Thread: Recursive function crashes.

Ok. I'm pretty new to C (and programming in general). I started 5 months ago. I have this code:

Code:

#include <stdio.h>
#include <stdlib.h>

/* I'm Lazy */
typedef unsigned long int BIG;

BIG Sum(BIG n);

int main(int argc, char *argv[])
{
	BIG n;
	
	if(argc<2) /*check for argument*/
		return(0);
	
	n=atoi(argv[1]); /*convert to an integer*/
	printf("\nAnswer = %d\n",Sum(n));
	return(0);
}

BIG Sum(BIG n)
{
	BIG result; /*final result variable*/
	
	if (n>0)
	{	
		printf("Calling Sum(%d)\n",n);
		result = n+Sum(n-1);
		/* after all Sum() functions are called, they start returning, returning, returning... :-) */
		printf("Sum() is returning. Partial result= %d\n",result);
		return(result);
	}
	else /*indicates the end of calling all the Sum()  functions*/
	{
		printf("\nFinished Calling Sum(). Return 0\n\n" );
		return(0);
	}
}

It will get a number from the user (inputed as an argument) and it will sum all the numbers from 1 to the number. Example: 4 -> 4+3+2+1 = 10.

I entered some printf() functions to understand how recursiveness works, and I'm all set on that part. The only problem is when I put a number like 99,999. The function gets called, and called, and called 'till 34,906 and then the program crashes and exits.

Why does this happen? I mean, I can also do it like this:

Code:

BIG Sum(BIG n)
{
	BIG result;
	for(result=0;n>0;n--)
	{
		result+=n;
	}
	return(result);
}

But why do recursive functions die like that? Thanks a lot.

ok so... there is no way to increase this stack space inside the program then?

Also, that leaves me a little bit worried.

The book I'm learning with (C All-in-one Desk Reference for Dummies) shows how to scan directories in the computer by using recursion... does this mean that this program is also limited? How could I scan all directories and subdirectories without using recursion?

Originally Posted by samus250

Well... I guess that there is always a way to make something work

I am using Bloodshed Dev-C++ (which I think uses MinGW or something like that...)

Just a couple more questions that have gone through my head since I first started programming:

1. One of the first C programs I wanted to do was a program that gave you the factorial value of a number. But... it seems that these variables are kind of limited. If they get too long, they get completely distorted and the computer spits out nonsense. Is there any ways to fix huge calculations? The program can only calculate up to 170! using unsigned doubles, larger than that, it explodes.

There are BIG number libraries that handle digits until they're pouring out both ears, not to worry. The interesting part for a young coder is "Can you program your own big number functions?". This is not trivial, but it is satisfying. Hint: base 10 number system. Reverse the numbers, then work them with the operand, from the one's column, the 10's, etc.

2. If I declare too many variables, does the program run out of stack space, or do the variables depend on your computers RAM?

You can run out of stack space. You can also run out of heap space, program space, constant space, depending on your rig's OS and your compiler. The variables may depend on RAM, or on the amount of virtual memory that your program is allocated.

Running out of stack space is a rarity in a program.

3. Am I wasting my time learning C, or is it really the most powerfull language out there? Is there any other language considered the language of the future, or can I do practically anything in C?

C is a medium high level language. Powerful, concise, and easy gets down to low-level work because it corresponds closely with the underlying hardware, and it runs VERY quickly, because of that. Also, it's procedural - you don't have to think up classes and data constructs and methods, for "objects", If you want to "talk on the phone", you just pick up the receiver and start dialing. You don't need to start defining what a phone is, in your program.

OOP is fine, but has a steep learning curve, less re-usability of code than you'd expect, and lots and lots of new ways to go wrong - VERY wrong.

If run-time is not an issue, languages like Python, Java, or Ruby are excellent choices, IMO. You just can't code a chess game with them, because they're like snails compared to C/C++. (or any other computationally intensive game).

I just want to feel that I'm learning something usefull :-)

examples:

1) I have a team fantasy football league, and I want to show some stats, scores, etc., with a program, including downloading data automatically. Python, Java, or Ruby would be my choice, because it's way easier, and the run-time is not an issue. The program only runs twice a week, at most. I will be far more productive in my time, with Python or Ruby, than in C/C++, for this.

2) I have a Radiologist friend. He'd like me to write a program to improve the images he has digitized, for better contrast. Over the years, his department has collected hundreds of thousands of images. I'd definitely use C for this one.

3) I'd like to code a program to manipulate the music I play in ways that I can select, when I press a foot pedal, in near-real time. C to the rescue, here.

Most programming languages were made for a particular purpose. They may have a general use, as well, but each has a real strength. C is system programming. Fortran was mathematics. Cobol was to make me ill, I mean for business. Pascal was for teaching programming, as was BASIC. Smalltalk is great for AI, Java for cross-platform development, and so on.

Whatever project you're working on, you want to choose a language that is strong in that area, and can meet or exceed the requirements of the project.

Originally Posted by iMalc

Recursion is perfect for directory scanning. You should never run into a problem doing that because nobody has directorys 1000+ levels deep. If you did then a lot of things would have difficulty with it because some OS functions only accept up to a certain number of characters, even though they were enlarged in later versions.

Couldn't you get around the maximum number of characters by changing the current directory as you traverse up and down?

--
As far as samus's question about recursive directory traversal, the stack only needs to store as many frames as the max depth of the directory structure:

Code:

             0                   1 \
       1           2             2  \ Maximum stack space used
    3     4     5     6          3  / 
  7  8   9 10 11 12 13 14        4 /

Originally Posted by oogabooga

Couldn't you get around the maximum number of characters by changing the current directory as you traverse up and down?

That wouldn't change the length limit of the total directory path, for deeper directories. The total path always starts with the drive letter. Anything less is a partial path only, needed to keep your sanity when using the DOS window.

Thanks a lot guys, you really helped. I'll now have to think about making a DIR Scanner that prints all files and their sizes very neatly in a file and the BIG numbers functions. Thanks for the idea Adak!