C programming help for beginning C class?

Question

I'm having trouble with the equations needed. we have learned the loops so far and i think this problem wants me to use the loops, and it looks like i need if-else as well. Here is the problem.

Write a program that asks the user to supply three positive integers k, m and n, with k being greater than 1. the program should compute the sum of all the integers between m and n that are divisible by k.

I don't know how to tell C that that i only want an integer evenly divisible by another. and then, i'm not quite sure how to get the addition of those integers done.

doh! forgot about modulus, and thanks for giving me another example of how to utilize the remainder. I didn't quite see why I would need it before.

Answer 1

int sum = 0;
for (int i=m; i<=n; i++)
if ((i % k) == 0) sum += i;

Use the "modulus" operator ("%"). It's the remainder of the first number, when divided by the second. When this is zero, the first number evenly divides by the second one.

So, for example,
5 % 3 = 2,
6 % 3 = 0,
7 % 3 = 1

And if you don't want to include m or n in the list, change the loop from:
for (int i=m; i<=n; i++)
... to:
for (int i=m+1; i
And make sure m is the smaller of the two numbers. You can swap them if not.

=====================
As an alternative, you don't have to check every single number. You could write:
for (int i=(n-(n%k)); i<=m; i+=k)
if (i >=n) sum += i;

... which starts i off as the biggest multiple of k less than or equal to n, and increments i by k in each loop iteration.

You can get rid of the if statement if you initialize i a little better:
for (int i=((n-1+k)-((n-1)%k)); i<=m; i+=k) sum += i;

Answer 2

You must ensure that the loop terminates (so that it doesn't last forever). This is how:

#include

void main()
{
int m,n,k , i ,sum = 0;

cout << "Enter k: ";
cin >> k;
if( k <= 1)
cout << "k must be greater than 1";
else
{
cout << "Enter m: ";
cin >> m;
cout << "Enter n: ";
cin >> n;
if(m > n)
{
m = m + n;
n = m - n;
m = m - n;
}
for(i=m ; i< n; i++)
{
if(i % k == 0)
sum +=i;
}
cout << "Sum is " << sum;
}
}

You have to check if m>n otherwise the loop

if(i = m; i< n; i++)

will never terminate.
We do this by swapping m and n if m>n
This is the classic way of swapping 2 numbers with only 2 variables:

m = m + n;
n = m - n;
m = m - n;

This is a complete program - compile and run this to see for yourself.

Answer 3

if (i % k)
{ i is evenly divided by k }
else
{ there is a remainder after the divide }