5.8. Selection

Selection allows us to ask questions, and take different actions depending on the answers. We use selection every day as we go about our duties. If it’s cold out we put on some extra clothes. If it’s raining we grab our umbrella before we leave the house. These are examples of the kinds of decisions we make. More formally we can put this into an if statement. if it is raining, then we take our umbrella.

Javascript allows us to ask questions like this using an if statement as well. An if statement is often used in Javascript to check an input value from a text box to make sure that it is a good value. For example if colors can only be in the range from 0 to 255 we could check the value of an input box as follows:

In this example we have one function that can check the values for any of our color text input boxes. Once again we have the interesting issue of how does the checkme function know which text box it is supposed to check? When we are setting up a call from html we can pass this as a special parameter that refers to the object that the onchange is connected to.

If you want to get a little practice, add another if statement to checkme that ensures the value entered by the user is greater than or equal to zero.

5.9. Guessing Game

Here is a simple application that uses selection to allow you play the old guessing game.

The computer will select a number between 1 and 100. You can type in your guess into the box and the computer will tell you when you are right, or if your guess is too high or too low.

To select our number we will use another of Javascripts builting facilities to make up a random number. This is called Math.random() It generates a random number between 0 and 1, so we’ll have to do a little bit of work to turn that into a random number between 1 and 100.

Things to do:

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