Archive
Prison break
Exercise
The big boss of Montreal Prison celebrates his 50th birthday next week. For this special occasion he came up with the idea to let some prisoners go. In the prison there are 600 cells, one person in each. The cells are numbered from 1 to 600. The prison guard should go through all cells and open the doors. Then he goes to the 2nd cell and flips the lock at every second cell (that is, close the door if it was open and open it if it was closed). Then go the 3rd cell and flip the lock of every third cell, etc. This procedure should be repeated with every cell. Question: who are those lucky guys who get release, i.e. which cells remain open at the end?
Example with 8 cells:
00000000 initialization, all cells are closed 11111111 step 1, flip every lock 10101010 step 2, flip every 2nd lock 10001110 step 3, flip every 3rd lock 10011111 step 4, flip every 4th lock 10010111 step 5, flip every 5th lock 10010011 step 6, flip every 6th lock 10010001 step 7, flip every 7th lock 10010000 step 8, flip every 8th lock
Reverse an integer
Exercise
Take an integer and reverse its digits. The result is also an integer. Example: 83657 becomes 75638.
Solution
#!/usr/bin/env python
def reverse_int(n):
return int(str(n)[::-1])
n = 83657
print n # 83657
print reverse_int(n) # 75638
Summary: convert the number to string, reverse the string, then convert it back to integer. Details: 83657 -> str(83657) returns "83657" which is a string -> reverse it, we get "75638" -> int("75638") converts it to an integer.
Notes
If you want to concatenate a string and an integer, first you need to convert the integer to string. Example:
n = 83657 print "The value of n is " + n # error, won't work print "The value of n is " + str(n) # OK
Fibonacci numbers
Exercise
Now an easy one. Calculate the first N Fibonacci numbers, where F0 = 0, F1 = 1, …, Fn = Fn-1 + Fn-2. Write a function that receives N as a parameter and returns a list with the first N Fibonacci numbers. Example: fib(10) would produce [0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55]. Notice that F0 = 0 is included in the result.
Solution
#!/usr/bin/env python
# fibonacci.py
def fib(n):
assert(n >= 0)
li = []
a, b = 0, 1
for i in range(n+1):
li.append(a)
a, b = b, a+b
return li
if __name__ == "__main__":
print fib(10)
Here we solved the problem in an iterative way, but you can do it with recursive calls too.
Links
See http://www.scriptol.com/programming/fibonacci.php for a comprehensive list how to implement it in any language.
Reverse a file
Exercise
Take a text file and reverse it the following ways: (1) reverse characters in each line, and (2) reverse the order of lines too. Let’s see an example:
Input:
#!/usr/bin/env python print "Please, reverse me completely!"
Output:
"!yletelpmoc em esrever ,esaelP" tnirp nohtyp vne/nib/rsu/!#
Pretty print an integer
Exercise: Take an integer and print it in a pretty way, i.e. use commas as thousands separators. Example: 1977 should be 1,977.
Solution:
#!/usr/bin/env python
def numberToPrettyString(n):
"""Converts a number to a nicely formatted string.
Example: 6874 => '6,874'."""
l = []
for i, c in enumerate(str(n)[::-1]):
if i%3==0 and i!=0:
l += ','
l += c
return "".join(l[::-1])
#
if __name__ == "__main__":
number = 6874
print numberToPrettyString(number) # '6,874'
The idea is simple. Consider the number 1977. Convert it to string ("1977") and reverse it ("7791"). Start processing it from left to right and after every third character add a comma: "7" -> "77" -> "779," (comma added) -> "779,1". Now reverse the string ("1,977"). Done.
Links
- http://stackoverflow.com/questions/1823058/how-to-print-number-with-commas-as-thousands-separators-in-python-2-x (for some more possible solutions)
Update (20131125)
There is an easier way. You can do it with string formatting too:
>>> n = 1977
>>> "{:,}".format(n)
'1,977'
Thanks to Krisztián B. for the tip.
Reverse a string
Exercise #1: Take a string and reverse its characters. For instance “ab12” => “21ba”.
Solution:
#!/usr/bin/env python s = 'Python adventures' print s # Python adventures print s[::-1] # serutnevda nohtyP
Slice notation has the form [start:stop:step]. By default, start is at the beginning of a sequence, stop is at the end, and step is 1. So the slice [::-1] returns the full sequence in reverse order.
Exercise #2: Decide if a word is a palindrome.
Solution:
#!/usr/bin/env python
def is_palindrome(str):
return str == str[::-1]
print is_palindrome('1367631') # True
print is_palindrome('Python') # False
ASCII table
Exercise: Print out the ASCII table.
Solution:
#!/usr/bin/env python
for char in range(256):
print "%d: %c" % (char, char)
Note that range(N) creates a list of elements in the interval [0, N), that is N is excluded. In the example, it will be 0..255.
Output (special characters are removed):
0:
1:
2:
3:
4:
5:
6:
7:
8:
9:
10:
11:
12:
13:
14:
15:
16:
17:
18:
19:
20:
21:
22:
23:
24:
25:
26:
27:
28:
29:
30:
31:
32:
33: !
34: "
35: #
36: $
37: %
38: &
39: '
40: (
41: )
42: *
43: +
44: ,
45: -
46: .
47: /
48: 0
49: 1
50: 2
51: 3
52: 4
53: 5
54: 6
55: 7
56: 8
57: 9
58: :
59: ;
60: <
61: =
62: >
63: ?
64: @
65: A
66: B
67: C
68: D
69: E
70: F
71: G
72: H
73: I
74: J
75: K
76: L
77: M
78: N
79: O
80: P
81: Q
82: R
83: S
84: T
85: U
86: V
87: W
88: X
89: Y
90: Z
91: [
92: \
93: ]
94: ^
95: _
96: `
97: a
98: b
99: c
100: d
101: e
102: f
103: g
104: h
105: i
106: j
107: k
108: l
109: m
110: n
111: o
112: p
113: q
114: r
115: s
116: t
117: u
118: v
119: w
120: x
121: y
122: z
123: {
124: |
125: }
126: ~
127:
128:
129:
130:
131:
132:
133:
134:
135:
136:
137:
138:
139:
140:
141:
142:
143:
144:
145:
146:
147:
148:
149:
150:
151:
152:
153:
154:
155:
156:
157:
158:
159:
160:
161: ¡
162: ¢
163: £
164: ¤
165: ¥
166: ¦
167: §
168: ¨
169: ©
170: ª
171: «
172: ¬
173:
174: ®
175: ¯
176: °
177: ±
178: ²
179: ³
180: ´
181: µ
182: ¶
183: ·
184: ¸
185: ¹
186: º
187: »
188: ¼
189: ½
190: ¾
191: ¿
192: À
193: Á
194: Â
195: Ã
196: Ä
197: Å
198: Æ
199: Ç
200: È
201: É
202: Ê
203: Ë
204: Ì
205: Í
206: Î
207: Ï
208: Ð
209: Ñ
210: Ò
211: Ó
212: Ô
213: Õ
214: Ö
215: ×
216: Ø
217: Ù
218: Ú
219: Û
220: Ü
221: Ý
222: Þ
223: ß
224: à
225: á
226: â
227: ã
228: ä
229: å
230: æ
231: ç
232: è
233: é
234: ê
235: ë
236: ì
237: í
238: î
239: ï
240: ð
241: ñ
242: ò
243: ó
244: ô
245: õ
246: ö
247: ÷
248: ø
249: ù
250: ú
251: û
252: ü
253: ý
254: þ
255: ÿ
Python Adventures 