Long Hand Multplication In C++

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I wrote this code for simulating multiplication of 4 bit binary numbers stored in arrays BeforeDecimal1 & BeforeDecimal2. The result of multiplication is stored in arrays s[]. The problem is this code isn't providing correct answer. I tried to figure out the problem but worthless, so now I seek the advice of experts.Here is the code for manipulating Long Hand Multiplication:

I am trying to do this:

VB

1000

x

1110
-------------------

   0000

  1000

 1000

1000
-------------
= 1110000

C++

int i=3,x=0,carry=0;
while(true)
{
    if(BeforeDecimal2[i]!=0)
    {
      for(int j=x;j>=0;j--)
      {
        if(s[j]==1 && BeforeDecimal1[j]==1 && carry==0)
        {
          carry=1;
          s[j]=0;
        }
        else if(s[j]==1 && BeforeDecimal1[j]==0 && carry==1)
        {
          carry=1;
          s[j]=0;
        }
        else if(s[j]==0 && BeforeDecimal1[j]==0 && carry==1)
        {
          carry=0;
          s[j]=1;
        }
        else if(s[j]==0 && BeforeDecimal1[j]==0 && carry==0)
        {
          carry=0;
          s[j]=0;
        }

        x++;
      }
    }
    else
    {
      for(int h=7;h>=0;h--)
      {
        if(h==0)
        {
          BeforeDecimal1[0]=0; // that is inserting zeros from the right
        }
        else
        {
          BeforeDecimal1[h]=BeforeDecimal1[h-1];
          BeforeDecimal1[h-1]=0;
        }
      }
    }
    if(i==0)
      break;

    i--;
  }

Regards

Indexation and else if in the same line

Posted 1-Dec-12 21:36pm

Alfred16

Updated 2-Dec-12 8:04am

Nelek

v3

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Comments

Philippe Mori 2-Dec-12 8:36am

First of all, it look like you are running your loop in the wrong direction. You should start your multiplication with least significant bits.

Then your if does handles all cases. With 3 bits, you have 8 cases to handle. And it whould be much easier if you would check if you have 2 or more 1 bit for the carry and do an XOR for the other bit.

Nelek 2-Dec-12 13:57pm

Please accept a tip.
When posting code that goes so deep in the indexation. You should "search and replace" tabulators by two or three "empty spaces". It will increase the readability a lot, making it easier for other users to solve your problem/question.
Another point, get used to always use the brackets, specially if you want to make such nested constructions. You can mess things up very easy if you don't, because you loose the scope of what is being executed and what not.
And please, stop using:
else
if

and use the correct
else if

I have edited the code. Please have a look now, to see the points I mean.

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OriginalGriff · Answer 1 · 2012-12-01T21:57:00

I'm sorry to say this, but...that is pretty much unreadable.

I'm not sure what algorithm you are using, but I doubt if it included quite so many nested if's.
I think the first thing you need to do is look at revising the indentation of that lot, because it is seriously confusing at the moment.
Try starting by changing:

C++

if(reverse==4 && carry==1 && temp==0)
{
    s[reverse]=1;
}
else
    if(reverse==4 && carry==0 && temp==1)
    {
        s[reverse]=1;
    }
    else
        if(reverse==4 && carry==0 && temp==0)

To

C++

if(reverse==4 && carry==1 && temp==0)
{
    s[reverse]=1;
}
else if(reverse==4 && carry==0 && temp==1)
{
    s[reverse]=1;
}
else if(reverse==4 && carry==0 && temp==0)

So that it is clearer without running off the right hand side of the page.
Then look at commenting - I have absolutely no idea why you are doing the tests you are, and without any idea of the algorithm you are trying to implement I can't work it out from that code.

Sorry, but this need to be made readable, and explained a bit better before I can help!

CPallini · Answer 2 · 2012-12-02T10:55:00

Bit long, but should work:

C

#include <stdio.h>

void mul(int bd1[4], int bd2[4], int s[8])
{
  int i,j;

  for (i=0;i<8;i++)
    s[i] = 0;

  for (i=0; i<4; i++)
  {
    int c = 0;
    if ( bd2[3-i] )
    {
      for (j=0; j<4; j++)
      {
        if ( s[i+3-j] == 1)
        {
          if (bd1[3-j] == 1)
          {
            if (c==1)
            {
              s[i+3-j] = 1;
              c = 1;
            }
            else
            { //c == 0
              s[i+3-j] = 0;
              c = 1;
            }
          }
          else
          { // bd1[3-j] == 0
            if (c==1)
            {
              s[i+3-j] = 0;
              c = 1;
            }
            else
            { //c == 0
              s[i+3-j] = 1;
              c = 0;
            }
          }
        }
        else
        {// s[i+3-j] == 0
          if (bd1[3-j] == 1)
          {
            if (c==1)
            {
              s[i+3-j] = 0;
              c = 1;
            }
            else
            { //c == 0
              s[i+3-j] = 1;
              c = 0;
            }
          }
          else
          { // bd1[3-j] == 0
            if (c==1)
            {
              s[i+3-j] = 1;
              c = 0;
            }
            else
            { //c == 0
              s[i+3-j] = 0;
              c = 0;
            }
          }
        }
      }
    }
  }
}

int main()
{
    int i;
  int bd1[]={1,0,0,0};
  int bd2[]={1,1,1,0};
  int s[8];
  mul(bd1,bd2,s);


  for(i=0;i<8;i++)
    printf("%d", s[i]);
  printf("\n");
  return 0;
}

Long Hand Multplication In C++

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