Conditions n341.tplt 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!!");