CS 116 Tutorial 3 (Solutions): Strings, Input/Output

Reminders:

Assignment 03 is due on Wed, Feb 5th at 10 AM

Write a function closest_integer that has no arguments, but instead reads in a floating point number from console input with a prompt "What's the number?", and returns the closest integer to that number. This function rounds ties up, so that:

closest_integer()

What's the number?:  0.5

will return 1,

while both of these (below)

`closest_integer()`	`closest_integer()`
`What's the number?: -0.5`	`What's the number?: 0.3`

will return 0.

DO NOT use round in your solution.

Solution:

import check


def closest_integer():
    '''
    returns the integer closest to the value entered by the user, always 
    round 0.5 up to the next largest integer.
    Effects: Reads in a float from console input
    
    closest_integer: None -> Int
    
    Examples:
    if the user types in 0, closest_integer returns 0
    if the user types in 0.3, closest_integer returns 0
    if the user types in -1.5, closest_integer returns -1
    if the user types in -1.8, closest_integer returns -2
    if the user types in 1.8, closest_integer returns 2
    '''
    n = float(input ("What's the number?"))
    base_number = int(n)
    after_decimal = n - int(n)
    if after_decimal < -0.5:
        return base_number-1
    elif after_decimal >= 0.5:
        return base_number+1
    return base_number

# additional tests:
check.set_input('-1.3')
check.expect('-1.3', closest_integer(), -1)
check.set_input('0.5')
check.expect('0.5', closest_integer(), 1)
check.set_input('0')
check.expect('0', closest_integer(), 0)
check.set_input('0.3')
check.expect('0.3', closest_integer(), 0)
check.set_input('-1.5')
check.expect('-1.5', closest_integer(), -1)
check.set_input('-1.8')
check.expect('-1.8', closest_integer(), -2)
check.set_input('1.8')
check.expect('1.8', closest_integer(), 2)

Write a function create_date that consumes nothing, but takes keyboard input. The program has three prompts: "Enter the year: ", "Enter the month: " and "Enter the day: ". The function then returns a date in the form "dd/mm/yyyy", where dd is a 2 digit number (between 01 and 31, depending on the month), mm is a 2 digit number (between 01 and 12) and yyyy is a 4 digit number.

For example,
create_date() Enter the year: 1996 Enter the month: 06 Enter the day: 17 => "17/06/1996"

Use string methods and string formatting (using {}) to complete the question

Solution:

import check


def create_date():
    '''
    prompts the user for three strings, reads in the year, the month 
    and the day respectively for each string, and returns the date in 
    the form "dd/mm/yyy".
    Effects: Prints three prompts to the user, reads in a year, a month and a day.

    create_date: None -> Str

    Examples:
    if the user enters "1996", "06", "17" when create_date() is called, 
    "17/06/1996" is returned
    if the user enters "2016", "05", "25" when create_date() is called, 
    "25/05/2016" is returned
    '''
    year = input("Enter the year: ")
    month = input("Enter the month: ")
    day = input("Enter the day: ")
    new_format = "{0}/{1}/{2}".format(day, month, year)
    return new_format

# Tests
check.set_input("1996", "06", "17")
check.expect("17/06/1996", create_date(), "17/06/1996")
check.set_input("2016", "05", "25")
check.expect("25/05/2016", create_date(), "25/05/2016")
check.set_input("2000", "01", "01")
check.expect("01/01/2000", create_date(), "01/01/2000")

Write a function fill_the_string that consumes a non-empty string s and a positive integer n, and returns an n-letter string that consists of copies of s, where the last one is perhaps a partial copy.

For example,
fill_the_string("love", 12) => "lovelovelove" fill_the_string("truth", 12) => "truthtruthtr"

Solution:
```
import check


def fill_the_string(s, n):
    '''
    returns an n-letter string made up of copies of s; the final copy might 
    not be a complete copy.
    
    fill_the_string: Str Nat -> Str
    requires: s is non-empty, n > 0

    Examples:
    fill_the_string("n", 0) => ""
    fill_the_string("love", 12) => "lovelovelove"
    fill_the_string("truth", 12) => "truthtruthtr"
    '''
    num_full_copies = n // len(s)
    leftover_bits = n % len(s)
    return s * num_full_copies + s[:leftover_bits]

# Tests:
check.expect("n is 1", fill_the_string("n", 1), "n")
check.expect("One full copy", fill_the_string("love", 4), "love")
check.expect("Three full copies", fill_the_string("love", 12), "lovelovelove")
check.expect("Partial copy", fill_the_string("love", 2), "lo")
check.expect("Full copies+partial copy", fill_the_string("truth", 12), 
             "truthtruthtr")
check.expect("Long full copies+partial copy", fill_the_string("love", 22), 
             "lovelovelovelovelovelo")
```
A few comments:
1) There are much longer ways to write this function, which are certainly fine.
2) Notice: It's not okay to name the function fill-the-string. Scheme is unusual in allowing the - character in the name of a function or variable. Python doesn't allow it. The underscore character is okay.
3) We used integer division here when we divided n by len(s). That gave us the right number of complete copies of s we needed.
4) If we'd had s * n // len (s), it wouldn't have worked. We would have first computed s * n (the string with n copies of s), and then been supposed to use // to divide this by len(s). But this gives this error:
TypeError: unsupported operand type(s) for //: 'str' and 'int'
5) You might have had the idea of using floating point multiplication, and tried to multiply s by a float. But that doesn't work, either:
>> "love" * 3.5
TypeError: can't multiply sequence by non-int of type 'float'

Write a recursive function sum_up that has no parameters but takes input from the keyboard. This function prompts the user with "Enter the amount of numbers to sum: ", followed by "Enter an integer: " which will read input the amount of times as the number entered before. The function then prints a message "The sum is n", where n is the sum of all the integers that were entered.

For example,
Enter the amount of numbers to sum: 4 Enter an integer: 3 Enter an integer: 56 Enter an integer: 7 Enter an integer: 8 The sum is 74

Solution:

import check


def read_and_sum(total,current_amount_read,total_amount_to_read):
    '''
    returns the sum of input numbers by prompting messages and reads
      in a series of integers
    Effects: Prints prompts to the user and reads in a series of integers
    read_and_sum: Nat Nat Nat -> Nat
    '''
    if current_amount_read == total_amount_to_read:
        return total
    else:
        read_in = input("Enter an integer: ")
        total = total + int(read_in)
        current_amount_read = current_amount_read + 1
        return read_and_sum(total,current_amount_read,total_amount_to_read)

def sum_up():
    '''
    prints out a message with the sum of all input integers by prompting
      messages and reads in a series of integers, with the initial integer
      corresponding to the amount of integers to be summed
    Effects: Prints prompts to the user and reads in a series of integers,
      prints out a message with the sum of all input integers
    sum_up: None -> None
    Examples:
      If the user input is ["1', "1"], sum_up() prints "The sum is 1"
      If the user input is ["4", "3", "56", "7", "8"],
        sum_up() prints "The sum is 74"
    '''
    amount_to_read = int(input("Enter the amount of numbers to sum: "))
    total_sum = read_and_sum(0,0,amount_to_read)
    print("The sum is {0}".format(total_sum))
    return                   ## optional

# Tests
check.set_input("1","1")
check.set_screen("The sum is 1")
check.expect("sum is 1", sum_up(), None)

check.set_input("4", "3", "56", "7", "8")
check.set_screen("The sum is 74")
check.expect("sum is 74", sum_up(), None)

check.set_input("5", "1", "2", "3", "4", "5")
check.set_screen("The sum is 15")
check.expect("sum is 15", sum_up(), None)

We've seen the function str.count() in lectures. Using recursion, implement a version of the function, called my_string_count(s, c) where s is any string, and c is a string of length one. my_string_count(s, c) will return the number of times that the character c appears in the string s.

For example,
my_string_count("hello world", "l") => 3 my_string_count("abracadabra", "e") => 0 my_string_count("", "e") => 0

Solution:

import check

def my_string_count(s, c):
    '''
    returns the number of times that the single character c appears in s
    
    my_string_count: Str Str -> Nat
    requires: len(c) == 1
    
    Examples: 
    my_string_count("", "a") => 0
    my_string_count("Hello World", "l") => 3
    my_string_count("The tragedy of Darth Plagueis the Wise", "t") => 3
    '''
    if s == "":
        return 0
    elif s[0] == c:
        return 1 + my_string_count(s[1:], c)
    else:
        return my_string_count(s[1:], c)
    
check.expect("empty s", my_string_count("", "s"), 0)
check.expect("single instance", my_string_count("super", "s"), 1)
check.expect("repeated instance", my_string_count("systematic", "s"), 2)
check.expect("no instance", my_string_count("long time, no see", "c"), 0)
check.expect("special characters (!)", my_string_count("Wow!", "!"), 1)
check.expect("numeral string", my_string_count("1729", "2"), 1)
check.expect("large repetition", my_string_count("abcd"*30, "b"), 30)
check.expect("capitals", my_string_count("AaaAaksAaa", "A"), 3)

CS 116: Introduction to Computer Science 2

CS 116 Tutorial 3 (Solutions): Strings, Input/Output

Reminders: