Getting the values of linked list with loops - c

Question

I have to create a function who can get the value of a matrix wich take the form of a double linked list. Here is the structures of the matrix
typedef struct row {
unsigned int indiceRow;
struct row * next;
struct col * data;
} row;
typedef struct col{
double value;
unsigned int indiceColumn;
struct col * next;
} col;
typedef struct matrix{
int nRows;
int nCols;
struct row * rowFirst;
}matrix;
the structure matrix represent the top of the linked list and contain the total number of rows and columns and a variable row wich point to the first node of the list of row nodes. the row nodes contain the number of the row of the matrice, a variable row called next wich represent the next line of the matrix and a variable data point to another list of col nodes. Those col nodes contains the number of the column, the value at those coordonates(row,column) and the a col next. only the values different of zero have to be in the col linked list.
To get the value of a precise point of the matrix I created the function sp_get. It take a structure matrix, the line and column I'm looking for and a double variable as argument. It returns 0 when it works and update the variable double *val with the value I'm looking for.
int sp_get( struct matrix *mat, unsigned int rows, unsigned int col, double *val){
row * temps = (row*)malloc(sizeof(row));
temps = mat->rowFirst;
while(temps->indiceRow!= rows){
temps = temps->next;
}
while(temps->data!= NULL && temps->data->indiceColumn!= col && temps->data->next!=NULL){
temps->data = temps->data->next;
}
if(temps->data->indiceColumn == col){
*(val) = temps->data->value;
}
else{
*(val) = 0.0;
}
return 0;
First I create a row variable to run through the matrix, then I look for the good row and then for the good column. If I can't find the good column it means that the value is 0.
When I use the function to look for one value, it works well, and always return the good value.(tempMatrix is a matrix variable and contain the linked list)
double * vall =(double*)malloc(sizeof(double));
sp_get(tempMatrix, 2, 3, vall);
but when I'm using the function with a double loop for I don't have the same results and I can't not explain why...
double * vall =(double*)malloc(sizeof(double));
int i;
int j;
for(i=1;i<=tempMatrix->nRows;i++){
for(j=1; j<=tempMatrix->nCols;j++){
sp_get(tempMatrix,i,j,vall);
printf(" %f ", *(vall));
}
printf("\n");
}
Here are the result I get with the loops
and here are the results I should get
It might be a proble of memory leak, I don't know where it comes from.
Thanks in advance for your help!

Answer 1

Just in sp_get alone the following problems abound:
Memory the first two lines.
Anytime you see something like this in successive lines in C:
ptr = malloc(...)
ptr = <something else>
it is always a memory leak.
Updating the column header rather than simply enumerating it
Once you find the row you seek, you then do this:
while(temps->data!= NULL &&
temps->data->indiceColumn!= col &&
temps->data->next!=NULL)
{
temps->data = temps->data->next;
}
Ask yourself, what is temps->data = ... actually updating? It is changing the temps->data pointer to point to its own next, which means what temps->data pointed to prior is gone. That's fine if temps->data is a temporary pointer, but it isn't. It is the data member in the row struct you worked so hard to find in the prior loop.
Potential NULL pointer dereference
You may think having this:
while(temps->data!= NULL &&
temps->data->indiceColumn!= col &&
temps->data->next!=NULL)
for the while-condition in your loop will harbor safety from temp-data being NULL for the code that follows:
if(temps->data->indiceColumn == col)
{
*(val) = temps->data->value;
}
but if it did, then why bother with the first clause (which is correct, btw). It appears the addition of the last clause (temps->data->next!=NULL) was an effort to stave off crashes. That isn't the way to do it.
Minor: Hiding type col with parameter col
Needs little explanation. See your var names.
Minor: There is no need to dynamically allocate the out-parameter as you're using it.
Your code do to this:
double * vall =(double*)malloc(sizeof(double));
int i, j;
for(i=1;i<=tempMatrix->nRows;i++)
{
for(j=1; j<=tempMatrix->nCols;j++)
{
sp_get(tempMatrix,i,j,vall);
printf(" %f ", *(vall));
}
printf("\n");
}
Can just as easily do this:
double val = 0.0;
int i, j;
for(i=1;i<=tempMatrix->nRows;i++)
{
for(j=1; j<=tempMatrix->nCols;j++)
{
sp_get(tempMatrix,i,j,&val); // note address-of operator
printf(" %f ", val);
}
printf("\n");
}
Updated sp_get
I'm pretty sure this is what you're trying to do. The following will return 0 if the indexed values found and retrieved, otherwise it returns -1 and the out-parameter is set to 0.0.
int sp_get( struct matrix const *mat, unsigned int rows, unsigned int cols, double *val)
{
// prime to 0.0
*val = 0.0;
if (!mats)
return -1;
// walk the row table
struct row const *row_ptr = mat->rowFirst;
while (row_ptr && row_ptr->indiceRow != rows)
row_ptr = row_ptr->next;
// leave now if we didn't find the row.
if (!row_ptr)
return -1;
struct col const *col_ptr = row_ptr->data;
while (col_ptr && col_ptr->indiceColumn != cols)
col_ptr = col_ptr->next;
if (!col_ptr)
return -1;
*val = col_ptr->value;
return 0;
}
Note we modify nothing in the actual matrix, so the entire thing, including all pointers we use to index within it, can be const (and should be).
Best of luck.