Debugging the Squares Example Program

The following program (SQUARES) uses the Fortran Console project type. The SQUARES program reads formatted data from the file datafile.dat and displays the calculated results. With the source code shown below, it does not generate the expected results:

    PROGRAM SQUARES
       INTEGER INARR(10), OUTARR(10), I, K
 ! Read the input array from the data file.
       OPEN(UNIT=8, FILE='datafile.dat', STATUS='OLD')
       READ(8,*,END=5) N, (INARR(I), I=1,N)
   5   CLOSE (UNIT=8)

 ! Square all nonzero elements and store in OUTARR.
       K = 0
       DO I = 1, N
	 IF (INARR(I) .NE. 0) THEN
 ! Is the error in this DO loop?
	    OUTARR(K) = INARR(I)**2
	 ENDIF
       END DO

 ! Print the squared output values.  Then stop.
       PRINT 20, N
   20  FORMAT (' Total number of elements read is',I4)
       PRINT 30, K
   30  FORMAT (' Number of nonzero elements is',I4)
       DO, I=1,K
         PRINT 40, I, OUTARR(K)
   40    FORMAT(' Element', I4, ' has value',I6)
       END DO
   END PROGRAM SQUARES

The formatted file datafile.dat currently contains one record that includes the following:

• An INTEGER count of the number of array elements, value 4
• The values for each of the four INTEGER array elements

To view the values of this formatted data file in the Microsoft visual development environment, use the Open item in the File menu.

When executed without array bounds checking (set by the /check:nobounds option), the output appears as follows:

When the program was executed with array bounds checking on, the output appears as follows:

You can either build this program from the command line or within the visual development environment (see Preparing Your Program for Debugging). This example assumes a project workspace already exists.

To debug this program:

1. From the Compaq Visual Fortran program folder, click Developer Studio to start the visual development environment.
2. In the File menu, click Open Workspace.
3. Click the FileView pane in the Workspace window. If the Workspace window is not displayed, click Workspace in the View menu.
4. Edit the file squares.f90: double-click its file name in the FileView pane. The screen appears as follows:

   

   The following toolbars are shown:

   • Build toolbar
   • Standard toolbar
   • Fortran toolbar

   To change the displayed toolbars, select Customize in the Tools menu and click the Toolbars tab. You can move toolbars by dragging the anchor (double vertical line on the left of the toolbar).

5. Click the first executable line to set the cursor position. In this case, click the beginning of the OPEN statement line:

      OPEN(UNIT=8, FILE='datafile.dat', STATUS='OLD')
   

6. Click the Set/Remove Breakpoint (open hand symbol) button in the Build toolbar:

   

   The red circle in the left margin of the text editor/debugger window shows where a breakpoint is set.

7. This example assumes you have previously built your application (see Preparing Your Program for Debugging).

   In the Build menu, click the Start Debug, Go item:

   

8. The debugger is now active. The current position is marked by a yellow arrow at the first executable line (the initial breakpoint):

   

   The Debug menu appears on the visual development environment title bar in place of the Build menu. If not displayed previously, the Debug toolbar appears.

   If needed, you can set another breakpoint, position the cursor at the line where you want to add or remove a breakpoint and do either of the following:

   • In the Build toolbar, click the Set/Remove Breakpoint button.
   • In the Edit menu, click Breakpoints. A dialog box allows you to set or clear breakpoints, evaluate expressions, and perform other functions.

   Step through the lines of source code. You can do this with the Debug menu item Step Over (as shown) or the Step Over button on the Debug toolbar:

   

9. Repeat the Step Over action and follow program execution into the DO loop. Repeat the Step Over action until you are at the end of the program. Position the cursor over the variable K to view its value (called Data Tips):

   

   The error seems related to the value of variable K!

10. In the text editor, add the line K = K + 1 as follows:

     ! Square all nonzero elements and store in OUTARR.
           K = 0
           DO I = 1, N
             IF (INARR(I) .NE. 0) THEN
             K = K + 1       ! add this line
             OUTARR(K) = INARR(I)**2
             ENDIF
           END DO
    

11. You have modified the source, so you need to rebuild the application:
    
    • In the Debug menu, click Stop Debugging
    • In the Build menu, click Build Squares.exe
    • In the Build menu, click Execute Squares.exe or click the exclamation point (!) on the Build toolbar.

    The output screen appears as follows:

    

12. Although the program generates better results, you can examine the values of both the input array INARR (read from the file) and the output array OUTARR that the program calculates. In the text editor window, the previously set breakpoint remains set.

    In the Build menu, click the Start Debug, Go item.

13. To view the values of certain variables as the program executes, we need to display the Variables or the Watch window. In the View menu, click the Debug Windows, Variables window item:

    

14. In the Variables window, click the Locals tab to display the values of your local variables:

    

    You can view the values of the local variables by using the Locals tab, including the arrays (click the plus sign).

    The Variables window displays a Context menu (after the word Context:). The Context menu can help you debug exceptions.

    The Locals tab does not let you display module variables or other non-local variables. To display non-local variables, display the Watch window:

    

15. Although this example does not use module variables or non-local variables, you can drag a variable name into the Watch window so the variable can be displayed. The Watch window allows you to display expressions.

    In the text editor window, select the variable name INARR (without its subscript syntax), drag it, and drop it into the Watch window:

    

16. Also drag the OUTARR array name to the Watch window. Click the Plus sign (+) to the left of the OUTARR variable's name to display the values of its array elements.
17. Execute lines of the program by using the Step Over button on the Debug toolbar. As the program executes, you can view the values of scalar variables with the data tips feature and view the values of arrays (or other variables) in the Watch window.

    When the program completes execution, the screen appears as follows:

    

If a Disassembly window (shows disassembled code with source-code symbols) unintentionally appears, click the Step Out button on the debugger toolbar (or select the Step Out item in the Debug menu) as needed to dismiss the Disassembly window.

If you have the Visual Fortran Professional or Enterprise Edition, you can use the Array Viewer to display and graph multidimensional array element values.

For more information:

• On using breakpoints, see Using Breakpoints in the Debugger.
• On viewing different types of Fortran data, see Viewing Fortran Data Types in the Debugger.
• On displaying array values in the Array Viewer, see Using the Array Viewer in the Debugger.
• On locating errors in your program, see Locating Run-Time Errors in the Debugger.
• On additional debugger capabilities, see the Debugger section of the Visual C++ Users Guide.