![Example: Reordering a String
def last_name_first(s):
"""Returns: copy of s in form 'last-name, 'first-name'
Precondition: s is in form 'first-name last-name'
with one blank between the two names"""
end_first = s.find(' ')
first_name = s[:end_first]
# Cut out the last name
# Glue them together with a comma
9/15/22 Algorithm Design 16](https://image.slidesharecdn.com/presentation-08-240829064055-c5297ce5/85/Python-Lecture-slides-topic-Algorithm-design-16-320.jpg)
![Case Study: Strings
• We can slice strings (s[a:b])
• We can glue together strings (+)
• We have a lot of string methods
§ We can search for characters
§ We can count the number of characters
§ We can pad strings
§ We can strip padding
• Sometimes, we can cast to a new type
9/15/22 Algorithm Design 18](https://image.slidesharecdn.com/presentation-08-240829064055-c5297ce5/85/Python-Lecture-slides-topic-Algorithm-design-18-320.jpg)
![Stubbed Returns
def last_name_first(s):
"""Returns: copy of s in form 'last-name, 'first-name'
Precondition: s is in form 'first-name last-name'
with one blank between the two names"""
end_first = s.find(' ')
first = s[:end_first]
# Cut out the last name
# Glue them together with a comma
return first # Not the final answer
9/15/22 Algorithm Design 20](https://image.slidesharecdn.com/presentation-08-240829064055-c5297ce5/85/Python-Lecture-slides-topic-Algorithm-design-20-320.jpg)
![Example: last_name_first
def last_name_first(s):
"""Returns: copy of s in the form
'last-name, first-name'
Precondition: s is in the form
'first-name last-name' with
with one blank between names""”
first = first_name(s)
# Cut out the last name
# Glue together with comma
return first # Stub
def first_name(s):
"""Returns: first name in s
Precondition: s is in the form
'first-name last-name' with
one blank between names"""
end = s.find(' ')
return s[:end]
9/15/22 Algorithm Design 24](https://image.slidesharecdn.com/presentation-08-240829064055-c5297ce5/85/Python-Lecture-slides-topic-Algorithm-design-24-320.jpg)
More Related Content
Similar to Python Lecture slides topic Algorithm design (20)
More from MuhammadIfitikhar (10)
Recently uploaded (20)
Python Lecture slides topic Algorithm design
- 2. Announcements For This Lecture Assignment 1 • Due TOMORROW § Due before midnight § Submit something… § Last revision Sep. 23 • Grades posted Sunday • Complete the Survey § Must answer individually Getting Help • Can work on it in lab § But still have a new lab § Make sure you do both • Consulting Hours § But expect it to be busy § First-come, first-served • One-on-Ones still going § Lots of spaces available 2 9/15/22 Algorithm Design
- 3. Announcements For This Lecture Assignment 1 • Due TOMORROW § Due before midnight § Submit something… § Last revision Sep. 23 • Grades posted Sunday • Complete the Survey § Must answer individually Getting Help • Can work on it in lab § But still have a new lab § Make sure you do both • Consulting Hours § But expect it to be busy § First-come, first-served • One-on-Ones still going § Lots of spaces available 3 9/15/22 Algorithm Design Will post Assignment 2 Friday. This is a handwritten assignment.
- 4. What Are Algorithms? Algorithm • Step-by-step instructions § Not specific to a language § Could be a cooking recipe • Outline for a program Implementation • Program for an algorithm § In a specific language § What we often call coding • The filled in outline 9/15/22 Algorithm Design 4 • Good programmers can separate the two § Work on the algorithm first § Implement in language second • Why approach strings as search-cut-glue
- 5. Difficulties With Programming Syntax Errors • Python can’t understand you • Examples: § Forgetting a colon § Not closing a parens • Common with beginners § But can quickly train out Conceptual Errors • Does what you say, not mean • Examples: § Forgot last char in slice § Used the wrong argument • Happens to everyone § Large part of CS training Proper algorithm design reduces conceptual errors 9/15/22 Algorithm Design 5
- 6. Testing First Strategy • Write the Tests First Could be script or written by hand • Take Small Steps Do a little at a time; make use of placeholders • Intersperse Programming and Testing When you finish a step, test it immediately • Separate Concerns Do not move to a new step until current is done 9/15/22 Algorithm Design 6
- 7. Testing First Strategy • Write the Tests First Could be script or written by hand • Take Small Steps Do a little at a time; make use of placeholders • Intersperse Programming and Testing When you finish a step, test it immediately • Separate Concerns Do not move to a new step until current is done Will see several strategies. But all built on this core idea. 9/15/22 Algorithm Design 7
- 8. Using Placeholders in Design • Strategy: fill in definition a little at a time • We start with a function stub § Function that can be called but is unfinished § Allows us to test while still working (later) • All stubs must have a function header § But the definition body might be “empty” § Certainly is when you get started 9/15/22 Algorithm Design 8
- 9. A Function Stub def last_name_first(s): """Returns: copy of s in form 'last-name, 'first-name' Precondition: s is in form 'first-name last-name' with one blank between the two names""" # Finish the body “Empty” 9/15/22 Algorithm Design 9
- 10. But it Cannot Really Be Empty def last_name_first(s): # Finish the body • A function definition is only valid with a body § (Single-line) comments do not count as body § But doc-strings do count (part of help function) • So you should always write in the specification Error 9/15/22 Algorithm Design 10
- 11. An Alternative: Pass def last_name_first(s): pass • You can make the body non-empty with pass § It is a command to “do nothing” § Only purpose is to ensure there is a body • You would remove it once you got started Fine! 9/15/22 Algorithm Design 11
- 12. Ideally: Use Both def last_name_first(s): """Returns: copy of s in form 'last-name, 'first-name' Precondition: s is in form 'first-name last-name' with one blank between the two names""" pass Now pass is a note that is unfinished. Can leave it there until work is done. 9/15/22 Algorithm Design 12
- 13. Outlining Your Approach • Recall the two types of errors you will have § Syntax Errors: Python can’t understand you § Conceptual Errors: Does what you say, not mean • To remove conceptual errors, plan before code § Create outline of the steps to carry out § Write in this outline as comments • This outline is called pseudocode § English statements of what to do § But corresponds to something simple in Python 9/15/22 Algorithm Design 13
- 14. Example: Reordering a String def last_name_first(s): """Returns: copy of s in form 'last-name, 'first-name' Precondition: s is in form 'first-name last-name' with one blank between the two names""" # Find the space between the two names # Cut out the first name # Cut out the last name # Glue them together with a comma 9/15/22 Algorithm Design 14
- 15. Example: Reordering a String def last_name_first(s): """Returns: copy of s in form 'last-name, 'first-name' Precondition: s is in form 'first-name last-name' with one blank between the two names""" end_first = s.find(' ') # Cut out the first name # Cut out the last name # Glue them together with a comma 9/15/22 Algorithm Design 15
- 16. Example: Reordering a String def last_name_first(s): """Returns: copy of s in form 'last-name, 'first-name' Precondition: s is in form 'first-name last-name' with one blank between the two names""" end_first = s.find(' ') first_name = s[:end_first] # Cut out the last name # Glue them together with a comma 9/15/22 Algorithm Design 16
- 17. What is the Challenge? • Pseudocode must correspond to Python § Preferably implementable in one line § Unhelpful: # Return the correct answer • So what can we do? § Depends on the types involved § Different types have different operations § You should memorize important operations § Use these as building blocks 9/15/22 Algorithm Design 17
- 18. Case Study: Strings • We can slice strings (s[a:b]) • We can glue together strings (+) • We have a lot of string methods § We can search for characters § We can count the number of characters § We can pad strings § We can strip padding • Sometimes, we can cast to a new type 9/15/22 Algorithm Design 18
- 19. Early Testing • Recall: Combine programming & testing § After each step we should test § But it is unfinished; answer is incorrect! • Goal: ensure intermediate results expected § Take an input from your testing plan § Call the function on that input § Look at the results at each step § Make sure they are what you expect • Add a temporary return value 9/15/22 Algorithm Design 19
- 20. Stubbed Returns def last_name_first(s): """Returns: copy of s in form 'last-name, 'first-name' Precondition: s is in form 'first-name last-name' with one blank between the two names""" end_first = s.find(' ') first = s[:end_first] # Cut out the last name # Glue them together with a comma return first # Not the final answer 9/15/22 Algorithm Design 20
- 21. Working with Helpers • Suppose you are unsure of a step § You maybe have an idea for pseudocode § But not sure if it easily converts to Python • But you can specify what you want § Specification means a new function! § Create a specification stub for that function § Put a call to it in the original function • Now can lazily implement that function 9/15/22 Algorithm Design 21
- 22. Example: last_name_first def last_name_first(s): """Returns: copy of s in the form 'last-name, first-name' Precondition: s is in the form 'first-name last-name' with with one blank between names""" # Cut out the first name # Cut out the last name # Glue together with comma # Return the result 9/15/22 Algorithm Design 22
- 23. Example: last_name_first def last_name_first(s): """Returns: copy of s in the form 'last-name, first-name' Precondition: s is in the form 'first-name last-name' with with one blank between names""” first = first_name(s) # Cut out the last name # Glue together with comma return first # Stub def first_name(s): """Returns: first name in s Precondition: s is in the form 'first-name last-name' with one blank between names""" pass 9/15/22 Algorithm Design 23
- 24. Example: last_name_first def last_name_first(s): """Returns: copy of s in the form 'last-name, first-name' Precondition: s is in the form 'first-name last-name' with with one blank between names""” first = first_name(s) # Cut out the last name # Glue together with comma return first # Stub def first_name(s): """Returns: first name in s Precondition: s is in the form 'first-name last-name' with one blank between names""" end = s.find(' ') return s[:end] 9/15/22 Algorithm Design 24
- 25. Concept of Top Down Design • Function specification is given to you § This cannot change at all § Otherwise, you break the team • But you break it up into little problems § Each naturally its own function § YOU design the specification for each § Implement and test each one • Complete before the main function 9/15/22 Algorithm Design 25
- 26. Testing and Top Down Design def test_first_name(): """Test procedure for first_name(n)""" result = name.first_name('Walker White') introcs.assert_equals('Walker', result) def test_last_name_first(): """Test procedure for last_name_first(n)""" result = name.last_name_first('Walker White') introcs.assert_equals('White, Walker', result) 9/15/22 Algorithm Design 26
- 27. A Word of Warning • Do not go overboard with this technique § Do not want a lot of one line functions § Can make code harder to read in extreme • Do it if the code is too long § I personally have a one page rule § If more than that, turn part into a function • Do it if you are repeating yourself a lot § If you see the same code over and over § Replace that code with a single function call 9/15/22 Algorithm Design 27
- 28. Exercise: Anglicizing an Integer • anglicize(1) is “one” • anglicize(15) is “fifteen” • anglicize(123) is “one hundred twenty three” • anglicize(10570) is “ten thousand five hundred def anglicize(n): """Returns: the anglicization of int n. Precondition: 0 < n < 1,000,000""" pass # ??? 9/15/22 Algorithm Design 28
- 29. Exercise: Anglicizing an Integer def anglicize(n): """Returns: the anglicization of int n. Precondition: 0 < n < 1,000,000""" # if < 1000, provide an answer # if > 1000, break into hundreds, thousands parts # use the < 1000 answer for each part , and glue # together with "thousands" in between # return the result 9/15/22 Algorithm Design 29
- 30. Exercise: Anglicizing an Integer def anglicize(n): """Returns: the anglicization of int n. Precondition: 0 < n < 1,000,000""" if n < 1000: # no thousands place return anglicize1000(n) elif n % 1000 == 0: # no hundreds, only thousands return anglicize1000(n/1000) + ' thousand' else: # mix the two return (anglicize1000(n/1000) + ' thousand '+ anglicize1000(n)) 9/15/22 Algorithm Design 30
- 31. Exercise: Anglicizing an Integer def anglicize(n): """Returns: the anglicization of int n. Precondition: 0 < n < 1,000,000""" if n < 1000: # no thousands place return anglicize1000(n) elif n % 1000 == 0: # no hundreds, only thousands return anglicize1000(n/1000) + ' thousand' else: # mix the two return (anglicize1000(n/1000) + ' thousand '+ anglicize1000(n)) 9/15/22 Algorithm Design 31 Now implement this. See anglicize.py