Lists of lists: last element of list overwritten by first element of another

This is a discussion on Lists of lists: last element of list overwritten by first element of another within the C Programming forums, part of the General Programming Boards category; Hi, I have a global list that contains smaller lists of char arrays. I have an issue where when Iím ...

  1. #1
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    Lists of lists: last element of list overwritten by first element of another

    Hi,

    I have a global list that contains smaller lists of char arrays. I have an issue where when Iím reading back the inner lists the last element of one list seems to point to first element of the next.

    So my data looks like the below (values separated by commas with the pairs separated by tabs. The last pair in a line is the same as the first). When I read the first list back instead of seeing ď456.678,678.98Ē as the last element in the list. I see ď435.67,234.98Ē twice: at the end of the first list and start of the other. I have debugged when the list is populated and can see the correct values going in so I canít figure whatís happening.

    456.678,678.98 123.45,345.56 256.67,789.98 456.678,678.98
    435.67,234.98 123.65,342.56 987.78,678.34 435.67,234.98

    Code to fill the list:

    Code:
    obstacle_list = op_prg_list_create();
    	
    while (fgets(line, sizeof(line), obstaclePositions_traj_file) )
    {
    	token = strtok(line, "\n"); //Pull the string apart into tokens using the \n
    	input = op_prg_list_create();
    
    	for( token = strtok(line, "\t"); token ; token = strtok (NULL, "\t") )
    	{
    	  test_token = strdup(token);
    	  op_prg_list_insert(input, test_token, OPC_LISTPOS_TAIL);
    	}     
    		
    	op_prg_list_insert(obstacle_list,input, OPC_LISTPOS_TAIL);
    	free(test_token);
    }
    Code to read back the list:

    Code:
    size_ob_list = op_prg_list_size (obstacle_list);
    line_coord_list = op_prg_list_create();
    for (k = 0; k <size_ob_list; k++)
    {
    	line_coord_list = (List*)op_prg_list_access (obstacle_list, k);
    	count_inner_list = op_prg_list_size (line_coord_list);
    	for (j=0; j< count_inner_list; j++)
    	{
    		coords = (char*)op_prg_list_access (line_coord_list, j);
    		printf("%c", coords);		
    	}
    }
    Any advice appreciated.

  2. #2
    and the hat of wrongness Salem's Avatar
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    > free(test_token);
    Yes, why do you do this?
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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  3. #3
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    That was it - Thank you very much Salem!

  4. #4
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    Code:
    while (fgets(line, sizeof(line), obstaclePositions_traj_file) )
    fgets() will read at most one '\n', which will be the last character in 'line' if it exists.

    Code:
    token = strtok(line, "\n"); //Pull the string apart into tokens using the \n
    Since there is only one '\n' in 'line' and it is at the end, strtok() will as a side effect change it to a '\0'. I.e. this line just removes the '\n' at the end of the 'line' using a rather unusual way.

    Code:
    for( token = strtok(line, "\t"); token ; token = strtok (NULL, "\t") )
    The initialization part of the for-loop throws away the result from the previous strtok()-call, i.e. the previous call is completely unnecessary if you use strtok() with the delimiter "\t\n" in your for-loop. This will split the string into tokens separated by either '\t' or '\n'.

    Code:
    size_ob_list = op_prg_list_size (obstacle_list);
    line_coord_list = op_prg_list_create();
    for (k = 0; k <size_ob_list; k++)
    {
        line_coord_list = (List*)op_prg_list_access (obstacle_list, k);
    It looks like you are accessing your lists using indices which is not very efficient. Why don't you use a simple loop which advances a pointer from one element to the next using the pointer to the next element?

    Code:
    coords = (char*)op_prg_list_access (line_coord_list, j);
    printf("%c", coords);
    Don't you get a warning when you compile? 'coords' seems to be a char pointer, but '%c' expects a char.

    Bye, Andreas

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