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String Methods - Thinking Like Computers - Lecture Slides, Slides of Artificial Intelligence

Bidhan Chandra Krishi ViswavidyalayaArtificial Intelligence

During the course work of Thinking Like Computers, we study the key concept of artificial intelligence. The main points in these lecture slides are:String Methods, Javascript Arrays, Separate Counters, Array of Counters, Dice Stats Page, Implementing Algorithms, Sequential Search, Binary Search, Index Variables, Writing Code, Swap List Elements

Typology: Slides

2012/2013

Uploaded on 04/24/2013

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CSCI 100
Think Like Computers
Lecture 22
Fall 2008
Last Time …
•String methods
charAt(i)
substring(i,j)
search(“ ”)
•Javascript arrays
a[i] : element at index i.
Can be modified
Strings to array: str.split(“ ”)
Something to remember
•Index starts at 0
•A range (i, j) includes all numbers from i to
j-1 (exclude j)
•This actually made things easy:
i=0; while ( i<a.length ) { … }
There are j-i items in range (i, j)
•Loops over index is the same as loops
over a counter
Example: Dice Stats
•recall our web page for
simulating repeated dice rolls
and recording the number of
times that a specific total was
obtained
statistical analysis predicts that,
given a large number of dice
rolls, the distribution of totals will
closely mirror the percentages
listed below
Approach 1: Separate
Counters
•to obtain a valid distribution of dice totals, we
would need to simulate a large number of rolls
and simultaneously count the occurrences of
every total
this can be accomplished by defining 11 counters,
each corresponding to a particular total
however, this would be extremely tedious
Approach 1: Separate
Counters requires separate
assignment
statements for all 11
counters
requires a
cascading if-else
statement with 11
cases
not easily
generalized – what
if we wanted to use
8-sided dice?
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CSCI 100

Think Like Computers

Lecture 22

Fall 2008

Last Time …

  • String methods Š charAt(i) Š substring(i,j) Š search(“ ”)
  • Javascript arrays Š a[i] : element at index i. ƒ Can be modified Š Strings to array: str.split(“ ”)

Something to remember

  • Index starts at 0
  • A range (i, j) includes all numbers from i to j-1 (exclude j)
  • This actually made things easy: Š i=0; while ( i<a.length ) { … } Š There are j-i items in range (i, j)
  • Loops over index is the same as loops over a counter

Example: Dice Stats

  • recall our web page for simulating repeated dice rolls and recording the number of times that a specific total was obtained Š statistical analysis predicts that, given a large number of dice rolls, the distribution of totals will closely mirror the percentages listed below

Approach 1: Separate

Counters

  • to obtain a valid distribution of dice totals, we would need to simulate a large number of rolls and simultaneously count the occurrences of every total Š this can be accomplished by defining 11 counters, each corresponding to a particular total Š however, this would be extremely tedious

Approach 1: Separate

Counters

Š requires separate assignment statements for all 11 counters

Š requires a cascading if-else statement with 11 cases

Š not easily generalized – what if we wanted to use 8-sided dice?

Approach 2: Array of

Counters

  • instead of representing each counter as a separate variable, we can define the counters as items in an array Š all 11 counters can be stored in an array and initialized via a single assignment statement Š any individual counter can be accessed and updated via its corresponding index ƒ since the first possible total is 2, its count is stored in index 0 ƒ the next possible total, 3, has its count stored in index 1 ƒ … ƒ for an arbitrary roll, its count is stored in index roll-

Approach 2: Array of

Counters

Š the resulting code is shorter, simpler, and easier to generalize

Dice Stats Page

  • code for maintaining statistics on repeated dice rolls can be integrated into a Web page Š the number of rolls to be simulated is entered by the user into a text box Š a button is defined to call the code for repeatedly simulating the roll and maintaining stats, with the text box contents as input Š the numbers of rolls for each total are then displayed in a (scrollable) text area

Implementing Algorithms

  • Sequential search
  • words = [“if”, “else”, “function”, “while”, “for”, “var”, “return”];
  • How to look for “function” in words?
  • function SeqSearch(list, desired) Š SeqSearch(words, “function”)

function SeqSearch(list, desired) //// Assumes: listReturns: index is of an list array where desired of items first appears, // or -1 if not found { var index; index = 0; // START AT FIRST ITEM while (index < list.length ) { // AS LONG AS ITEMS LEFT if (list[index] = desired ) { // IF NEXT ITEM IS DESIRED ONE, return index; // THEN RETURN WHERE FOUND } index = index + 1; // GO ON TO NEXT ITEM } return -1; // IF FAILED TO FIND, RETURN - }

Any error?

Binary Search

  • What is the idea?
  • Need: an already sorted list
  • [“Charlie”, “Dave”, “Jack”, “Jerald”, “Jerroll”, “Laura”, “Lois”, “Winnie”]
  • binarySearch(list, target)

Writing the Code

  • Inside the loop:
  • (1) compare list[mid] and target
  • if ( list[mid] == target ) return mid;
  • (2) // otherwise, update the range
  • else if ( list[mid] > target ) {
  • right = mid;
  • } else {
  • left = mid+1;
  • }
  • (3) don’t forget the mid:
  • mid = Math.floor( (left+right)/2 );

Writing the Code

  • If loop is finished (did not return during the loop)
  • Then target is not in the list!
  • return -1;
  • That’s it!

How to Sort?

  • Any idea?
  • list = [ 3, 20, 11, 13, 8, 10, 6, 2];

First, How to Swap?

  • You’ll need a temporary variable:
  • var temp;
  • temp = x;
  • x = y;
  • y = temp;
  • Notice the sequence: first save/write, then you can modify (and lose its original value)

Swap List Elements

  • Swap elements at index i and j:
  • var temp;
  • temp = list[i];
  • list[i] = list[j];
  • list[j] = temp;