Help with pointer to function problem

**rob90** · 05-28-2010

Hi everyone, a few months ago my professor gave us a problem which I didn't manage to solve, then I forgot about it but now I found it again and I'm still stuck with it, the problem is:
given a prototype: int sum (int (*f) (int), int start, int end);
/* Post sum(f,start,end)= ∑i = start, i = start+1, .... i = end f(i) *\
Implement recursively the funcion sum and show how it has to be called in order to output
∑i = 1, ... i = N g(i) where g is a function which prototype is int g(int).

I wrote this code:

Code:

#include <stdio.h>

int g(int n) 
{
	return n*(n+1) / 2;
}
int sum(int (*f)(int), int start, int end)
{	
	if(start==end) return f(start);
	return sum(f, start+1, end);
}
int main()
{
	int start, end, result;	
	
	scanf("%d%d", &start, &end);	
	
	result = sum(g, start, end);	
	
	printf("%d\n", result);	
	
	return 0;
}

However I'm not sure this is the right way to solve it, if someone can give me a few hints I would really appreciate it since i guess my professor is going to give us a similar problem in the next exam, thanks

**tabstop** · 05-28-2010

You're only calculating the last value. When you do the recursion, you split your list up into (start) and (start+1, ..., end) -- you find the value of the last bit, but not the first bit.

**rob90** · 05-28-2010

Originally Posted by tabstop

You're only calculating the last value. When you do the recursion, you split your list up into (start) and (start+1, ..., end) -- you find the value of the last bit, but not the first bit.

Do you mean that I just have to modify the sum function this way?

Code:

int sum(int (*f)(int), int start, int end)
{
	if(start == end) return f(start);
	printf("%d\n", f(start));
	return sum(g, start+1, end);
}

So that for each value of start it outputs its summation? Thanks for the help anyway

**tabstop** · 05-28-2010

Originally Posted by rob90

Do you mean that I just have to modify the sum function this way?

Code:

int sum(int (*f)(int), int start, int end)
{
	if(start == end) return f(start);
	printf("%d\n", f(start));
	return sum(g, start+1, end);
}

So that for each value of start it outputs its summation? Thanks for the help anyway

No, I mean you need to actually add it up.

**rob90** · 05-28-2010

Here:

Code:

int g(int n) 
{
	return n;
}
int sum(int (*f)(int), int start, int end)
{	
	if(start==end) return f(start);
	return f(start) + sum(f, start+1, end);
}

Am I right?

**tabstop** · 05-28-2010

That looks a lot more like adding.

**MK27** · 05-28-2010

Code:

int sum(int (*f)(int), int start, int end)
{	
	if(start==end) return f(start);
	return sum(f, start+1, end);
}

This will just return the original value of f(end), ie. it's meaningless. I presume you want to accumulate a value somehow (what does that big greek letter mean?*

), either by using "return start + sum(....)" or else via a static variable inside sum.

* seriously, in fact -- anyone know of anywhere where all those symbols are listed with their meaning?

**KBriggs** · 05-28-2010

* seriously, in fact -- anyone know of anywhere where all those symbols are listed with their meaning?

They are just letters, the meaning can differ with context.

**rob90** · 05-28-2010

I was just using Gauss formula in order to calculate the summation of end without actually adding each value using recursion, I understand it now, thanks tabstop for the help

**MK27** · 05-28-2010

Originally Posted by KBriggs

They are just letters, the meaning can differ with context.

Hmmmph. So I would actually have to study formal math or sumthin', is what yer saying.

**KBriggs** · 05-28-2010

Heh, not quite. There are some that have a commonly accepted meaning if there is a lack of context clues. Sigma usually means sum for example. But most symbols are just that, without any inherent meaning beyond what they are given in a particular situation.

**tabstop** · 05-28-2010

Originally Posted by MK27

Hmmmph. So I would actually have to study formal math or sumthin', is what yer saying.

There's always Greek letters used in mathematics, science, and engineering - Wikipedia, the free encyclopedia

**MK27** · 05-28-2010

Originally Posted by tabstop

There's always Greek letters used in mathematics, science, and engineering - Wikipedia, the free encyclopedia

Well I'll be -- thanks tabstop. So in this case the epsilon indicates a set with a definable order to it?

**KBriggs** · 05-28-2010

Originally Posted by MK27

Well I'll be -- thanks tabstop. So in this case the epsilon indicates a set?

Haha and looking at that list you see my point ^_^

I haven't seen even a tenth of those uses for any of those symbols in my life.

o in this case the epsilon indicates a set?

Epsilon? If you mean the symbol in the first post, it is Sigma, and it represents a sum from i = 1 to N of some function g(i) a far as I can make out, though the OP's use of the symbol is a little confusing, so I could be off the mark there.

**claudiu** · 05-28-2010

It can actually mean a sum from anything to anything, not even necessarily in discrete steps. All of these details are usually specified underneath and above the sigma.

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