puppybasic intro ch. 14 - errors

green text is being edited and should not be considered  
relevant to the tutorial. for a while, it will make up  
the bulk of this section.
  
black text has been edited to be more accurate with  
regards to puppybasic.

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errors                                                                   
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                  an intro to puppybasic                                 
                                                                         
                  chapter 14: errors                                     
                                                                         
                                                                         
                                                                         
                                                                         
                                                                         
                                                                         
                                                                         
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errors                                                                   
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you could do well to always trap and handle errors, i almost never       
bother. if you write your program well, you might manage to avoid it.    
this applies less to large programs. of course, sometimes error trapping 
is quite useful.                                                         
                                                                         
whether you use error handling or not, your program, upon encountering   
an error, can tell the user the name of the error. if you do not trap    
your errors, the program will end as well. if you do use error handling, 
you have the option of telling the user a more friendly interpretation   
of the problem:                                                          
                "I'm sorry, no file by that name exists in this folder." 
                                                                         
                                                                         
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errors                                                                   
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or even fixing it so the program can continue:                           
                                                                         
"That file does not exist. Create? (Y/N)"                                
                                                                         
you can even trap errors to detect which screen modes don't work. as     
mentioned in the graphics section, there are several screen modes in     
puppybasic, and they start with SCREEN 0, or text-only mode and range   
from 0 to 13. so, you can tell your program to go to all the possible  
screen modes: (none of this is true, this tutorial is being edited.)                                                                  
                                                                         
                                                                         
                                                                         
                                                                         
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errors                                                                   
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okscreen$ = SPACE$(14)                                                   
ON ERROR GOTO oops                                                       
FOR q = 0 TO 13                                                          
SCREEN q                                                                 
NEXT q: nomorec = 1                                                      
                                                                         
and then when it tries to SCREEN [not an available mode] it will cause   
an error 5, and GOTO the "oops" routine:                                 
                                                                         
                                                                         
                                                                         
                                                                         
                                                                         
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errors                                                                   
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oops:                                                                    
IF nomorec = 0 THEN                                                      
IF ERR = 5 THEN MID$(okscreen$, q + 1) = "x"                             
END IF                                                                   
RESUME NEXT                                                              
                                                                         
which adds an "x" under the part of the variable okscreen$ for that      
mode.                                                                    
                                                                         
                                                                         
                                                                         
                                                                         
                                                                         
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errors                                                                   
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the command RESUME NEXT tells the program "don't worry about that last   
error..." and the program then runs the next line, which is the "NEXT q" 
of the FOR... NEXT loop.                                                 
                                                                         
when you're done trying all the modes, you can go back to SCREEN 0,      
which is the default, and should always work:                            
                                                                         
SCREEN 0: WIDTH 80 'WIDTH 80 should always work, otherwise, remove it.   
                                                                         
then print a guide for the numbers 0 to 13:                              
                                                                         
                                                                         
                                                                         
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errors                                                                   
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PRINT "          1111"                                                   
PRINT "01234567890123"                                                   
                                                                         
and under that:                                                          
                                                                         
PRINT: PRINT okscreen$                                                   
END 'don't forget to put an END statement between                        
'your program and routines.                                              
                                                                         
now your program will put a little "x" under the modes that don't work   
on your computer!                                                        
                                                                         
                                                                         
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errors                                                                   
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if we went over this section too fast: here is a review:                 
                                                                         
ON ERROR... GOTO linelabel                                               
                                                                         
tells the program what routine to goto when an error occurs. there is no 
"ON ERROR... GOSUB."                                                     
                                                                         
IF ERR=n THEN                                                            
                                                                         
will do something in response to the specific error... (you can also use 
"SELECT CASE ERR" for handling this part.)                               
                                                                         
                                                                         
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errors                                                                   
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RESUME NEXT                                                              
                                                                         
goes back to the part of the program where the error occured, but it     
goes to the first line AFTER the problem.                                
                                                                         
note: error code 5 is "illegal function call" - the error you get when 
you try to change to an unavailable mode, although there are other 
possible causes.  
                                                                         



                                                                         
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errors                                                                   
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there are at least a couple other commands for error handling, but       
unless you want them enough to look them up, you probably won't ever     
need them.                                                               
                                                                         
                                                                         
                         -= end of chapter 14 =-                         
                                                                         
                                                                         
                                                                         
                                                                         
                                                                         
                                                                         
                                                                         
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an intro to puppybasic                                      (chapter 14) 
this content is public domain.                          
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