Thread: Sum help

Write a program that for the integers from 1 to 100, inclusive:

1) determines the sum
2) counts how many of the integers from the end of the summation it takes to reach at least half of the total sum. That is, when calculating 100 + 99 + 98 + ... + n = total_sum/2, how many integers does it take to reach n?



I have this half way done.

Output I am getting:

The sum from 1 to 100 is 5050
Half the sum is 0

Code:

#include <stdio.h>

int main()
{
        int i;
        int sum;

        for( i = 1; i <= 100; i++)
        {
                sum += i;
        }
        printf("The sum from 1 to 100 is %d\n", sum);

        for (i = 1; i <= 100; i++)
        {
                sum /= i;
        }
        printf("Half the sum is %d\n", sum);
}

How do I divide the sum by 2 to get 2525? I'm having trouble with it.

Thanks for your help.

"/ 2"?

Why are you dividing for the second part? All that's happening is you're dividing more and more (making the number smaller and smaller) and C just truncates it to 0 by the end of it all.

So I should change

int sum;

to

int sum = 0;?

That would be one of your (potential) issues, yes. The other issue is previously mentioned: sum /= 2 might be okay, but then you went ahead and put it in a for loop. It will continuously divide the answer by i up the loop ends. In your case, that's 100 times. Say your sum is 500. You are dividing that number by !100, which is a number with 157 places. 500 divided by !100, as far as C is concerned, is 0.

Making some progress

My Output now:

The sum from 1 to 100 is 5050
Half the sum is 50

Code:

#include <stdio.h>

int main()
{
        int i;
        int sum = 0;

        for( i = 1; i <= 100; i++)
        {
                sum += i;
        }
        printf("The sum from 1 to 100 is %d\n", sum);

         sum /= i;

        printf("Half the sum is %d\n", sum, i);
}

I want it to say "Half the sum is 2525"

Got it

Output:

The sum from 1 to 100 is 5050
Half the sum is 2525

Code:

#include <stdio.h>

int main()
{
        int i;
        int sum = 0;
        int x = 2;

        for( i = 1; i <= 100; i++)
        {
                sum += i;
        }
        printf("The sum from 1 to 100 is %d\n", sum);

         sum /= x;

        printf("Half the sum is %d\n", sum );
}

Now I am looking output to something like this

The sum of the last 30 integers in the
range 1 to 100 is 2565.

Would I use a for loop?

I would use a for loop. Just have it start at 100 and end at 70.

You don't need to create a new variable to do the division.

sum /= 2

Also, you could just have the counter just start counting for numbers >= 70.

if(i >= 70) sum += i

Or just another for loop starting at 70 like suggested.

Doesn't work but am I on the right track?

Code:

#include <stdio.h>

int main()
{
        int i;
        int sum = 0;

        for( i = 1; i <= 100; i++)
        {
                sum += i;
        }
        printf("The sum from 1 to 100 is %d\n", sum);

                 sum /= 2;

        printf("Half the sum is %d\n", sum );

        for( i = 70; i <= 100; i++)
        {
                sum += i;
        }
        printf("The sum of the last 30 integers in the
range 1 to 100 is %d\n", sum);
}

How do I change the sum?

I am trying to find the sum of the last 30 integers.

So the sum from 70 to 100 = ?