Thread: New to programming, help with recursive functions.

Just started programming in C and I'm using SAMS Teach Yourself C in 21 Days (I'm not holding them do that time frame though!)

Anyway, chapter 5 is all about functions which are fine and I understand. However, they briefly mention recursive functions. Now I understand the theory, the function calling itself until a condition is reached but I just can't follow in me head their example. I'm nearly there but . . . ah!

This is their code from an exercise question, can someone help me understand why the function doesn't return the value of 1 in every case?

Code:

#include <stdio.h>

int three_powered( int power );

main()
{

int a = 4;
int b = 9;

printf("\n3 to the power of %d is %d", a, three_powered(a));
printf("\n3 to the power of %d is %d\n", b, three_powered(b));

}

int three_powered( int power )
{

  if ( power < 1 )
    return ( 1 );
  else
    return( 3 * three_powered( power - 1 ));
	
}

Well, look at the function and look at the parameter passed. If the passed parameter was 1, then it would return 1 immediately and quit. Let's say the parameter is 3, so the first part of the "if statement" is ignored and the "else" is executed. Then the next time, the function is "called" with a parameter of 2, and so on until the parameter is 1, and then the execution will stop and the value from the "recursion" is returned.

Clear?

Originally Posted by HitchHiker

Just started programming in C and I'm using SAMS Teach Yourself C in 21 Days (I'm not holding them do that time frame though!)

Anyway, chapter 5 is all about functions which are fine and I understand. However, they briefly mention recursive functions. Now I understand the theory, the function calling itself until a condition is reached but I just can't follow in me head their example. I'm nearly there but . . . ah!

This is their code from an exercise question, can someone help me understand why the function doesn't return the value of 1 in every case?

Code:

#include <stdio.h>

int three_powered( int power );

main()
{

int a = 4;
int b = 9;

printf("\n3 to the power of %d is %d", a, three_powered(a));
printf("\n3 to the power of %d is %d\n", b, three_powered(b));

}

int three_powered( int power )
{

  if ( power < 1 )
    return ( 1 );
  else
    return( 3 * three_powered( power - 1 ));
	
}

when you call three_powered(4):
power<1? NO -- > return 3*three_powered(3);
three_powered(3) --> return 3*three_powered(2);
three_powered(2)--> return 3*three_powered(1);
three_powered(1)-->return 3* three_powered(0);
finally: three_powered(0) since 0<1 returns 1;

NOW YOU GO BACKWARD:
three_powered(0)-->returns 1;
three_powered(1)-->rreturn 3*three_powered(0)=3*1=3
three_powered(2)--> return 3*three_powered(1)= 3*3=9
three_powered(3)-->return 3*three_powered(2)=3*9=27
three_powered(4)-->return 3*three_powered(3)=3*27=81

when you'll understand how the functions are called using the stack it will be easier to understand recursion.. bye!

when you call three_powered(4):
power<1? NO -- > return 3*three_powered(3);
three_powered(3) --> return 3*three_powered(2);
three_powered(2)--> return 3*three_powered(1);
three_powered(1)-->return 3* three_powered(0);
finally: three_powered(0) since 0<1 returns 1;

This is as far as I got. Obviously it returns 1 at the end, couldn't work out how it could return any other value. I think I understand now, seems the book has forgotten to tell me about the stack. Maybe it will go into it later and things will click more.

Thanks.

Mea Culpa --- old age is ruining my eyesight (and my ability to reason logically).