15-110 PA7 Sample Solutions - Spring 2018

1.

def unsigned_value(bitstring):
    total = 0
    for i in range(16):
        if bitstring[i] == '1':
            total = total + 2**(15-i)
    return total

2.

def to_positive(bytestring):
    if bytestring[0] == "0":
        return bytestring
    bytelist = list(bytestring)
    i = 0
    for i in range(len(bytelist)):
        if bytelist[i]=='1':
            bytelist[i] = '0'
        else:
            bytelist[i] = '1'
    i = 7
    while bytelist[i]=='1':
        bytelist[i] = '0'
        i = i - 1
    if i >= 0:
        bytelist[i] = '1'
    return "".join(bytelist)

3.

def bin_to_hex(bitstring):
    bin = ['0000', '0001', '0010', '0011', '0100', '0101', '0110', '0111',
           '1000', '1001', '1010', '1011', '1100', '1101', '1110', '1111']
    hex = ['0','1','2','3','4','5','6','7','8','9','A','B','C','D','E','F']
    result = ""
    for i in range(0, 16, 4):
        fourbits = bitstring[i:i+4]
        j = bin.index(fourbits)
        result = result + hex[j]
    return result

4.

def forward(c, i):
    position = ord(c) - 65
    new_position = (position + i) % 26
    return chr(new_position + 65)

def capitalize(s):
    new_s = ""
    for i in range(len(s)):
        if i == 0 or s[i-1]==' ':
            new_s = new_s + chr(ord(s[i]) - 32)
        else:
            new_s = new_s + s[i]
    return new_s