outline

Overview

• Can computers think?
• Jacquard's loom
  - Sequential logic
  - beginning, middle and end
• Example:
     Morning weather report
     - If it's cool out today, I'll wear a sweater

Logic

• Humans understand different kinds of syntax to get to the semantics
• Computers - compare some variable to a value or another variable
• Conditions - evaluated as true or false

Example

• temperature < 65
• temperature is a variable with a starting value
• (<) is a comparison operator
• Last part of the conditional statement is often a value
  - value must be same type as the variable
  - could also be a variable

Evaluate

• What is cool to the computer?
• Translate the vague concept 'cool' into a condition the computer can understand

Branching - If Statement

• 
• Which way the train goes is dependent on a switch in the track.
• The conditional statement is the switch.

Simple JavaScript
   -If Statement

• //set up my variable
  var temperature = 0;
  temperature = prompt ("What is the temperature?");
  
  //be sure we have an integer to work with
  temperature = parseInt(temperature);
  
  //check the temperature
  if (temperature < 65){
     //this will ONLY happen when temp is less than 65
     alert ("Put a sweater on!!");
  }//end if
  
  //no matter what the temp, we're wearing pants
  alert ("Put some pants on.");

The Else Clause

• If statement useful, but could be improved.
• Example:
     -if we do not wear a sweater, we should wear something
  
     If it is cool out today,
        wear a sweater
     otherwise,
        wear a shortsleeve shirt

JavaScript Example

• var temperature = 0;
  
  temperature = prompt("What is the temperature?");
  temperature = parseInt(temperature);
  
  if (temperature < 65){
     alert ("Wear a sweater!!);
  } else {
     alert ("Wear short sleeves!!");
  } // end if

Else Clause

• Happens only when the condition is evaluated as FALSE.
• Else doesn't make sense without an 'if' statement.
• If-else commonly used.
• "{…}" used to combine lines, do not use a semicolon at the end of the statement using the brackets
      - this causes your condition to be ignored!

Multiple Branching

• Sometimes your decisions will be more complex than a simple 'if' statement.
• Example:
      if it's warm outside,
       wear short sleeves
      if it's cool,
       wear a sweater,
      if it's cold,
       wear a jacket

Multiple 'If' Statements

• 
  
• You can use three different 'if' statements
  
  OR
  
  You can use 'nested if' statements

The Switch Structure

• Some languages support a special 'multiple-branch' structure
• Basic-like languages use "select case"
• Many C-like languages use the switch statement

Loops

• 
  Allows certain parts of code to repeat based on the status of a condition
  
  New syntax -- while statement.
  The while statement checks the condition and repeats the code between {…} symbols if the condition is TRUE.

Sentry variables and Initialization

• Start by creating a variable
• Giving the variable an initial value is initializing the variable
• Variables used in loops are often known as sentry variables

Example

• //Note that I initialized stillRacing incorrectly
    var stillRacing = "N";
  
  //This condition will only be checked once, and it will be false
    while (stillRacing == "Y"){
     //the body of this loop will NEVER be activated
     alert ("zooming around the track.");
     //see if we're still on
     stillRacing = prompt("Is the race still on? (Y/N)");
    }//end while
  
  alert ("Nice race!!");

Preventing Endless Loops

• Inside a loop, we MUST ensure that there is some way of exiting the loop
• We have to make it possible for the sentry variable to change values

Example

•  var stillRacing = "Y";
  
   while (stillRacing == "Y"){
     alert("zooming around the track");
  
     //forgot to check if we are still racing!!!
  
   }// end while
   alert ("Nice race!!");
  
• This program will go on FOREVER!!!

Endless Loops

• The condition statement checks to see if the condition is TRUE
• In the previous example, the condition is true
• Because the condition is true, the body of the loop still thinks we are "zooming around the track"
• Whenever you make a loop, it is your obligation to ensure that the sentry variable is changed so it is possible to exit the loop

The For Loop

• Variation of the loop we have already used
• Difference - we have added a counter
• Needs four pieces of information:
   - name of counting variable
   - starting value of the variable
   - ending value of the variable
   - how to increment the variable

For Loop

• The for statement needs three parts:
   1.  initialize the variable
   2.  a condition
   3.  how to increment the variable
• Once the loop is set up, we don't have to worry about it in the body - it automatically increments

Example

• var lap = 1;
  
  for (lap = 1; lap <= 10; lap ++){
     alert ("Now on lap " + lap);
  }// end for
  
  alert ("Good race!!");