• C

# multiply and divide by adding and subtracting in C programing language

how can i build a program in C programing language, that  multiplies two user given numbers , by only adding numbers , and divides, by only subtracting the same numbers?
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Commented:
>> how can i build a program in C programing language, that  multiplies two user given numbers , by only adding numbers

For x times y, can you use a loop, and repeatedly add the same number y to itself x amount fo times ?

>> and divides, by only subtracting the same numbers?

Similarly for division, except that in this case, you need to use division just to find the value you need to subtract, so what you're asking is a bit weird.
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Author Commented:
that much, i had imagined. i would like some help with an example of C code. but thanks anyway.
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Commented:
int mult(int a, int b){
int p=0;
unsigned u = (unsigned)a;
while( u ){
if( u&1 ){ p+=b; }
b<<=1;
u>>=1;
}
return p;
}
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Commented:
Actually, it is more interesting than that.  Assuming no multiply or divide at all...

Starting with the user-supplied number (presumably in decimal) - how to convert this to binary without using div or mult?

Better to leave it in decimal and simulate long division and long multiiplication using repeated subtraction and addition.   This is much more efficient than brute-force repeated subtraction and addition (in binary).  You do have to be smart about carrying and borrowing digits.

I've written extended precision decimal arithmetic functions using this technique.  Nice thing is that you can do any number of digits, just like doing arithmetic by hand.
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Commented:
``````#include<iostream>
02	using namespace std;
03	int main()
04	{
05	    int a, b, result, i;
06
07
08	    cout<<"Enter first number:"<<endl;
09	    cin>>a;
10	    cout<<"Enter second number:"<<endl;
11	    cin>>b;
12
13	    result = 0;
14	    i = 0;
15
16	    do { result+=a; i++; } while(i<b);
17
18
19	    cout << "Your result is: " << result <<endl;
20	    system("PAUSE");
21	}
``````
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Commented:
Solution above does not work properly for b == 0.

Use a simple "for" loop.

99 percent of the time, do/while loops are the wrong thing to use.
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Commented:
division using subtraction:-

``````#include <iostream.h>
int main()
{
int x;
int y;
int rem;
int quotient = 0;

cout<<"input first number:";
cin>>x;
cout<<"input second number:";
cin>>y;

for(int i=0; i<y; i--)
{
quotient -=x;
}

cout<<"Quotient is:"<<quotient<<"Remainder is:"<<rem;
return 0;
}
``````
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Commented:
Still just scratching the surface.

Solution above for division does not handle negative x and y.

All these solutions assume integer numbers (no decimal point).  Boring!

And the division could be expected to generate some digits to the right of the decimal point rather than a simple remainder.

Is this someone's homework assignment we're doing?
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Author Commented:
its not a homework of some one , its a exercise i was trying to resolve, and cant seem to put it to work while learning C programing language.
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Commented:
Hi. If you would like to show us your code, we'll help you figure out why it isn't working.  There are so many ways to solve this problem.
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Author Commented:
thank you for all your answers, it gave me plenty information which allowed me to figure out what was wrong and fix it.
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