Recursive Programming -- Power of function

I am using Stephen Prata's C Primer Plus (5th Edition) to teach myself C Programming. I am not a student (I wish), and am seeing myself stuck!

(bear with me, this is my first post so I hope I get the formatting right)

I have this code:

Code:

---

#include <stdio.h>

double power(double a, int b);  /* ANSI prototype */

int main(void)
{
  double x, xpow;
  int n;

  printf("Enter a number and the integer power");
  printf(" to which\nthe number will be raised. Enter q");
  printf(" to quit.\n");
  while (scanf("%lf%d", &x, &n) == 2)
  {
      xpow = power(x,n);      /* function call          */
      printf("%.3g to the power %d is %.5g\n", x, n, xpow);
      printf("Enter next pair of numbers or q to quit.\n");
  }
  printf("Hope you enjoyed this power trip -- bye!\n");
  return 0;
}

// test runs:
// 1 & 1 == 1
// 5 & 5 == 3125
// 6 & 7 == 2.7994e+005

double power(double a, int b) 
{
  double pow = 1.0;
  int i;
 
        if ((b == 0) && (a == 0)) pow = 1.0;
        else if (a == 0) pow = 0.0;
        else if (b > 0)
      for(i = 1; i <= b; i++)
      pow *= a;
  else    /* b < 0 */
      pow = 1.0 / power(a, - b);
  return pow; 
}

---

I am learning Recursion and the books suggest transforming the above using a recursive approach; I just cant get my head around it. any suggestions would be highly appreciated.

You should concentrate on the loop

Code:

---

for(i = 1; i <= b; i++)
      pow *= a;

---

on each iteration it multiplys the old value to a


instead of the loop you can call power recursively to calculate the previous value, multiply it with a and return result

would you mind showing me in code?

OOPS -- noticed I posted this in C++ and not the C section. sorry

First, format this so that me/you/we know what we're looking at. That, and get your original in better shape before going for a recursive.

Code:

---

/*Decide between tabs/spaces, and stick to it.*/
double power(double a, int b) 
{
        double pow = 1.0;
        int i;

/* This if() has no effect, pow is already 1.0...*/
        if((b == 0) && (a == 0)) pow = 1.0;
/* This if() is partially incorrect. 0^0 != 0, but it's a special case.*/
        else if (a == 0) pow = 0.0;
        else if (b > 0)
        {
                for(i = 1; i <= b; i++)
                        pow *= a;
        }
/* The comment on the else is wrong. b may == 0 here - if it does, this will stack fault. */
        else /* b < 0 */
                pow = 1.0 / power(a, - b);
        return pow; 
}

---

Quote:

---

Originally Posted by Cactus_Hugger

First, format this so that me/you/we know what we're looking at. That, and get your original in better shape before going for a recursive.

Code:

---

/*Decide between tabs/spaces, and stick to it.*/
double power(double a, int b) 
{
        double pow = 1.0;
        int i;

/* This if() has no effect, pow is already 1.0...*/
        if((b == 0) && (a == 0)) pow = 1.0;
/* This if() is partially incorrect. 0^0 != 0, but it's a special case.*/
        else if (a == 0) pow = 0.0;
        else if (b > 0)
        {
                for(i = 1; i <= b; i++)
                        pow *= a;
        }
/* The comment on the else is wrong. b may == 0 here - if it does, this will stack fault. */
        else /* b < 0 */
                pow = 1.0 / power(a, - b);
        return pow; 
}

---

---

Technically

Code:

---

else if (b > 0)
        {

---

should be

Code:

---

else if (b > 0) {

---

This subtle point in style only matters if you are doing coding for a major corporation. But whatever. I would have written the function like:

Code:

---

double power( double x, int n )
      {
      if ( n > 0 )
        return x * power( x, n- 1 );
      else if ( n < 0 )
        return ( 1/x ) * power( x, n + 1 );
      else
        return 1;
      }

---

Excellent feedback -- and so fast! I am impressed; Also highly impressed about the depth of knowledge and help! :)

Quote:

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Originally Posted by Overworked_PhD

Technically

Code:

---

else if (b > 0)
        {

---

should be

Code:

---

else if (b > 0) {

---

This subtle point in style only matters if you are doing coding for a major corporation.

---

What in the world are you talking about?

Quote:

---

Originally Posted by Overworked_PhD

Technically

Code:

---

else if (b > 0)
        {

---

should be

Code:

---

else if (b > 0) {

---

This subtle point in style only matters if you are doing coding for a major corporation.

---

(To clarify) Technically, both are correct. The compiler won't care two bits which is used. The only thing to care is a human. Religious wars have been started over which brace style is the best. Looking through some of the code I have, it seems that some of the SDL sub libraries are written using yours. It also appears that libpng and wxWindows are written in mine. (In the small samples of code that I checked, at least.) Neither is particularly predominant, and is the choice of the project developers/leaders. I perfer the former as it aligns the braces, letting me easily match braces, and spaces things out vertically a bit more. See One True Brace Style on wikipedia.

Quote:

---

Originally Posted by Cactus_Hugger

(To clarify) Technically, both are correct. The compiler won't care two bits which is used. The only thing to care is a human. Religious wars have been started over which brace style is the best. Looking through some of the code I have, it seems that some of the SDL sub libraries are written using yours. It also appears that libpng and wxWindows are written in mine. (In the small samples of code that I checked, at least.) Neither is particularly predominant, and is the choice of the project developers/leaders. I perfer the former as it aligns the braces, letting me easily match braces, and spaces things out vertically a bit more. See One True Brace Style on wikipedia.

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Wrong. The style I mentioned is PREDOMINANT in BSD derived code. The other PREDOMINANT form is having the braces indented to mimic LISP. This style appears in GNU software.