Problem with garbage value in string

[stdq]

Hello. I wrote a program that tries to tokenize a mathematical expression, inserting the tokens in a list of strings. The list is as follows:

typedef struct listOfStrings {
    char **array;
    int size;
} ListOfStrings;

There is even a function to initialize the listOfStrings. The thing is: I'm printing a token every time it is complete and every time it is inserted in the list. The output is okay. However, when all tokens are processed and I call function print_list_of_strings to print the tokens again, the first token is printed with a leading garbage value if the input for the program is "3 + 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3". How is this possible? The code for printing the list is as follows:

void print_list_of_strings( const ListOfStrings *const lPtr )
{
    int i;
    int numberOfElements = lPtr->size;
 
    if ( numberOfElements != 0 )
    {
        for ( i = 0 ; i < numberOfElements ; ++i )
        {
            printf( "%s\n", *( ( lPtr->array ) + i ) );
        }
    }
    else
    {
        printf( "---\n" );
    }
}

The list just prints --- if it's empty, although this isn't the case for the program I'm writing. Also, if the input is "1 + 2", everything goes fine.

The code for inserting at the list is:

int insert_at_end_of_list_of_strings( ListOfStrings *lPtr, const char *const str )
{
    int lengthOfStr = strlen( str );
    int numberOfElements = lPtr->size;
 
    if ( ( ( *( lPtr->array + numberOfElements ) ) = ( char * )malloc( ( lengthOfStr + 1 ) *
        sizeof( char ) ) ) == NULL )
    {
        return 0;
    }
 
    memcpy( *( lPtr->array + numberOfElements ), str, ( lengthOfStr + 1 ) );
 
    ++( lPtr->size );
 
    printf( "%s inserted!\n", *( lPtr->array + numberOfElements ) );
 
    return 1;
}

[weaknessforcats]

You didn't post the code to initialize listOfStrings.

I assume it does something like a) malloc an array of char* and b) set listOfStrings.size to the number f char* allocated.
Presumably, all allocated char* are set to 0.

You would add a string (str) to this array by:

array[0] = str;

so I'm not sure what all the malloc and memcpy is about. Are you making a copy in addition to adding the string to the array? It looks like that would give you headaches in memory management and knowing which string to use (the original or the copy).

Maybe you could post your initialization function.

Also, I am not seeing how you know how much of the array is used. All I see is the size, which is most likely the number of elements but nothing about which elements are available.

[stdq]

Hello! I'm using malloc to allocate space to the new string to be added to the list. To be honest, I'm not sure I'm doing it right. Here is the code for the initialization - it returns 1 on success or 0 on failure:

int initialize_list_of_strings( ListOfStrings *lPtr )
{
    if ( ( lPtr->array = ( char ** )malloc( sizeof( char * ) ) ) == NULL )
    {
        return 0;
    }
 
    lPtr->size = 0;
 
    return 1;
}

[stdq]

I think I figured it out. I added the following statement to the beginning of the function of insertion:

lPtr->array = ( char ** )realloc( lPtr->array, ( numberOfElements + 1 ) * sizeof( char * ) );

[weaknessforcats]

lPtr->array = ( char ** )malloc( sizeof( char * ) ) )

This code will allocate one pointer. You can have one string in this array.

The allocating code does not initialize the pointer to zero. In the programming busimess, a zero pointer is OK a non-zero pointer is assumed to point to valid data.

You might consider allocating enough pointers so that you don't need to realloc every time. Maybe you start off with 30 pointers and you initialize them all to zero.

You would add a string just by traversing the array looking for the first zero pointer and replace it with the address of your string. If there are no zero pointers, the array is full so you can now reallocate and add more pointers.

You insert in the list by:

for (int i = 0; i < lPtr->size; ++i)
{
    if (lPtr->array[i] == 0)
    {
       lPtr->array[i] = str;
    }
}
 

You delete from the list by:

 free(lPtr->array[i]);
     lPtr->array[i] = 0;
 

You might consider a linked list of char* and eliminate the array altogether. This way additions and deletions are handled by adding and removing elements of the list. This method is the one most commonly used.

[stdq]

Thanks for your input! I guess I still have much to learn. I was trying to use a data structure of my own, but I intend to study data structures, in particula the book from Sedgewick. Thanks again!