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CS149 – Strings

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Model 1 Character Arrays

In Python the type String encapsulates an array of characters so a single character is a String of length 1.

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letter = ‘a’

array = “cat”

word = “dog”

“A”

“dog”

“cat”

letter→

array→

word→

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Indexes

  • What is the index of 'd' in the string above? What is the index of 'g'? In general, what is the index of the last character of a string?

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A

“dog”

“cat”

letter→

array→

word→

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Memory Stack Diagrams

  • Draw a memory diagram for the given code. Each variable should be a name next to a box containing its value.

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Link to Java Visualizer

str = “Hi!”��let = ‘X’

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num = -1

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hmm = str

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Modifying strings

b = "Hello, There!"

print(b[6:10]) # prints "The"

print(b[:5]) # prints "Hello"

print(b[7:]) # prints "There!"

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a = " Hello, World! "

print(a.strip()) # prints "Hello, World!"�a = "Hello, World!"

print(a.replace("H", "J")) # prints ['Jello', ' World!']

a = "Hello, World!"

print(a.split(",")) # prints ['Hello', ' World!']

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String Methods

If txt contains the string "hello world", then what is the return value of the following

String method calls?

a) txt.index(“ll”)

b) txt.endswith("ld")

c) len(txt)

d) txt[4: 7]

e) txt.upper()

capitalize() - Converts the first character to upper case

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count() - Returns the number of times a specified value occurs in a string

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endswith() - Returns true if the string ends with the specified value

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find() - Searches the string for a specified value and returns the position of where it was found

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index() - Searches the string for a specified value and returns the position of where it was found

isdecimal() - Returns True if all characters in the string are decimals

lower() - Converts a string into lower case

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replace() - Returns a string where a specified value is replaced with a specified value

split() - Splits the string at the specified separator, and returns a list�upper() - Converts a string to upper case

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Common Mistakes

1. Write the output of each program in the space under the table above. What is the logic

error you see when you run Program A?

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2. In Program A, what is returned by the string method? What happens to the return value?

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3. Describe two different ways you can fix the logic error in #17.

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4. What is the output of program B if you enter letter as the input

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Program A

Program B

greeting = “hello world”

greeting.upper()

print(greeting)

line = input(“Enter a word:”)

letter = line[1]

print(letter)

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Issues

Trying to get the check the last element of a blank string can create an issue if you use -1 indexing

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For example:

text = ""

last = text[-1]

print(last)

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This will produce a runtime error (we saw this in the Eight ball lab). If you are looking at this

you might be able to use the endswith() to check and ending character.

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txt = "Hello, welcome to this class."

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last = txt.endswith(".")

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print(x)

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Or you can do an if len(text) > 0 before pulling that last character.

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  • Acknowledgements

Parts of this activity are based on materials developed by Chris Mayfield and Nathan Sprague.

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