Section 3 Solutions

String Slicing

s[1:6]
s[:2] or s[0:2]
s[6:9]
s[6:] or s[6:10]
s[6] or s[6:7]
s[:] or s[0:10] (or just s)

String Function Warmup

              
def defront(s):
    if len(s) >= 2:
        return s[2:]
    return s

def x_end(s):
    found = s.find('x')
    if found != -1:
        return s[found:]
    return ''

def pre_colon(s):
    colon = s.find(':')
    if colon != -1:
        return s[:colon]
    return s

def at_word(s):
    at1 = s.find('@')
    if at1 != -1:
        at2 = s.find('@', at1 + 1)
        if at2 != -1:
            return s[at1 + 1:at2]
    return ''

CoHome Delivery

          
def contains_email(line):
    """
    This function checks to see if an email address is present in a given line of a message. 
    It checks for the '@' symbol. If one is present, it checks that it is part of an email address, 
    meaning it must have at least one character before and after it. If an email address is found,
    the function returns True and it returns False otherwise. 
    """
    at_location = line.find('@')
    if at_location == -1:
        return False

    if line[at_location - 1] == ' ' or line[at_location + 1] == ' ':
        return False

    return True

def contains_phone_number(line):
    """
    This function searches a given line of a message for a 10 digit sequence of numbers to check if a phone
    number is present. If a phone number is present, it returns True and it returns False otherwise. 
    """
    for idx in range(len(line)):
        if line[idx].isdigit():
            possible_phone_number = line[idx:idx + 10]
            if len(possible_phone_number) == 10 and possible_phone_number.isdigit():
                return True 
        
    return False

def is_message_valid(filename):
    """
    This function searches a file for an email address or phone number using the helper methods defined above. 
    If the file contains a phone number or email address, it is considered invalid and the function should 
    return False. If it contains neither a phone number or email address, the message is valid and should 
    return True.
    >>> is_message_valid('100284.txt')
    True
    >>> is_message_valid('100282.txt')
    True
    >>> is_message_valid('100280.txt')
    False
    """
    with open(filename, 'r') as f:
        for line in f:
            if contains_email(line):
                return False 
            if contains_phone_number(line):
                return False 
    
    return True