How to get to an outer while

This is a discussion on How to get to an outer while within the C Programming forums, part of the General Programming Boards category; Hi everyone. I've written a search and replace routine in C99. I think it should work, but there is a ...

  1. #1
    Registered User
    Join Date
    Feb 2011
    Posts
    126

    How to get to an outer while

    Hi everyone. I've written a search and replace routine in C99. I think it should work, but there is a point marked by the // where I want to restart the outer while loop. How do I do this?

    Richard (TIA)

    Code:
    char *ptokens[][2] = 
    {
       { "Find this" , "Replace with this" } ,
       { "Another one" , "Here's the replacement." } ,
       { 0 , 0 }
    } ;
    
    int searchandreplace ( char* psource , char* pdestination ) 
    {
    	while ( *psource != 0 )
    	{
    		int i = 0 ;
    
    		while ( ptokens [i][0] != 0 )
    		{
    			char* ptoken = strstr ( psource , ptokens [i][0] ) ;
    			if ( ptoken == psource )
    			{
    				strcpy ( pdestination , ptokens [i][1] ) ;
    				psource += strlen ( ptokens [i][0] ) ;
    				pdestination += strlen ( ptokens [i][1] ) ;
    				// start outer while again
    			}
    			else
    				i++ ;
    		}
    
    		*pdestination ++ = *psource ++ ;
    	}
    
    	*pdestination = 0 ;
    
    	return 0 ;
    }
    Last edited by Richardcavell; 03-05-2011 at 12:37 AM. Reason: fix stupid bug

  2. #2
    Registered User
    Join Date
    Jun 2005
    Posts
    5,883
    There is actually no direct way to jump back out through two levels of nested loops.

    There are various indirect ways.

    The simplest way, although it would make "anti-goto" zealots incredibly unhappy, is to place a label at the start of the outer loop, and then "goto that_label;".

    Another way might involve using negative values of i as an indicator that the code needs to start again;
    Code:
    while ( *psource != '0' )
    	{
    		int i = 0;
    
    		while ( i >= 0 && ptokens [i][0] != 0)
    		{
    			char* ptoken = strstr ( psource , ptokens [i][0] ) ;
    			if ( ptoken == psource )
    			{
    				strcpy ( pdestination , ptokens [i][1] ) ;
    				psource += strlen ( ptokens [i][0] ) ;
    				pdestination += strlen ( ptokens [i][1] ) ;
    				i = -1;   /*  signal breakout */
    			}
    			else
    				i++ ;
    		}
    
    		if (i >= 0) *pdestination ++ = *psource ++ ;
    	}
    More generally, however, it would be better to restructure your code to reduce (or, better, eliminate if you can) the need to back out of multiple nested loops. Code that arbitrarily backs out of multiple nested loops tends to be somewhat difficult to understand, and therefore harder to get right - and rather fragile if you need to change the logic later on.
    Right 98% of the time, and don't care about the other 3%.

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