Debugging methods

2. Errors in logic

Look at the following sentence:

My pencil just rode a bike to London

While it is a grammatical sentence, it makes no sense. Pencils don't ride bikes. Somewhere, the programmer writing this sentence has made a logical error. Only the programmer can spot these logic errors because the computer will just do exactly as it has been told.

Here is a very common logic error.

Infinite loops

This pseudocode has a logic error

                 Line 30:            mynumber = 1
                 Line 31:              WHILE mynumber != 2  
                 Line 32:                PRINT mynumber
                 Line 33:              END WHILE

The mynumber variable is set to 1 in line 30, and nothing ever changes it - so the code loops around forever. The code below guarantees the while loop can end because of the increment mynumber command at line 33

                 Line 30:            mynumber = 1
                 Line 31:              WHILE mynumber != 2  
                 Line 32:                PRINT mynumber
                 Line 33:                INCREMENT mynumber
                 Line 34:              END WHILE