# Some functions that uses strings
#Strings versus arrays.

A = ["a","b","c"]
S = "abc".

S[2]  # ok
len(S) # ok

# S[2] = "e"    # not ok.
# S.append("d")  # not ok
S = S+"c"
S = S+str(3)
S = "aa"+S      # OK.  "+" is non-destructive.

# Strings as objects

S = "oldold"
S = S.replace("old","new") # gives "newnew"

# Strings, unlike arrays, are immutable.

# function to reverse a string
def revstr(S):
    R = ""  # string to be built
    i = 0
    while i<len(S):
        R = S[i] + R  # add to the front of R
        i +=1
    return R
#revstr

# function to convert a string to all lower-case
# ord('A') is 65.  This function doesn't change the parameter string,
# it BUILDS a new string.
def lowercase(S):
    L = ""
    i = 0
    while i<len(S):
        asc = ord(S[i])  # ascii code for S[i] character
        if (asc>=65 and asc<=90): # upper case
            asc = asc+32  # asc now code for a lower-case character
        lc = chr(asc)     #convert code back to character
        L = L+lc          # add char to string
    #while
    return L
#lowercase

# function to test if two strings are case-insensitively equal:
# (semi-class exercise)
def ciequals(A,B):
    la = lowercase(A)
    lb = lowercase(B)
    return la==lb
    # same as if la==lb: return True
    #         else: return False
#ciequals
##### This is an example of a BOOLEAN FUNCTION: returns True or False


# Sometimes, it is convenient to translate a string into an array of
# chars, change it, then convert it back.  We will use these functions
# a lot:
def strToArray(S):  # convert (return) array of chars to represent S
    A = []
    i = 0
    while i<len(S):
        A.append(S[i])
        i +=1
    return A

# number-base conversion
def intTobin(x): #convert x to binary string "0110"
    S = ""
    while x>0:
        d = x % 2
        S = str(d) + S
        x = x/2
    #while
    return S
#intTobin
# try it on 23


def binToint(S):
    answer = 0
    i = len(S)-1
    power = 0
    while i>=0:
	  if S[i]=="1":
	     answer += 2**power
	  power +=1
	  i = i-1
    #while
    return answer
#btoi