to check for membership in a sequence.

'n' in 'banana'

'ana' in 'banana'

'seed' in 'banana'

String Comparisons

For strings, we already know the + operator concatenates, and == tests for equality.

It turns out that the relational operators < and > have been implemented to reflect lexicographic ordering (i.e., indicate whether something comes earlier or later in a dictionary).

'apple' < 'banana'

'apple' > 'banana'

'banana' == 'apple'

'apple' < 'banana' < 'orange'

'app' < 'apple'

'apples' < 'apple'

'apple' < 'Apple'

'alpha' < 'Bravo'

"Apple".lower() == "apple"

String Formatting in Python

This notebook covers the "old style" of string formatting (using printf-like syntax). This style of formatting is documented here. Essentially we use the % operator:

• when applied to integers, % is the modulus operator
• when the first operand is a string, % is the format operator

Details for the "new style" are covered in the Python 3 Documentation here. Although more powerful, this style is also more object-oriented (not covered here).

nBananas = 27
"We have %d bananas." % nBananas

noSuch = "kiwis"
'We are out of %s today.' % noSuch

"%f" % 0.0

"%f" % 1.5

"%.10f" % (1/7)

caseCount = 42
caseContents = "peaches"
print("We have %d cases of %s today." %(caseCount, caseContents))

For additional details, see the online documentation.

Next lesson: 1.4 tuples, lists, dictionaries, and sets!