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Software Design Using C++



Fourth Intermediate Windows Forms Example



The Goal


We wish to create a Windows App that displays the square roots of the first so many positive integers. Let's make it so that the user can specify how many integers are desired. Here is a picture of the resulting app showing 25 roots. This picture of the app with 40 roots illustrates the fact that the user can scroll through the list of roots if the list is too long to all be seen at once. The particular control that we use to hold the integers and their roots is called a ListBox.

Getting Started


Create a new C++ Windows Forms App project and name it Roots or similar. Resize the resulting form so that it is about the same dimensions as seen in the picture of the running application. In the Properties Window, change the MaximizeBox property to false so that the user will not be able to maximize the application. Change the Text property to "List of Square Roots" (without the quotes). You should not need to change any other properties for the overall form.

Adding the Controls


From the picture of the running app, you can see that we need a label at the top left. Drag one from the Toolbox to your form and change its Text property to "Number of ints to use:" (again without the quotes). To the right of it put a TextBox control and change its name to NumberBox. This box will be used to hold the number of integers for which we want square roots. To the right of this box put a button and change its name to StartButton. Change its Text property to "Start". You have already used controls like this in the previous examples and should have little trouble adding these controls.

Next, we turn to the new control, the so-called ListBox. Drag one of these from the Toolbox to your form, just under the 3 controls already present. Resize it so that it fills most of the section of the form below the 3 controls. Change the name of this control to RootListBox.

Save all of your files. Also right click on the new project in Solution Explorer and set this project to be the startup project. You might even want to build and run the project to be sure that nothing went wrong so far.

The Code


In Design View, double click on the button on your form so as to produce an outline (prototype) for your click handler. In Form1.h it should look like this: 


private: 
   System::Void StartButton_Click(System::Object * sender, System::EventArgs * e);

Then, in Form1.cpp place the code for this new function. Use the following: 


System::Void Form1::StartButton_Click(System::Object * sender, System::EventArgs * e)
   {
   long k, Count;
   double Root;
   String * Info;

   RootListBox->Items->Clear();
   Count = NumberBox->Text->ToInt32(NULL);
   for (k = 1; k <= Count; k++)
      {
      Root = Math::Sqrt(k);
      Info = S"   ";
      Info = String::Concat(Info, k.ToString());
      Info = String::Concat(Info, S"\t   ");
      Info = String::Concat(Info, Root.ToString());
      RootListBox->Items->Add(Info);
      }
   }

How does this code work? The Info variable is used to hold a string that we build up to hold the data that we want to display on an individual line of the ListBox. To be more precise, it is a pointer to this string as you can tell from the asterisk in String * Info. Notice how we start by calling a Clear function on the Items collection in RootListBox. We are again being a bit loose with our terminology since RootListBox is really a pointer to the ListBox and RootListBox->Items is a pointer to the Items collection object. That is why we see the use of the arrow notation instead of the dot notation to pick out a field or function. In any case, the Clear function is used to wipe out old data whenever the user clicks on the button. If the user clicks the button 2 or more times we do not want the output appended together. Rather, we just want to see the output for the last set of square root calculations.

We next extract the Text from NumberBox and use the ToInt32 function to convert this string to an integer. We should really check to see if this conversion works or fails, but we will delay the handling of exceptions like this to the First Advanced Windows Forms Example.

We then use a FOR loop to go through the first so many positive integers and insert a line of data into the ListBox for each such integer. Note that we use long integers since the user might want to find a huge number of square roots. The Math::Sqrt(k) function call obviously calculates the square root of integer k.

The string manipulations may be new to you. The S" " is a convenient way to create a string literal, in this case containing 3 blank characters. We have our Info string pointer set to point to this string of 3 blanks. Then we concatenate onto the end of this string the result of converting the current integer to a string. Then we concatenate the literal string S"\t ". Note that the \t indicates a tab character. It and the blanks in this literal string are used for spacing between the integer and its square root. Finally we use Root.ToString() to convert our calculated square root to string form and concatenate this as well.

The Add function is then used to add the string we have build up, the one that Info points to, to the Items collection in RootListBox. Note again the use of the arrow notation since both RootListBox and Items are really pointers to objects and not the objects themselves.

Try It Out


Save all of your files, then build your new project. If there are syntax errors, fix them and build again. Then run your program by using Debug, Start Without Debugging. Try it out and make sure that it works correctly.

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Author: Br. David Carlson
Last updated: March 24, 2007