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Introduction to Programming with Python Ignas Rudaitis, 2020 Draft version 2

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In what ways are programs like (or unlike) sequences of actions?

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A B C D E F G Sequence of actions A B C D E F G A() B() C() D() E() F() G() Code

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A B C D E F G Sequence of actions A B C D E F G A() B() C() D() E() F() G() Code

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A B C D E F G Sequence of actions A B C D E F G A() B() C() D() E() F() G() Code

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A() B() for i in range(5): D() E() F() G() Code A B C D E F G A B C D E Loop (for ... in range) D E D E D E D E F G 5 times

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A() B() for i in range(5): D() E() F() G() Code A B C D E F G A B C D E Loop (for ... in range) D E D E D E D E F G 5 times

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A() B() for i in range(5): D() E() F() G() Code A B C D E F G A B C D E Loop (for ... in range) D E D E D E D E F G 5 times

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A() B() for i in range(5): D() E() F() G() Code A B C D E F G A B C D E Loop (for ... in range) D E D E D E D E F G 5 times

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A B C D E H I F G + - A B C D E H I A B C F G H I Conditional (if ... else) A() B() if C(): D() E() else: F() G() H() I() Code

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A B C D E H I F G + - A B C D E H I A B C F G H I Conditional (if ... else) A() B() if C(): D() E() else: F() G() H() I() Code

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A B C D E H I F G + - A B C D E H I A B C F G H I Conditional (if ... else) A() B() if C(): D() E() else: F() G() H() I() Code

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This action might result in two distinct outcomes, e.g. the action “check if it’s raining outside”. The remainder of the action sequence then depends on the particular outcome.

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Conditional within a loop A B C F G D E + - 3 times A B C F C B F C B F G A B D F C B F C B F G E , , A B D F C B F C B F G E , A B D F B F C B F G E D E , ... A() for i in range(3): if B(): C() else: D() E() F() G() Code 4 more

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Conditional within a loop A B C F G D E + - 3 times A B C F C B F C B F G A B D F C B F C B F G E , , A B D F C B F C B F G E , A B D F B F C B F G E D E , ... A() for i in range(3): if B(): C() else: D() E() F() G() Code 4 more

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Conditional within a loop A B C F G D E + - 3 times A B C F C B F C B F G A B D F C B F C B F G E , , A B D F C B F C B F G E , A B D F B F C B F G E D E , ... A() for i in range(3): if B(): C() else: D() E() F() G() Code 4 more

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A G 3 times - do something A() for i in range(3): do_something() G() Code

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A G 3 times - do something B C F D E - what exactly? A() for i in range(3): do_something() G() Code

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A B C F C B F C B F G A B D F C B F C B F G E , , A B D F C B F C B F G E , A B D F B F C B F G E D E , ... 4 more In total, 2 × 2 × 2 = 8 distinct sequences of actions are possible here, given that in every one of the three repetitions action B might yield a different outcome.

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Transporting data with variables

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Display Ask for text: Enter your name. Insert at the beginning: Hello, Display

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The result of this action may be any piece of text, which will fill the slot as soon as the action completes. Ask for text:

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The slots in these actions are associated using colors and arrows. This is to indicate that once a slot’s content changes, an identical change also takes place in all others that are associated.

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Display Ask for text: Enter your name. Insert at the beginning: Hello, Display

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Enter your name. Display Ask for text: Enter your name. Insert at the beginning: Hello, Display

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Display Ask for text: Enter your name. Insert at the beginning: Hello, Display

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OK Display Ask for text: Enter your name. Insert at the beginning: Hello, Display

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R OK Display Ask for text: Enter your name. Insert at the beginning: Hello, Display

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Ro OK Display Ask for text: Enter your name. Insert at the beginning: Hello, Display

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Rom OK Display Ask for text: Enter your name. Insert at the beginning: Hello, Display

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Rome OK Display Ask for text: Enter your name. Insert at the beginning: Hello, Display

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Romeo OK Display Ask for text: Enter your name. Insert at the beginning: Hello, Display

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Romeo OK Display Ask for text: Enter your name. Insert at the beginning: Hello, Display

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Display Ask for text: Enter your name. Romeo Insert at the beginning: Hello, Romeo Display Romeo

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Display Ask for text: Enter your name. Romeo Insert at the beginning: Hello, Romeo Display Romeo

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Display Ask for text: Enter your name. Hello, Romeo Insert at the beginning: Hello, Hello, Romeo Display Hello, Romeo

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Display Ask for text: Enter your name. Hello, Romeo Insert at the beginning: Hello, Hello, Romeo Display Hello, Romeo

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Hello, Romeo Display Ask for text: Enter your name. Hello, Romeo Insert at the beginning: Hello, Hello, Romeo Display Hello, Romeo

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display('Enter your name.') t = ask_for_text() t = 'Hello, ' + t display(t)

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If, for instance, we put a u instead of t here, like we just did, it would mean that this action’s slot would receive a “different color” and would therefore be no longer synchronized with the rest of them, unless... display('Enter your name.') t = ask_for_text() u = 'Hello, ' + t display(t)

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...unless we were to use u elsewhere as well ‒ in which case, another process of synchronization would occur as well, this time among the blue slots. display('Enter your name.') t = ask_for_text() u = 'Hello, ' + t display(t) If, for instance, we put a u instead of t here, like we just did, it would mean that this action’s slot would receive a “different color” and would therefore be no longer synchronized with the rest of them, unless...

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Example: cleaning up the desktop

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Go to Create folder Count files, of which there are: Desktop Oldies 0 Repeat times 0 Select file # 1 Is the file older than ? 1 Jan 2015 Cut yes no Go to Oldies Paste Go up a level Increase by one: 1

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0 0 1 1

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go_to('Desktop') create_folder('Oldies') n = count_files() k = 1 for i in range(n): select_file(k) if file_older_than('1 Jan 2015'): cut() go_to('Oldies') paste() go_up_a_level() else: k += 1

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n = count_files() k = 1 for i in range(n): select_file(k) k += 1

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What kinds of actions will be available in “real-world” programming?

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# “Listen” for a string of text from the user (up to Return) text = input() # Add “rocks!” to the end of the string, with an extra space # at the beginning and another one at the end text += ' rocks! ' # Triple the string (attach two copies of it at the end) text *= 3 # Show the final string to the user print(text)

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# “Listen” for a string of text from the user (up to Return) line = input() # Add “rocks!” to the end of the string, with an extra space # at the beginning and another one at the end line += ' rocks! ' # Triple the string (attach two copies of it at the end) line *= 3 # Show the final string to the user print(line)

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# “Listen” for a string of text from the user (up to Return) cookie = input() # Add “rocks!” to the end of the string, with an extra space # at the beginning and another one at the end cookie += ' rocks! ' # Triple the string (attach two copies of it at the end) cookie *= 3 # Show the final string to the user print(cookie)

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# “Listen” for a string of text from the user (up to Return) zxcvbnm = input() # Add “rocks!” to the end of the string, with an extra space # at the beginning and another one at the end zxcvbnm += ' rocks! ' # Triple the string (attach two copies of it at the end) zxcvbnm *= 3 # Show the final string to the user print(zxcvbnm)

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Actions (functions) y = input() Asks for a line of text; saves it to y. print(x) Displays a line with the text in x. y = x.replace(a, b) Takes a piece of text x, replaces all occurrences of a within it with the piece b, and saves the result to y. y = len(x) Measures the length of the text in x; saves it to y. y = x[k] Extracts the (k + 1)-th character from the text in x; saves it to y. y = a + b Attaches the text in b to the end of the text in a, saving the result to y. y = a * k Repeats the piece of text in a (k times), saving the result to y.

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Sequencing actions through function application

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Ask for text: In the text string , replace with , giving Display kitten Ask for text:

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OK Ask for text: In the text string , replace with , giving Display kitten Ask for text:

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S OK Ask for text: In the text string , replace with , giving Display kitten Ask for text:

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Se OK Ask for text: In the text string , replace with , giving Display kitten Ask for text:

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Sea OK Ask for text: In the text string , replace with , giving Display kitten Ask for text:

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Sea l OK Ask for text: In the text string , replace with , giving Display kitten Ask for text:

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Sea li OK Ask for text: In the text string , replace with , giving Display kitten Ask for text:

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Sea lio OK Ask for text: In the text string , replace with , giving Display kitten Ask for text:

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Sea lion OK Ask for text: In the text string , replace with , giving Display kitten Ask for text:

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Sea lions OK Ask for text: In the text string , replace with , giving Display kitten Ask for text:

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Sea lions OK Ask for text: In the text string , replace with , giving Display kitten Ask for text:

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Ask for text: In the text string , replace with , giving Sea lions Display kitten Ask for text: Sea lions

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OK Ask for text: In the text string , replace with , giving Sea lions Display kitten Ask for text: Sea lions

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l OK Ask for text: In the text string , replace with , giving Sea lions Display kitten Ask for text: Sea lions

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li OK Ask for text: In the text string , replace with , giving Sea lions Display kitten Ask for text: Sea lions

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lio OK Ask for text: In the text string , replace with , giving Sea lions Display kitten Ask for text: Sea lions

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lion OK Ask for text: In the text string , replace with , giving Sea lions Display kitten Ask for text: Sea lions

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lion OK Ask for text: In the text string , replace with , giving Sea lions Display kitten Ask for text: Sea lions

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Ask for text: lion In the text string , replace with , giving Sea lions Display lion kitten Ask for text: Sea lions

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Ask for text: lion In the text string , replace with , giving Sea lions Display lion kitten Ask for text: Sea lions

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Ask for text: lion In the text string , replace with , giving Sea lions Display Sea kittens lion kitten Sea kittens Ask for text: Sea lions

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Sea kittens Ask for text: lion In the text string , replace with , giving Sea lions Display Sea kittens lion kitten Sea kittens Ask for text: Sea lions

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Ask for text: lion In the text string , replace with , giving Sea lions Display Sea kittens lion kitten Sea kittens Ask for text: Sea lions

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= input() Sea lions  Ask for text: lion In the text string , replace with , giving Sea lions Display Sea kittens lion kitten Sea kittens

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= input() lion = input() Sea lions   In the text string , replace with , giving Sea lions Display Sea kittens lion kitten Sea kittens

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ž = .replace( , ) Sea lions lion kitten Sea kittens    Display Sea kittens = input() lion = input() Sea lions

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= input() lion ž = .replace( , ) Sea lions print( ) Sea kittens lion kitten Sea kittens = input() Sea lions    

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x = input() y = input() z = x.replace(y, 'kitten') print(z) = = input() lion ž = .replace( , ) Sea lions print( ) Sea kittens lion kitten Sea kittens = input() Sea lions

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ž = .replace( , ) Sea lions print( ) Sea kittens lion kačiuk Sea kittens = input() Sea lions = input() lion x = input() y = input() z = x.replace(y, 'kitten') print(z) =

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= input() lion ž = .replace( , ) Sea lions print( ) Sea kittens lion kitten Sea kittens = input() Sea lions x = input() y = input() z = x.replace(y, 'kitten') print(z) =

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= input() lion ž = .replace( , ) Sea lions print( ) Sea kittens lion kitten Sea kittens = input() Sea lions x = input() y = input() z = x.replace(y, 'kitten') print(z) =

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= input() lion ž = .replace( , ) Sea lions print( ) Sea kittens lion kitten Sea kittens = input() Sea lions x = input() y = input() z = x.replace(y, 'kitten') print(z) =

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= input() lion ž = .replace( , ) Sea lions print( ) Sea kittens lion kitten Sea kittens = input() Sea lions

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= input() lion ž = .replace( , ) Sea lions print( ) Sea kittens lion kitten Sea kittens

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ž = input().replace(input(), ) print( ) Sea kittens kitten Sea kittens

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print( ) Sea kittens ž = input().replace(input(), ) Sea kittens kačiuk

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print(input().replace(input(), )) kitten

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print(input().replace(input(), 'kitten'))

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print(input().replace(input(), 'kitten')) x = input() y = input() z = x.replace(y, 'kitten') print(z) =

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Thank you for your attention!