type cast problem

[tikelele]

I wrote a little pointer program and I just have an error that I don't get.

this line

numSize = ceil( (float) (max - min) / interval ) + 1;

is supposed to make numSize 6 when:

int min = 0
int max = 10
int interval = 2

But I get numSize = 1026

seems to work fine like this:

numSize = ((max - min) / interval ) + 1;

the reason I need the ceil in there is because if

int min = 0
int max = 10
int interval = 3

I need 5 and not four.

I am using visual C++ to compile and debug (I tried cygwin but vista does not ply nice).
Is the problem specific to VC++?

[robatino]

Are you sure you're printing the value of numSize properly, and not doing something like using the wrong printf() type specifier? Also, ceil() takes a double argument, so you might want to cast to double instead of float (not that it matters here).

Edit: Another reason for using (double) instead of (float) is that the maximum integer that can be represented exactly by a float is fairly small, since a 32-bit IEEE 754 float only has 6 or 7 decimal digits of precision.

[tikelele]

No here is what I am doing.

{
	int max;
	int min;
	int interval;
	int numSize;
	int i;
	int *num;
	int *numSquare;

	printf( "Enter a minimum value: ");
	scanf( " %d", &min);
	printf( "Enter a maximum value: ");
	scanf( " %d", &max);
	printf( "Enter an interval: ");
	scanf( " %d", &interval);
	
	
	numSize =  ceil( (float) (max - min) / interval )  + 1;
	
	num = malloc(numSize * 2);
	numSquare = malloc(numSize * 2);

	for (i = 0; i < numSize; i++)
	{
		num[i] = min + (i * interval);
		numSquare[i] = num[i] * num[i];
		printf( "  %3d squared  =  %5d\n", num[i], numSquare[i]);
	}

	free (num);
	free (numSquare);
	num = NULL;
	numSquare = NULL;

	return 0;
}

[robatino]

1) What makes you think numSize is equal to 1026 with your specified input? The expression looks okay to me. I notice you don't have any code to print the value of numSize, though.

2) Whatever you're doing in allocating space for num and numSquare is horribly wrong. For one thing, since they're arrays of ints, you should have a sizeof(int) factor inside malloc() (or sizeof(*num) and sizeof(*numSquare) if you want). What exactly are you trying to do?

[tikelele]

1) What makes you think numSize is equal to 1026 with your specified input? The expression looks okay to me. I notice you don't have any code to print the value of numSize, though.

The output is way too many lines. If I use min 0 max 10 and interval 2 i should get 6 lines. when i use the ceil(....) I get hundreds. Also when I step into the debugger numSize has a value of 1026 at that line. might just be VC++

2) Whatever you're doing in allocating space for num and numSquare is horribly wrong. For one thing, since they're arrays of ints, you should have a sizeof(int) factor inside malloc() (or sizeof(*num) and sizeof(*numSquare) if you want). What exactly are you trying to do?

I am asking for a minimum number, a maximum number, and an interval (all integers). I need to Allocate two arrays big enough to hold the values defined by those numbers, safely.

Then i need to Initialize one array to have the values defined by those numbers (min, min+interval, min+2*interval, etc.) and the other array to have the square of those values.

[robatino]

Try just putting a printf() statement right after the assignment to numSize to see what it is. I'm pretty sure that right after you assign it, the value is what it should be. Also, if you want to allocate space for an array of N ints, the proper syntax is malloc(N*sizeof(int)) (you need the sizeof() there since malloc() takes the size in bytes and you need to tell it how many bytes are in each int, which is what sizeof(int) is).

[tikelele]

Thank you for the malloc() information.

I added the printf to see what was being placed in numSize is after assignment and it is indeed 1026. I am going to reboot to see if that helps.

[robatino]

I doubt if rebooting will help. Could you post the _entire_ piece of code you are compiling? There are a number of things that could be causing the problem with numSize that could be from code you're not posting.

[tikelele]

#include "stdafx.h"

int _tmain(int argc, _TCHAR* argv[])
{
	int max;
	int min;
	int interval;
	int numSize;
	int i;
	int *num;
	int *numSquare;

	printf( "Enter a minimum value: ");
	scanf( " %d", &min);
	printf( "Enter a maximum value: ");
	scanf( " %d", &max);
	printf( "Enter an interval: ");
	scanf( " %d", &interval);
	
	//numSize = ((max - min) / interval ) + 1; //this works
	numSize =  ceil( (float) (max - min) / interval )  + 1; //this doesnt
	//printf( " %d", numSize); //use this to show the value of numSize
	
	num = malloc(numSize * sizeof(int));	
	numSquare = malloc(numSize * sizeof(int));	

	for (i = 0; i < numSize; i++)
	{
		num[i] = min + (i * interval);
		numSquare[i] = num[i] * num[i];
		printf( "  %3d squared  =  %5d\n", num[i], numSquare[i]);
	}

	free (num);
	free (numSquare);
	num = NULL;
	numSquare = NULL;

	return 0;
}

That's it unfortunately

[dwks]

You really don't need all of that memory. You could get by with a for loop body like this:

		num = min + (i * interval);
		printf( "  %3d squared  =  %5d\n", num, num * num);

	//numSize = ((max - min) / interval ) + 1; //this works
	numSize =  ceil( (float) (max - min) / interval )  + 1; //this doesnt

Do you have the header file math.h included? If not, the compiler might not be aware of what ceil returns and assume it returns an int and mess up royally.

[tikelele]

You really don't need all of that memory. You could get by with a for loop body like this:

		num = min + (i * interval);
		printf( "  %3d squared  =  %5d\n", num, num * num);

Thank you. I understand.

	//numSize = ((max - min) / interval ) + 1; //this works
	numSize =  ceil( (float) (max - min) / interval )  + 1; //this doesnt

Do you have the header file math.h included? If not, the compiler might not be aware of what ceil returns and assume it returns an int and mess up royally.

That did it. I did not include it. Thank you greatly. rookie error (still wearing tightie whities - hope to be in boxers soon!) thanks again