# lab10.py
# YOUR NAME(S) AND NETID(S) HERE
# DATE COMPLETED HERE
"""Loop functions for lab 10"""


def isprime(p):
    """Returns: True if p a prime, False otherwise

    i.e. p is at least 2 and is divisible by only 1 and itself

    Precondition: p is an int"""
    assert type(p) == int, `p`+' is not an int'

    if p < 2:
        return False    # CHANGE THIS EXPRESSION, IF NECESSARY

    if p == 2:
        return False    # CHANGE THIS EXPRESSION, IF NECESSARY


    # Return False if some integer in 2..p-1 divides p
    k = 0               # CHANGE THIS EXPRESSION, IF NECESSARY

    # inv: p is not divisible by integers in 2..k-1
    while k < p:
        if False:       # CHANGE THIS EXPRESSION
            return False
        k= k+1

    # no integer in 2..p-1 divides p
    return False        # CHANGE THIS EXPRESSION, IF NECESSARY


# HINT:  chr(x) turns an int into a letter
#        ord(x) turns a letter into a number
def prime_chars():
    """Returns: a string that contains each capital letter (in 'A'..'Z')
    whose numerical representation is prime.

    For example, since 67 is a prime, and the letter 'C',
    is represented by 67 (since ord('C') is 67), the string should
    contain 'C'.  But it should not contain 'B", since ord('B') is 66,
    not a prime."""
    s = NONE            # CHANGE THIS EXPRESSION

    c = ''              # CHANGE THIS EXPRESSION

    # inv: string s contains each capital in 'A'..c-1,
    #      where 'c-1' means the character alphabetically preceding
    #      c, whose numerical representation is prime
    while c <= ' ':     # CHANGE THIS EXPRESSION
        if False:       # CHANGE THIS EXPRESSION
            s = s + ''  # CHANGE THIS EXPRESSION

        # Increment to next letter
        # Change to number, increment, and back to letter
        c = chr(ord(c) + 1)

    # post: s contains each capital in 'A'..'Z' whose rep is a prime
    return s


def count_letter(c, s):
    """Returns: number of times character c appears in string s

    Precondition: s, c are strings. len(c) == 1"""
    assert type(s) == str, `s`+' is not a string'
    assert type(c) == str and len(c) == 1, `c`+' is not a character'
    # STUB: IMPLEMENT ME
    # you must use a while loop and must include its invariant
    # (don't use str's count function (except for testing))

    return 0 # CHANGE THIS, IF NECESSARY


# OPTIONAL FUNCTION
def count_all_letters(s1, s2):
    """Returns: number of times the characters in s1 appear in s2
    Each letter in s1 is treated separately.  Duplicates are ignored.

    Example: count_all_letters('aaab', 'ac') is 3

    Example: count_all_letters('aeiou', word) gives the number of
    vowels in string word (for some definitions of vowels)

    Precondition: s1, s2 are strings"""
    assert type(s1) == str, `s1`+' is not a string'
    assert type(s2) == str, `s2`+' is not a string'
    # STUB: IMPLEMENT ME
    # Hint: Don't nest one while loop in another; use count_letter as
    # a helper function
    return 0 # CHANGE THIS, IF NECESSARY