''' Purpose: do some functions produces lists from a string
'''

def separate( s ) : # s is the parameter (if we pass in the argument 'apple', then we return
    # ['a','p','p','l','e']
    ''' Returns a new list where each element in the list corresponds to a character of s
    '''
    # .split() by default separates on whitespace!
    # 'apple'.split() will become ['apple'] (We're not separating on whitespace there's none!)
    result = [] # accumulator - we will add characters of our string s to it one by one!
    for ch in s:    # Go through each character in string s
        result.append( ch ) # Add each character to our list one by one (each iteration)
    return result

    # result = result + [ch] # The + operator cannot be used to add things to a list
                            # if you do result + ch ^
                            # + can be used in number addition and string concatenation
                            # You do + when you want to glue two strings together
                            # The reason why result + [ch] works is this is saying
                            # make ch into a list ex. ['a'] and combine lists (result and
                            # [ch]) but the append way is best for understanding


def join( sl ) :
    ''' Returns a string composed of the strings in string list sl, where a blank is used
        to separate the strings from each other.
    '''
    # sl is a list of strings i.e. ['wa','hoo','wa']
    result = '' # accumulator - we want to build up and return a string!
    for s in sl: # Loop through each string in sl (the list passed in)
        result = result + s + " " # Update result to be the last version of result with the next
                              # string s concatenated onto it and a space after each string s
    n = len( result ) # 0 to n - 1 are the first index and last index and we want
                      # to get rid of the last space
                     # Remember slicing s[i:j] gets me s from i to j-1 !
    result = result[ 0 : n - 1 ] # gets the string from indexes 0 to n - 2 (no more space!)
    return result

def ints( ns ) :
    ''' Returns list of integers corresponding to the integer substrings in ns.
    '''
    # SUPER USEFUL I WOULD REMEMBER HOW TO DO THIS :)

    # We want to go from a string of numbers to a list of numbers!
    # "1 2 3" -> [1,2,3]

    result = [] # accumulator is going to be a list - build up list of numbers!
    list_numeric_strings = ns.split() # If ns is '1 2 3' now we have ['1','2','3']
    for numeric_string in list_numeric_strings: # Go through each numeric string ^
        nbr = int(numeric_string) # '1' -> 1 Convert numeric string to integer!
        result.append( nbr ) # Add nbr (integer version of numeric string to result)
    return result

        
def parse_phone_string( pn ) :
    ''' Returns a three-element integer list. The first element of the list is the pn area code,
        the middle element of the list is the pn prefix; the last element is the pn line number
    '''

    DIGITS = '1234567890' # All of our possible digits in a phone number

    # First go through and get the pn as a string of just the digits! Store it in
    # phone_number_string (ex. (201) 867-5309 -> 2018675309)
    phone_number_string = '' # Build up a string of just the numbers!
    for ch in pn: # pn is a string so let's go through each character in pn
        if ch in DIGITS:  # Check if each ch is a digit
            phone_number_string = phone_number_string + ch # Add the number to our phone number

    # First get the string of the phone number, slice the pieces, and then convert to numbers.
    # If you converted it to a number first, you can't go back and slice the number!

    # How can we use the phone_number_string and slicing?
    area_code   = phone_number_string[ 0 : 3 ] # gets s at indexes 0 to 2
    prefix      = phone_number_string[ 3 : 6 ] # gets s at indexes 3 to 5
    line_number = phone_number_string[ 6 : ] # gets s at indexes 6 all the way to the end


    area_code   = int( area_code )
    prefix      = int( prefix )
    line_number = int( line_number)

    result      = [area_code, prefix, line_number ] # Create a list of the three parts

    return result