Chapter8.ipynb

{
 "metadata": {
  "name": "Chapter8"
 },
 "nbformat": 3,
 "worksheets": [
  {
   "cells": [
    {
     "cell_type": "code",
     "collapsed": true,
     "input": [
      "fruit = 'mango'",
      "letter = fruit[0]"
     ],
     "language": "python",
     "outputs": [],
     "prompt_number": 1
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "print letter"
     ],
     "language": "python",
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "m"
       ]
      }
     ],
     "prompt_number": 2
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "len(fruit)"
     ],
     "language": "python",
     "outputs": [
      {
       "output_type": "pyout",
       "prompt_number": 3,
       "text": [
        "5"
       ]
      }
     ],
     "prompt_number": 3
    },
    {
     "cell_type": "code",
     "collapsed": true,
     "input": [
      "lastletter = fruit[-1]"
     ],
     "language": "python",
     "outputs": [],
     "prompt_number": 4
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "print lastletter"
     ],
     "language": "python",
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "o"
       ]
      }
     ],
     "prompt_number": 5
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "index = 0",
      "while index < len(fruit):",
      "    letter = fruit[index] ",
      "    print letter",
      "    index = index + 1"
     ],
     "language": "python",
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "m",
        "a",
        "n",
        "g",
        "o"
       ]
      }
     ],
     "prompt_number": 6
    },
    {
     "cell_type": "plaintext",
     "source": [
      "Exercise 8.1 Write a function that takes a string as an argument and displays the letters backward, one per line."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": true,
     "input": [
      "def backward(word):",
      "    index = len(word)-1",
      "    while index >= 0:",
      "        print word[index]",
      "        index = index -1",
      "        "
     ],
     "language": "python",
     "outputs": [],
     "prompt_number": 16
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "backward('motion')"
     ],
     "language": "python",
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "n",
        "o",
        "i",
        "t",
        "o",
        "m"
       ]
      }
     ],
     "prompt_number": 17
    },
    {
     "cell_type": "plaintext",
     "source": [
      "The following example shows how to use concatenation (string addition) and a for loop to generate an abecedarian series (that is, in alphabetical order). In Robert McCloskey\u2019s book Make Way for Ducklings, the names of the ducklings are Jack, Kack, Lack, Mack, Nack, Ouack, Pack, and Quack. This loop outputs these names in order:",
      "",
      "prefixes = 'JKLMNOPQ' ",
      "suffix = 'ack'",
      "for letter in prefixes: ",
      "    print letter + suffix",
      "",
      "The output is:",
      "Jack ",
      "Kack ",
      "Lack ",
      "Mack ",
      "Nack ",
      "Oack ",
      "Pack ",
      "Qack",
      "",
      "Of course, that\u2019s not quite right because \u201cOuack\u201d and \u201cQuack\u201d are misspelled. ",
      "",
      "Exercise 8.2 Modify the program to fix this error."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "prefixes = 'JKLMNOPQ' ",
      "suffix = 'ack'",
      "for letter in prefixes:",
      "    if letter == 'O' or letter == 'Q':",
      "        print letter + 'u' + suffix",
      "    else:",
      "        print letter + suffix"
     ],
     "language": "python",
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "Jack",
        "Kack",
        "Lack",
        "Mack",
        "Nack",
        "Ouack",
        "Pack",
        "Quack"
       ]
      }
     ],
     "prompt_number": 18
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "print fruit"
     ],
     "language": "python",
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "mango"
       ]
      }
     ],
     "prompt_number": 19
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "print fruit[:]"
     ],
     "language": "python",
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "mango"
       ]
      }
     ],
     "prompt_number": 20
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "print fruit[:3]"
     ],
     "language": "python",
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "man"
       ]
      }
     ],
     "prompt_number": 21
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "print fruit[3:]"
     ],
     "language": "python",
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "go"
       ]
      }
     ],
     "prompt_number": 22
    },
    {
     "cell_type": "code",
     "collapsed": true,
     "input": [
      "def find(word, letter): ",
      "    index = 0",
      "    while index < len(word):",
      "        if word[index] == letter:",
      "            return index ",
      "        index = index + 1",
      "    return -1"
     ],
     "language": "python",
     "outputs": [],
     "prompt_number": 23
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "find('physics','s')"
     ],
     "language": "python",
     "outputs": [
      {
       "output_type": "pyout",
       "prompt_number": 24,
       "text": [
        "3"
       ]
      }
     ],
     "prompt_number": 24
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "find('physics','t')"
     ],
     "language": "python",
     "outputs": [
      {
       "output_type": "pyout",
       "prompt_number": 25,
       "text": [
        "-1"
       ]
      }
     ],
     "prompt_number": 25
    },
    {
     "cell_type": "plaintext",
     "source": [
      "Exercise 8.4 Modify find so that it has a third parameter, the index in word where it should start looking."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": true,
     "input": [
      "def find(word, letter, start): ",
      "    index = start",
      "    while index < len(word):",
      "        if word[index] == letter:",
      "            return index ",
      "        index = index + 1",
      "    return -1"
     ],
     "language": "python",
     "outputs": [],
     "prompt_number": 101
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "find('physics','s',5)"
     ],
     "language": "python",
     "outputs": [
      {
       "output_type": "pyout",
       "prompt_number": 105,
       "text": [
        "6"
       ]
      }
     ],
     "prompt_number": 105
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "find('physics','s',2)"
     ],
     "language": "python",
     "outputs": [
      {
       "output_type": "pyout",
       "prompt_number": 103,
       "text": [
        "3"
       ]
      }
     ],
     "prompt_number": 103
    },
    {
     "cell_type": "plaintext",
     "source": [
      "The following program counts the number of times the letter a appears in a string:",
      "",
      "word = 'banana' ",
      "count = 0",
      "for letter in word:",
      "    if letter == 'a': ",
      "        count = count + 1",
      "print count",
      "",
      "This program demonstrates another pattern of computation called a counter. The variable count is initialized to 0 and then incremented each time an a is found. When the loop exits, count contains the result\u2014the total number of a\u2019s.",
      "",
      "Exercise 8.5 Encapsulate this code in a function named count, and generalize it so that it accepts the string and the letter as arguments."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": true,
     "input": [
      "def count(word,letter):",
      "    cnt = 0",
      "    for lett in word:",
      "        if lett == letter:",
      "            cnt = cnt + 1",
      "    print \"Found\",cnt,\"instances of '\",letter,\"' in '\",word,\"'\""
     ],
     "language": "python",
     "outputs": [],
     "prompt_number": 33
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "count('physics','s')"
     ],
     "language": "python",
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "Found 2 instances of ' s ' in ' physics '"
       ]
      }
     ],
     "prompt_number": 34
    },
    {
     "cell_type": "plaintext",
     "source": [
      "Exercise 8.6 Rewrite this function so that instead of traversing the string, it uses the three-parameter version of find from the previous section."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "def count(word, letter): ",
      "    cnt = 0",
      "    next = 0",
      "    while next >= 0:",
      "        next = find(word,letter,next+1)",
      "        cnt = cnt + 1",
      "    print cnt-1"
     ],
     "language": "python",
     "outputs": [],
     "prompt_number": 149
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "count('physsics','i')"
     ],
     "language": "python",
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "1"
       ]
      }
     ],
     "prompt_number": 152
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "count('Mississippi','s')"
     ],
     "language": "python",
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "4"
       ]
      }
     ],
     "prompt_number": 153
    },
    {
     "cell_type": "plaintext",
     "source": [
      "Exercise 8.9 A string slice can take a third index that specifies the \u201cstepsize;\u201d that is, the number of spaces between successive characters. A step size of 2 means every other character; 3 means every third, etc.",
      ">>> fruit = 'banana' ",
      ">>> fruit[0:5:2] ",
      "'bnn'",
      "",
      "A step size of -1 goes through the word backwards, so the slice [::-1] generates a reversed string.",
      "",
      "Use this idiom to write a one-line version of is_palindrome from Exercise 6.6."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": true,
     "input": [
      "def is_palindrome(word):",
      "    if not isinstance(word,str):",
      "        print \"This only works for strings\"",
      "        return None",
      "    return word == word[::-1]"
     ],
     "language": "python",
     "outputs": [],
     "prompt_number": 154
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "is_palindrome(\"sets\")"
     ],
     "language": "python",
     "outputs": [
      {
       "output_type": "pyout",
       "prompt_number": 155,
       "text": [
        "False"
       ]
      }
     ],
     "prompt_number": 155
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "is_palindrome(\"redivider\")"
     ],
     "language": "python",
     "outputs": [
      {
       "output_type": "pyout",
       "prompt_number": 156,
       "text": [
        "True"
       ]
      }
     ],
     "prompt_number": 156
    },
    {
     "cell_type": "code",
     "collapsed": true,
     "input": [],
     "language": "python",
     "outputs": []
    }
   ]
  }
 ]
}