CS116 - Module 3 - Strings and Input/Output

Transcription

1 Winter 2018 Reminder: if you have not already, ensure you: Read Think Python, chapters 8, 10.

2 Printing As we have seen in passing, print(whatever) writes its argument on the screen. It can be used with numbers: print(22/7) prints Also with strings: print('hello world!') prints hello world Multiples values can be printed, separated by spaces: print('pi is about', math.pi) prints pi is about print('python', 4, 'evr!') prints python 4 evr!

3 Printing and newline If you want to have more than one line in a single string, you can use a special character, '\n', which is called a newline character. This prints: print('hello,\nhow are you?\ni am fine.') Hello, How are you? I am fine. By default each print statement ends with a newline. You can change this by using the end='...' argument to print: print('foo') print('bar') This prints: foo bar print('foo', end='-xxx -') print('bar') This prints: foo-xxx-bar That is, by default, the print statement is like print(..., end='\n').

4 Printing and testing We have very limited capability for testing print statements. 1 In words, describe the expected output. Use check.set_screen: check.set_screen('cs 116 on three lines') 2 Use check.expect or check.within to test the value returned by the function. Remember that every function always returns some value; if you do not specify, it returns None. check. expect('just print', myprint3('cs116'), None) 3 When you run your code, it will display the actual output, and the expected output. Use your judgement to determine if there is an error. The check module does no comparisons of screen output. Output of testing: just print: PASSED just print ( expected screen output): CS116 on three lines just print ( actual screen output): CS116 CS116 CS

5 Design Recipes and print() Include a description of what is printed in the Purpose statement. Add a new section: an Effects statement (immediately after the purpose) to briefly indicate a value is printed. Examples should include a description of the actual values printed for that input. ## myprint3( thing) print thing 3 times. ## Effects: prints some text. ## myprint3: Any -> None ## Example: ## myprint3 ( ' CS116 ') => None, ## and prints ' CS116\ ncs116\ CS116\n ' def myprint3( thing): print( thing) print( thing) print( thing) Exercise: Complete the design recipe for f1 and f2. def f1(x): print (x + 1) def f2(x): return (x + 1)

6 Introducing Str Like in Racket, we have a datatype for holding a sequence of characters. A Str is a value made up of letters, numbers, blanks, and punctuation marks, all enclosed in quotation marks. Examples: 'hat' "this Str's quotes are double!" 'I said "Hello world".' String functions 'now' + 'here' => 'nowhere' len('foobar') => 6 'foo' * 2 => 'foofoo' 'corn' in 'unicorns' => True '' in 'anything' => True # string - append # string - length # multiplication is repeated addition! # check if a Str contains a substring. # every Str contains the empty Str.

7 Powerful tools for substrings ## # first is character zero. x = ' caterpillar' x[5:9] => 'pill' # basic substring from 5 to 9 x[:5] => 'cater' # start with first x[5:] => 'pillar' # go to last x[3:10:2] => 'epla' # 3 to 10 by steps of 2 x[-5:-2] => 'ill' # counts from end; last is -1

8 You cannot change a Str, you can only make a new one. s = 'BOLT' # now s[2] == ' L ' s[2] = 'A' # this will cause an error; you cannot change a string. s = s[:2] + 'A' + s[3:] # All the stuff before 2, then ' A ', then all the stuff after 2. # So now s contains ' BOAT '

9 Read the documentation! Like with math, you can list functions on str with dir(str). There are lots of useful functions; explore the possibilities. x = 'foobar' x. upper() => 'FOOBAR' # does not modify x. Returns a modified copy. str. upper(x) => 'FOOBAR' # alternative way to write the same thing. Exercise: s = 'abcde a b ' Given the value of s above, run each command and figure out what they do. s. find('a') # return first place ' a ' appears in s. s. find('a',1) # return first place ' a ' appears in s, after 1. s. split() # split on each space. s.split('a') # split on each ' a '. s. startswith('abc') # does it start with the given Str? s. endswith('b') # does it end with the given Str?

10 Str and recursion Exercise: per line. Write a function which takes a string as argument and prints it, one character

11 Exercise e.g. Exercise: Write a Python function userid that consumes a non-empty first name, middle name (which might be empty), and a non-empty last name, and constructs a userid consisting of first letter of the first name, first letter of the middle name, and the last name. Make the userid all lowercase. If it is longer than 8 characters, discard the extra characters. userid("harry", "James", "Potter") => "hjpotter" userid("hermione", "Jean", "Granger") => "hjgrange"

12 User Input via the input() function user_input = input() Program halts. User types, ending with Enter. input returns the Str typed by the user. You may include a prompt: answer = input('what is your name?')

13 User Input and the Design Recipe When a function includes a input() call, this must be described in the Purpose statement, and mentioned in the Effects statement. Describe what happens with the value entered by the user. ## interrogate() Ask the user some useful questions. ## Concatenate and return the answers. ## Effects: prints 3 prompts, and reads 3 lines using input(). ## interrogate: None -> Str ## Example: ## interrogate(), then user enters ' Arthur ', ' seek the grail ', ## and ' African or European? ' => ' Arthur:: seek the grail:: African or European? ' def interrogate(): name = input('what is your name?') quest = input('what is your quest?') velocity = input('what is the air - speed velocity of an unladen swallow?') return name + '::' + quest + '::' + velocity

14 Basic Input Exercise: Write a function greet which takes no arguments, but uses input to ask the user their name, then prints a greeting with the name. For example, have the computer display: Who goes there? Then if the user types Arthur, have it display: Hello, Arthur Include full design recipe!

15 Guessing game import random x = random.randint(1,100) Now x has a random value on the interval [1, 100]. Exercise: Write a function ask_guess(correct_answer) which consumes a Nat, reads a guess from input(), and prints a message indicating if the guess is equal to correct_answer, too small, or too big. To run your function so you don t know the correct answer, you may use: ask_guess(random.randint(1,100)) Reminder: input() returns a Str. You can use int(n) to convert. Exercise: Now modify ask_guess so it calls itself recursively until the user guesses the correct answer.

16 Testing with input We wish to be able to test code without having a user type stuff in. There is a special function check.set_input that loads a list of Str. Calls to input will read from this list. import check ## set_input simulates the user typing. check. set_input(['arthur', 'to seek the grail', 'African or European?']) check. set_screen('3 questions: What is name/ quest/ airspeed velocity?') ## this function returns a value. check. expect('test0', interrogate(), 'Arthur:: to seek the grail:: African or European?') If the list is too long, the test will not pass. If the list is too short, an error message will appear. Exercise: Use check.set_input and check.set_screen to test your greet function.

17 String formatting On any string you may run the.format() method. This will perform substitutions for you: 'count to {0}'.format(3) => 'count to 3' qstr = 'I am {0} and I {1}.' qstr. format('arthur', 'seek the grail') => 'I am Arthur and I seek the grail.' The first argument to format will replace in {0}, the next will replace {1}, and so on. '{3}{2} {1} {0} {2}'.format('not', 'are', 'nuts', 'coco') => 'coconuts are not nuts' It is not an error to have unused values: 'four is {4}'.format(1, 1, 2, 4, 14, 42, 132) => 'four is 14'...but you can t use more than there are. 'four is {4}'.format(1, 1, 2) causes IndexError: tuple index out of range.

18 Practice Exercise: Write the recursive function printfactor that completes askfactor. # askfactor() input a number to factorize; print its factorization. # Effects: prints a prompt, reads 1 line, prints 1 line. # askfactor: None -> None # Examples: askfactor() => None; # if user enters 2, it prints: 2 # if user enters 30, it prints: 2 * 3 * 5 # if user enters 60, it prints: 2 * 2 * 3 * 5 def askfactor(): n = int( input('what number shall I factor?')) printfactor(n, 2)

19 Practice Exercise: Write a recursive function print_airshow(n, k) that works as follows: # print_airshow(4, 0) => None, and prints: > -> --> ---> ====> ---> --> -> > Do not use any additional functions or add any parameters!

20 Goals of Module 3 Become comfortable with Str and methods on Str, including methods for formatting. Start using print, and understand the important difference between print and return. Be able to write and test programs that read data from the user using input. Before we begin the next module, please Read Think Python, chapters 8, 10.