CODEWITHSUNDEEP
c
Sandra
Sandra

Reputation: 77

Can help me with my C code?

I'm supposed to make a program which allows me to do this calculation: -5+10-20+40-80+160

I've done this so far:

const int START = -5, LIMIT = 160;

int somme = 0;  
int terme = START;

do {
    somme += terme;

    terme = terme * 2;

    terme = -terme;

} while(terme <= LIMIT);

printf ("equals %d\n\n", somme);

But when I run it it shows -215 and of course it's not the correct answer. I'd really appreciate your help.

Upvotes: 0

Views: 108

Answers (3)

ssm
ssm

Reputation: 5373

For a calculation like this, you should look at the expression more carefully. What you want is:

-5+10-20+40-80+160

= 5*(- 1 + 2 - 4 + 8 - 16 + 32)

= 5*( (-1)^1*(2^0) + (-1)^1*(2^1) + (-1)^1*(2^2) + (-1)^1*(2^3) + (-1)^1*(2^4) + (-1)^1*(2^5) )

where in C terms, a^b is equivalent to pow(a,b)

= 5 * sum over i ((-1)^(i+1) * 2^i ) where i goes from 0 to 5

Do you think that it would be easier to iterate over a variable i in a for loop? Ill leave this as an exercise.

Upvotes: 1

Matan Perry
Matan Perry

Reputation: 67

another cool way:

const int START = -5, LIMIT = 160;

int somme = 0;  
for(int terme = START; (terme<LIMIT && (-terme)<LIMIT)||(printf("equals //
                      %d\n\n",somme),0); terme = -(terme*2)){
somme += terme;
}

Upvotes: 0

VolAnd
VolAnd

Reputation: 6407

You should use absulute value of terme in condition of your loop, that better have to be PRE-CONDITION while:

#include <stdio.h>

#define ABS(X) (X>=0)?(X):(-X)

int main()
{
    const int START = -5, LIMIT = 160;

    int somme = 0;  
    int terme = START;

    while( ABS(terme) <= LIMIT )
    {
        somme += terme;

        terme = terme * 2;

        terme = -terme;
    } 
    printf ("equals %d\n\n", somme);
}

Upvotes: 1

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