Module 8 - Strings

Question 1

How are strings different from integers, floats, and booleans?

Answer 1

A string is a sequence - an ordered collection of other values, where each value is identified by an integer index

Question 2

How do you access each character in a string?

Answer 2

The bracket operator

Question 3

How would you access the first letter in banana?

Answer 3

fruit = ‘banana’
letter = fruit[0]
^ selects first character from fruit and assigns it to letter

Question 4

What is an index?

Answer 4

The expression in brackets

An integer value used to select an item in a sequence, such as a character in a string. In Python indices start from 0

Question 5

What types of values can you put into a string index?

Answer 5

Value MUST evaluate to an integer, but you can use numbers, operators, and expressions in the brackets: 
0
if i=1, can do i+1 
2*3 
etc..

Question 6

What is len? What is the syntax for it?

Answer 6

Function that returns the number of characters in a string
size=len(‘fruit’)
print(size)
5

Question 7

How would you get the last character of a string?

Answer 7

fruit = ‘banana’ #assign value ‘banana’ to var fruit
length = len(fruit) #assign length to var length
last = fruit[length-1] #assign last letter to var last
print(last) #displays ‘a’

remember, b is the zero’th letter!!

you can also use negative indices - fruit[-1] gives you last letter

Question 8

What does it mean to traverse?

Answer 8

To iterate through the items in a sequence, performing a similar operation on each.
For example, processing a string one character at a time

Question 9

What is concatenation?

Answer 9

String addition

Question 10

What is a string slice? What is the syntax?

Answer 10

AKA substring - a segment of a string is called a slice

string [start:end] returns the string from the start (default is 0) up to, but EXCLUDING, the end value

Question 11

How do you slice from the beginning or to the end?

Answer 11

If you want to slice from the beginning, leave first one blank, [ :3] – it defaults to 0 if blank
If you want to slice to the end, leave last one blank, [3;] – defaults to len(string) if blank

Question 12

What is the result if the first index is greater than or equal to the second index in a slice? i.e., [3:2] or [3:3]

Answer 12

An empty string

no characters and length 0 but still a string

Question 13

What happens if you leave both indices empty? i.e., [:]

Answer 13

The entire string is return - not sliced

Question 14

T or F: Strings are sequences (lists), so many of
the tools that work with sequences work
with strings

Answer 14

True!

Question 15

What happens if you try to use an index that is out of range for the string?

Answer 15

IndexError exception will occur

Question 16

sentence=input(“enter your essay”)
for letter in sentence:

Describe what “sentence”, “letter” and “in” are in the code above.

Answer 16

sentence is a SEQUENCE that is a string
letter is a variable that’s introduced in the for-loop
the “in” operator tells us to iterate thru each item in the sequence

Question 17

Which loop would you use when you need to iterate over the entire string (e.g., count the occurrences of a specific character)?

Answer 17

for loop

Question 18

Which loop would you use if you want to traverse a part of a string? How would you do it?

Answer 18

while loop
use len to stop loop if we reach the end of the string
use [ ] to refer to particular letters
need a counter to advance index

Question 19

What happens when you try to change part of a string?
For example,
greeting = “hello world”
greeting[0] = “j”

Answer 19

Error message - TypeError: ‘str’ object does not support item assignment

Question 20

T or F: Strings are immutable.

Answer 20

True - once they are created, they cannot be changed

The best you can do is create a new string that uses the original

Question 21

How would you change the first letter of greeting = “hello world” from h to j?

Answer 21

create a new variable

new_greeting = “j” + greeting[1:]

Question 22

When you put a loop into a function, what happens when you put a return statement INSIDE the loop?

Answer 22

When we get to the return statement, the function breaks out of the loop and returns immediately

Question 23

What’s the pattern of computation called when we traverse a sequence and return when we find what we are looking for?

Answer 23

Search

Question 24

If strings are immutable, how come you can concatenate them?

Answer 24

Concatenation doesn’t actually change the existing string, but rather creates a new string and assigns the new string to the previously used variable

Question 25

Slicing expressions involves a start and end value:
string [start : end]
Can it include a step value?
What are the rules for the values?

Answer 25

Yes, step values can be included

Values can be positive or negative integers (negative indexes used for end of string)

Question 26

How do you determine whether one string is contained in another string? What about not contained?

Answer 26

Use the ‘in’ operator
General format: string1 in string2
The strings can be string literals or variables referencing strings
You can use ‘not in’ operator for not contained

Question 27

What is the general format of a string method and operation?

Answer 27

string.method(arguments)

string is either the string (in quotes) or the variable that the string was assigned to

Question 28

T or F: string comparisons are case-sensitive

Answer 28

True!

lower and upper methods can be used for making case-insensitive string comparisons

Question 29

What is a method (i.e., string method)?

Answer 29

A method is similar to a function - it takes arguments and returns a value - but the syntax in different

Question 30

How many arguments does the find method take and what is the syntax?

Answer 30

string.find(“string or slice we’re searching for”, start, end)

not inclusive of end - stops right before it reaches ‘end’ value
start and end are optional arguments