Download 1.3.5 Strings Answer Key

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Activity 1.3.5 Strings Answer Key
Introduction
Text– it's all around us. Books, web pages, email,
text messages, papers for school. These are mostly
text, and most of it has been digitized, represented
with zeros and ones. Most programming languages
have a data type called a string. A string takes care of
the details for us, and we can just think of some text
as a string of characters. One character might be a
letter, number, or symbol, depending on the
character set of the representation standard.
Keep cats off string.
Why do you suppose computer scientists chose the
term string?
Procedure
1. Form pairs as directed by your teacher. Meet or greet each other to practice
professional skills and establish norms.
2. Launch Canopy and open an editor window.
3. If your teacher directs you to turn in your work with an IPython log, set the
working directory for the IPython session, turn on session logging, and title your
log.
In []: %logstart -ort JDoeBSmith1_3_5.log
In []: # Jane Doe 1.3.5 IPython log
4. Start a new program in the code editor by choosing File > New > Python file.
Save the file as JDoe_BSmith_1_3_5.py
5. In previous activities you learned that you can make decisions by evaluating
Boolean expressions. You also learned input and output.
In []: from __future__ import print_function
In []: if raw_input('One character: ') == '!':
...:
print('Wow', end='!')
One character: !
Wow!
© 2014 Project Lead The Way, Inc.
Computer Science Principles Activity 1.3.5 Strings Answer Key – Page 1
6. In addition to the native types we’ve seen so far (int, float, long, bool),
another type (str) represents strings of characters. You can use the function
type() to check the variable type of a variable or expression.
In []: slogan = 'My school is the best'
In []: type(slogan)
Out[]: str
In []: slogan
Out[]: 'My school is the best'
Which of these types can represent the number six million? A correct answer
should include all but the bool type but would still be correct by including the
bool type.
Data of type int, float, long, and str all could easily be used represent the number
six million. If represented as a string, it would have to be converted to another
type before arithmetic were performed with it. The point of the question is to
point out that numbers are represented with strings – such as in text files or
reading from user keyboard entry – and to give opportunity for discussion about
why arithmetic nevertheless doesn't use strings.
A student thinking beyond the intent of the question would also be correct in
thinking that six million could also be represented by a sequence of bools.
TFTTFTTTFFFTTFTTFFFFFFF
7. String literals are enclosed in single or double quotes. The opening and closing
quotes must match.
One of the following two inputs will produce an error. Try this, discuss both
outputs with your partner, and summarize your discussion.
In []: type('tr' + "y this")
In []: type('tr' + 5)
In []: #7. (Discuss and explain.)
You can't add a string to an int. A string is not treated as numeric data by various
operators and functions.
8. Strings are iterables. Iterables are sequences that can be counted in order, one
at a time, during iteration. Strings contain a sequence of characters, one after
another. The elements — including the spaces — are indexed, starting at 0.
'My school is the best'
Character
Index #
M
0
y
1
space
2
s
3
c
4
h
5
o
6
…
…
t
20
Try this, discuss the outputs with your partner, and summarize your discussion.
© 2014 Project Lead The Way, Inc.
Computer Science Principles Activity 1.3.5 Strings Answer Key – Page 2
In []: slogan[0]
Out[]: 'M'
# The first character
In []: slogan[2]
# Space is the third character.
In []: slogan[8]
# 'l' is the ninth character
In []: slogan[26]
# The string doesn't have a 27th character
In []: slogan[-2] # Unique to Python: index<0 counts from end
# 's' is the 2nd to last character.
In []: #8. (Discuss and explain.)
The 0 index gave the 1st character.
9. Python allows iterables to be sliced. To slice, use square brackets and two
indices separated by a colon. Python returns the iterable from the beginning
index up to but not including the ending index.
In []: slogan[0:5] # Note that slogan[5] is ‘h’
Out[]: 'My sc'
In []: slogan[5:21]
Out[]: 'hool is the best'
When slicing, you can omit the starting (or ending) index if you want to start at
the beginning (or end at the ending) of the string.
In []: slogan[:5]
Out[]: 'My sc'
Try to return 'best' by slicing the variable slogan, omitting the end index.
In []: # 9. Slicing
In []: (Write code to return 'best'.)
slogan[-4:] or slogan[17:]
10. Use slicing and concatenation to create your own sentence. Concatenation
involves pasting together two strings, one after another. Follow the example
here.
In []: slogan[:13] + 'awesome!'
Out[]: 'My school is awesome!'
Answers vary but should look like
variable[:] + 'fsd'
11. The len() function returns the number of elements in an iterable. The index of
the last element is always one less than the length of the iterable since the
indices begin at 0.
© 2014 Project Lead The Way, Inc.
Computer Science Principles Activity 1.3.5 Strings Answer Key – Page 3
In []: len(slogan)
Out[]: 21
Explain the output of the following inputs:
In []: activity = 'theater'
In []: len(activity)
In []: # 11a. (Discuss and explain.)
There are 7 characters.
In []: activity[0 : len(activity)-1]
In []: # 11b. (Discuss and explain.)
len(activity)-1 is 6, so the expression that was input is
activity[0:6], which omits the character at index 6, which is
the last and 7th character. That's why the interpreter returns
'theate' for the expression.
12. The in keyword can be used as a Boolean condition, returning True or False:
In []: 'test goo' in 'Greatest good for the greatest number!'
Out[]: True
In []: # 12. (Discuss and explain.)
This is True because 'test goo' is a substring within
'Greatest good'. (Note: the quote is a summary of John Stuart
Mill's utilitarian morality.)
13. A social media site offers a contest to write a humorous short paragraph. A
constraint on the creative format: the entry must include a question, a quote, a
compound sentence, and an exclamation. These would contain the characters ?,
", ,, and !, respectively.
Create a function how_eligible(essay) that returns 0 to 4, equal to the
number of these four characters that the essay included. As pair programmers
generate ideas for how to solve this problem, strategize and then code and test
iteratively.
In []:
Out[]:
In []:
Out[]:
how_eligible('This? "Yes." No, not really!')
4
how_eligible('Really, not a compound sentence.')
1
The 1.3.5 teacherSourceFiles.zip contains an example solution and a
function to autograde student submissions.
Conclusion
1. How many characters are in this sentence? Does it matter whether Python is
storing the string as one byte per character or four bytes per character?
The words contain 3, 4, 10, 3, 2, 4, and 8 letters. With the 6 spaces between words
and 1 character for the question mark, the sentence contains 41 characters.
© 2014 Project Lead The Way, Inc.
Computer Science Principles Activity 1.3.5 Strings Answer Key – Page 4
It is likely to be stored in 41 consecutive bytes, but this depends on how the string
is represented by a particular interpreter or compiler. The details usually don't
matter if your focus is at the higher level of abstraction of an algorithm that uses
the string.
2. This question asks you about something you have not learned. In fact, the
question is asking about details that go beyond what you will learn in this course.
However, wondering what is going on at a lower level of abstraction – and talking
about it – can be a useful strategy when learning about computing.
Describe what you think occurs in memory when the following code is executed.
In []: a = 'one string'
In []: b = 'another'
In []: c = a[:3] + ' and ' + b
In []: print c[6:10]
Answers will vary. What actually happens in memory? Students might not have the
conception that there are different types of memory: registers that are part of the CPU, a
cache that is part of the CPU, RAM, hard drive, and flash storage. The "correct" answer
to this question is not important. The purpose of the question is to discover students'
conceptions about memory, instructions, and variables. In fact, there is no single correct
answer for all of Python, because the memory representation of strings varies from
CPython to IronPython to Jython.
What actually happens in memory in CPython, which we are using? To dive down the
ladder of abstraction, you would look at the code for the Python interpreter, which is
written in C and maintained using the Mercurial version control system:
http://hg.python.org/cpython/file/1411df211159/Objects/bytearrayobject.c
Details that can be abstracted for the student:
The instructions from the keyboard enter RAM, are interpreted by the Python
interpreter, which is machine bytecode from a C compiler. Bytecode instructions are
executed by loading one instruction at a time into the CPU.
These instructions create three new addresses in the namespace look-up table in RAM.
The strings 'one string' and 'another' are copied from the program to the RAM. Two of
the namespace addresses point to the corresponding string objects, which include an
array of bytes, one character per byte. a[:3], ' and ' in the third instruction lead to two
more string objects being created, and a fifth string object is created for the
concatenated result. Python constructs yet a sixth string for the slice in the final
command, which is created, stored, and then accessed by the print command.
© 2014 Project Lead The Way, Inc.
Computer Science Principles Activity 1.3.5 Strings Answer Key – Page 5