Problem with loop in C

Question

i'm trying to compute "2^0 + 2^1 + 2^2 + ... + 2^14", using the following program(i'm a newbie and can only compute a exponent by multiply itself a certain times). The result should be 32767, but i ran it and got 270566475, i thought for long but can't figure out why...

#include <stdio.h>

int main(void)
{
   int i, e, exponent, sum;

   e = 1;
   exponent = 1;
   sum = 1;

   for (i = 1; i <=14; i++)
   {
       for (e = 1; e <= i; e++)
       {
          exponent *= 2;
       }

       sum += exponent;
   }

   printf("%d\n", sum);

   return 0;
}

So what's wrong with this??? Thanks!!!

Marcelo Cantos · Accepted Answer · 2011-04-30 14:11:27Z

7

You don't need the inner loop. Just execute exponent *= 2 once, directly inside the outer loop. BTW, I think you have to do it after the sum += ....

Also, you could start with sum = 0 and i = 0, which is closer to the math you described.

edited Apr 30, 2011 at 14:11

answered Apr 30, 2011 at 13:43

Marcelo Cantos

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1 Comment

asunnysunday Over a year ago

Yeah!~~~ After you told me this, i really don't know why i did it in that complex way, thankkkkks!

VoteyDisciple · Accepted Answer · 2011-04-30 13:43:45Z

2

Look at your inner loop by itself. It's trying to calculate, for one specific value of i, 2^i.

But exponent does not start at 1 every time. So you go into that loop with exponent already some very large value.

for (i = 1; i <=14; i++)
{
    exponent = 1;
    for (e = 1; e <= i; e++)
    {
        exponent *= 2;
    }

    sum += exponent;
}

Now you've reset exponent (which, to be clear, isn't the exponent at all but the calculated result) for each new power of 2.

answered Apr 30, 2011 at 13:43

VoteyDisciple

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2 Comments

Pushpendra Over a year ago

You don't have to use the inner loop as it can be done with the outer loop only, as answered above by Marcelo.....

asunnysunday Over a year ago

Oh, yes~~ initialization! Thank you very much!!

William A · Accepted Answer · 2011-04-30 14:05:13Z

2

If you have right to create a function it better to do it like this with a recursive function :

#include <stdio.h>

int power(int x, int exp) {
    if (exp == 0)
        return 1;
    else
        return x * power(x, exp-1);
}

int main (int argc, const char * argv[])
{
    int i;
    int sum = 0;
    for (i = 0; i <= 14; i++) {
        sum += power(2, i);
    }

    printf("%d",sum);
    return 0;
}

I hope it helps.

You just need one loop because each you already have the result of n-1 value. I had correct your code it works.

#include <stdio.h>

int main (int argc, const char * argv[])
{
    int i, e, exponent, sum;

    e = 1;
    exponent = 1;
    sum = 1;

    for (i = 1; i <= 14; i++)
    {
        exponent *= 2;
        sum += exponent;
    }

    printf("%d\n", sum);

    return 0;
}

Both codes work

edited Apr 30, 2011 at 14:05

answered Apr 30, 2011 at 13:59

William A

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5 Comments

William A Over a year ago

You can reuse this function to do for example 3^3, 4^4 any value you want (x^exp).

R.. GitHub STOP HELPING ICE Over a year ago

Don't recommend recursion where iteration is called for. (And likewise don't make complex solutions to simple errors.)

asunnysunday Over a year ago

Oh, thank you! but for me stuff like power, argc are symbols that are more mysterious than Latin now, hehe~~ thanks a lot!

William A Over a year ago

Your are welcome. The argc is automatically generated by the software.

William A Over a year ago

I didn't saw the comment before. For some people like recursion is faster and easier to set in some cases. And I know it takes place in the memory but for a small operation like this it's nothing.

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