Reading a polynomial...

[sam...]

Help please! I'm writing the main part of a program that utilizes several functions (in various files); I think I've got the functions (like poly.c that's mentioned here) but I'm confused about this.

I don't know if my question makes any sense...

I am having trouble with the part about inputting the coefficients.
I use "printf" to ask the user to input the coefficients; but how many coefficients am I asking for? Also, do I use scanf to detect the coefficients? How can I use scanf if I don't know the number of coefficients that have to be inputted?)


Write a program that:

4.a) Reads a polynomial (p) from the keyboard using scanf. You should
prompt the user to enter the degree of the polynomial (d). If d < 0
you should exit the program. If d >= 0 prompt the user to input all
coefficients from 0 to d. Input here is from the terminal keyboard,
not a file!

4.b) Exit with an error message if the coefficient of the monomial
with the highest degree is zero.

4.c) Using the functions you wrote in poly.c:

4.c.i) Compute the first (p') and second (p'') derivatives of p;

4.c.ii) Print the polynomials p, p' and p'';

4.c.iii) Compute a zero (y) of p using Newton's method starting at x = 1;

4.c.iv) If a zero of p has been found, print the zero (y) and the value
of p(y);
If a zero has not been found issue a message;
HINT: Use the function isnan() to check for a NAN!

4.c.v) Compute a zero (z) of p' using Newton's method starting
at x = 1;

4.c.vi) If a zero of p' has been found, print the zero (z) and the
value of p'(z); If a zero has not been found issue a message;
HINT: Use the function isnan() to check for a NAN!

4.c.vii) Evaluate p''(z) and inform the user if z is a point of
maximum, minimum or inflection. As we are computing
approximate zeros, you should compare p''(z) not with zero
but a small number, say 1E-4.

4.c.viii) Destroy all polynomials you created during the execution of
your program and print the value of poly_memory() to ensure
it is zero.

Thank you!

BY THE WAY, here's what I have (it's pretty basic b/c I'm not sure what the prompt is asking me to do):

#include <stdio.h>

main()
{
	int p, p', p'', d;
	
	printf("Enter degree (d) of polynomial:\n");
	scanf("%d", &d);
	if (d>=0)
	{
		printf("Input all coefficients from 0 to d:\n");
		int i;
		for (i = 0; i < d; i++)
		{
			scanf(
		}

		/* code */
	}else
	{
		if (d<0)
{
	printf("ERROR! d<0\n");
	return 0;
}
	}

[nimitzhunter]

You can use dynamic array to get data from scanf:

#include <stdio.h>
#include <stdlib.h>
int main()
{
  int d;
  double *p;  
  printf("Enter degree (d) of polynomial:\n");
  scanf("%d", &d);
  if (d>=0)
    {
      p = malloc((d+1)*sizeof(double));
      printf("Input all coefficients from 0 to d:\n");
      int i;
      for (i = 0; i < d; i++)
	{
	  scanf("%lf",&p[i]);
	}
      
      /* code */
      
    }
  free(p);
  return 0;
}

[sam...]

Thank you so much!
I think this should work.

What is free(p)?
I've never used it before..

[nimitzhunter]

Thank you so much!
I think this should work.

What is free(p)?
I've never used it before..

p is a dynamic array. You allocate memory with "malloc"

p = malloc((d+1)*sizeof(double));

After you finish using "p", you use "free" to free up the allocated memory.

[anduril462]

Quick note, you will probably need <= d, not < d, since you malloc'ed d+1 elements (having indexes from 0..d).

for (i = 0; i <= d; i++)

[nimitzhunter]

By the way, don't name your variable: p', p''. it's not valid name for C.

[sam...]

Thank you!! That makes this so much more clear!

I was wondering: what is the command to open a library file?
Our instructor provided us with access to one (for the functions) but i don't know how to access it.

[nimitzhunter]

Thank you!! That makes this so much more clear!

I was wondering: what is the command to open a library file?
Our instructor provided us with access to one (for the functions) but i don't know how to access it.

What library and functions are you talking about?

[sam...]

It's a file (libpoly.a) that was provided by him that I was instructed to copy into my own directory.
Here are his statements regarding this:

libpoly.a is a library file that contains compiled versions of all
functions you will develop in poly.c already compiled and bundled
in a library file by your instructor. You can use it to help you
develop your code.

To compile my program (final.c) with libpoly.a, I'm supposed to use
gcc -Wall final.c -L. -lpoly

[Bayint Naung]

Btw, the code nimitzhunter posted has a bug if d < 0.
Your instructor already told you what's the command to link with his library libpoly.a
Maybe there should also be a header file associated with the library. say poly.h?

[nimitzhunter]

Btw, the code nimitzhunter posted has a bug if d < 0.
Your instructor already told you what's the command to link with his library libpoly.a
Maybe there should also be a header file associated with the library. say poly.h?

That's part of his hw, he should be able to come up with the code for that.
Sam, if there is the header file you could use "#include" to incorporate it with your code:

#include "poly.h"

BY THE WAY, here's what I have (it's pretty basic b/c I'm not sure what the prompt is asking me to do)

The purpose of this code is just to find the relative max/min values of the polynomial you input. So, p is a poly that you have. Solving for y such that p'(y)=0 gives you the location of either a min or a max(you don't know which). Then you use p'' to determine whether y is min or max ( check wikipedia for the condition).