The contents of the first edit box are checked when the Check button is pressed. The form also has a button to check the contents of the first edit field. First of all, build a form with five edit fields and five corresponding labels, describing in which occasion the corresponding Edit component checks the input. In this example we're going to compare the effect of testing the input at different stages. We will see an example of the use of another even more sophisticated control for keyboard input, the MaskEdit component. To handle numerical input in an application you'll generally use specific components, as the SpinEdit or the UpDown controls available in Delphi. This example is meant as an exercise to discuss keyboard input and the input focus. The next example, Numbers (see the form in the following figure), shows some of the techniques you can use to handle numerical input with an Edit component, so you can compare them easily. Of course, you can improve it by combining it with one of the other techniques. This technique is not foolproof (a user can always paste some text into an edit box), but it works quite well and is easy to implement. You can look at the input stream to the edit box and stop any non-numerical input. There is another, radically different, solution to the problem of allowing only numerical input in an edit box. As you'll see, not all of these techniques work well. The second question is, when can we perform this test? Maybe when the value of the edit box changes, when the component loses focus, or when the user clicks on a particular button, such as the OK button in a dialog box. This sounds good, but what if the user types a letter when a number is expected? Of course, these conversion functions return an error code, so we can use it to test whether the user has really entered a number. To accomplish this, you can use the MaskEdit component (in the Additional page of the Components palette) or simply use an Edit component and then convert the input string into an integer, using the standard Pascal Val procedure or the Delphi IntToStr function. In general, it's quite common to ask users for numeric input, too. We saw in the previous example that it is very easy to use an Edit component to ask the user to input some text, although it must be limited to a single line.

Delphi numeric edit box code