Strings

a. Given a string s, return a new string made of all the chars from s which are 'd' or 'o' or 'g' (not case sensitive).

'xdxggy' -> 'dgg'  |  def doggy(s):
'DOggy' -> 'DOgg'  |
'abdGxo' -> 'dGo'  |
'xyz' -> ''        |

def doggy(s):
    result = ''
    for ch in s:  # for/i/range also fine
        low = ch.lower()
        if low == 'd' or low == 'o' or low == 'g':
            result += ch
    return result

b. Given a string s. Find the first '@' in s. If there is a digit 3 chars after the '@', such as the '6' in 'x@106', then return the numeric value of that digit doubled. If there is no '@' or no digit, then return -1. Use the s.find() function.

'x@106x'  -> 12  |  def digit2x(s):
'xx@ab51' -> 10  |
'x@abx' -> -1    |
'abc@'  -> -1    |
'abc'   -> -1    |

def digit2x(s):
    at = s.find('@')
    if at == -1:
        return -1
    if at + 3 < len(s) and s[at + 3].isdigit():
        return int(s[at + 3]) * 2
    return -1

c. Given a string s, find the first double left-bracket '[[' in s, and the first double right-bracket ']]'. If the double left-brackets are before the double right-brackets, then return a new string where the text between the brackets is replaced by the string 'NOPE'. If the left or right double brackets are missing or they are in the wrong order, then return the empty string. Do not use loops. Use s.find() and slices.

'xx[[hi there!]]zzz'  ->  |  def nope(s):
  'xx[[NOPE]]zzz'         |
'a[[]]b'  -> 'a[[NOPE]]b' |
'xx[[abc'  -> ''          |
'xx]]abc'  -> ''          |
'x]]ab[['  -> ''          |

def nope(s):
    left = s.find('[[')
    right = s.find(']]')
    if left == -1 or right == -1:
        return ''
    # Right before left = wrong order
    if right < left:
        return ''

    # Slice for each side, 'NOPE' in the middle!
    return s[:left + 2] + 'NOPE' + s[right:]