Thread: Correct way of doing this?

I am getting some strange results from the following code:

Code:

while (fgets(buffer, 2048, pfile) != NULL)
{
	sscanf(buffer, "%d %d", &stid[i], &stsc[i]);
	printf("%d		%d\n", stid[i], stsc[i]);
	i++;
}

As you can see, its reading from a file with this type of format:

Code:

item1 subitem1
item2 subitem2

But after the while loop, and I call stsc[0], it contains elements that are supposed to be in stid. But this only happens AFTER the loop. During the loop, the output is correct. I need to use the arrays later on for something else, and with this happening, this isn't possible. Can anyone see if I'm misusing memory in some way?

Well, here is my declaration

Code:

	int* stid		 = (int*)malloc(sizeof(int));
	int* stsc		 = (int*)malloc(sizeof(int));

I am not sure whether or not using a struct would be easier or harder. I currently read from a text file with a non-specified amount of lines. With this way, it should work, but doesn't.

Originally Posted by carrotcake1029

Well, here is my declaration

Code:

	int* stid		 = (int*)malloc(sizeof(int));
	int* stsc		 = (int*)malloc(sizeof(int));

This gives you ... one number each. If you want more, you need to malloc more memory than this.

Originally Posted by tabstop

This gives you ... one number each. If you want more, you need to malloc more memory than this.

Well, it may be so that it doesn't allocate enough, but it still doesn't solve my problem.
I am still for some reason getting data that is supposed to be in stid[] inside of stsc[].

If you want, you can take a look at my whole function right now.

Code:

int main(int argc, char* argv[])
{
	int* stid	= (int*)malloc(sizeof(int*));
	int* stsc	= (int*)malloc(sizeof(int*));
	char* input_file = (char*)malloc(sizeof(char*));
	char* buffer	 = (char*)malloc(sizeof(char*));
	FILE* pfile;
	int i = 0;											
	float avg = 0.0;
	int avgtot = 0;										
	int sort;
	int j;

	printf("Please enter a file to read "
		   "(default is scores.txt): \n");				

	scanf("%s", input_file);							

	printf("If you would like sort the data, type the corresponding numerical value:\n");
	printf("(ID = 0) ");
	printf("(Grade = 1) ");
	printf("(None = 2)\n");

	scanf("%d", &sort);

	pfile = fopen(input_file, "r");						

	if (pfile == NULL)									
	{
		printf("File could not be read, reverting to default.\n");				
		pfile = fopen(default_file, "r");				
	}

	if (pfile == NULL)									
	{
		printf("Error finding scores.txt\n");
		return 1;										
	}

	printf("\nStudent Letter grades:\n\n");
	printf("Student ID\t\tGrade\n");

	while (fgets(buffer, 2048, pfile) != NULL)			
	{
		sscanf(buffer, "%d%d", &stid[i], &stsc[i]);		
		printf("%d\t\t%s\n", stid[i], parsegrade(stsc[i]));
		avgtot += stsc[i];								
		i++;											
	}

	avg = (avgtot / i);									

	printf("\nAverage grade: %2.1f	%s\n",	avg, parsegrade(avg));

	for(i = 0; i < 5; i++)
	{
		printf("%d\n", stsc[i]);     //This is my test loop that is drawing strange results
	}

	fclose(pfile);										
	
	free(stid);											
	free(stsc);											
	free(buffer);										
	free(input_file);									


	return 0;
}

Originally Posted by carrotcake1029

Well, it may be so that it doesn't allocate enough, but it still doesn't solve my problem.

Except that it does, of course. You have room for one integer. You put two or five or however many numbers ... somewhere. Where are they going to go? Somewhere else.

(Since stid and stsc are allocated one after the other, they are probably next to each other in memory; so writing stid[1] actually writes into the space that is really stsc[0], and you write stid[2] into the space that you really stsc[1] -- sort of anyway, since neither stid[1] nor stsc[1] exist, since you only allocate one spot.)

Well, I am still getting the same results. Could you show me the correct way to allocate both arrays?

Easy as pie:

Code:

int* stid = malloc(N*sizeof(int));

and same for stsc, of course. The catch: you have to know N (the number of things) before you start.

Originally Posted by tabstop

Easy as pie:
The catch: you have to know N (the number of things) before you start.

Well, I don't, lol. Workarounds?

Originally Posted by carrotcake1029

Well, I don't, lol. Workarounds?

Guess a number that will be "big enough".

Use realloc to resize the memory every single time you get a new one.

If it's a file, read through it once to count how many lines there are, then rewind.

Ok, thanks for taking the time to explain it to me. Everything is working well now.