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//
// Name: Multiply 2 big numbers
// Description:Multiply 2 big numbers wh
// ich normal data types cannot handle.
// By: Nguyen Ngoc Giang
//
// Inputs:2 integer numbers
//
// Returns:Product of 2 numbers, and the
// number of digits of this product.
//
// Side Effects:-Each input number shoul
// d have less than 500 digits (actually, i
// t can be more further, but for the purpo
// se of memory, I adjust at 500).
-The program employs the use of itoa function. Some of C++ compilers (e.g. ANSI C) may not support this function. It works well in Visual C++.
//
//This code is copyrighted and has// limited warranties
#include <iostream.h>
#include <string.h>
#include <ctype.h>
#include <stdlib.h>
/*--------------------------------------- Function prototypes -------------------------*/
void reverseArray(int*, int*, int);
void print(int*, int);
void product(char*, char*, int*&, int*);
void create(int*&);
void destroy(int*&);
/*--------------------------------------- Function definition -------------------------*/
void reverseArray(int* source, int* destination, int length)
{
int i;
for (i = 0; i < length; i++)
destination[length-i-1] = source[i];
}
void print(int* num, int length)
{
if (num[0] != 0)
cout << num[0];
for (int i = 1; i < length; i++)
cout << num[i];
}
void create(int*& num, int length)
{
num = new int[length];
}
void destroy(int*& num)
{
delete[] num;
}
void product(char s1[], char s2[], int*& actualResult, int* actualLength)
{
int *num1, *num2, lengthTempResult;
int *n1, *n2;
char temp[2];
int i, j, k;
int length1, length2;
int** tempResult;
int *result;
length1 = strlen(s1);
length2 = strlen(s2);
//store 2 numbers in strings of integers
create(n1, length1);
for (i = 0; i < length1; i++) {
strncpy(temp, s1 + i, 1);
n1[i] = atoi(temp);
}
create(n2, length2);
for (i = 0; i < length2; i++) {
strncpy(temp, s2 + i, 1);
n2[i] = atoi(temp);
}
create(num1, length1);
create(num2, length2);
reverseArray(n1, num1, length1);
reverseArray(n2, num2, length2);
destroy(n1);
destroy(n2);
lengthTempResult = length1 + length2;
//allocate memory for tempResult
tempResult = new int*[lengthTempResult];
for (i = 0; i < lengthTempResult; i++)
create(tempResult[i], length2);
//allocate memory for result
create(result, lengthTempResult);
//allocate memory for actualResult;
create(actualResult, lengthTempResult);
for (j = 0; j < length2; j++) {
int rem = 0;
//fill 0s in the tempResult
for (k = 0; k < j; k++)
tempResult[k][j] = 0;
//calculate in reverse order
for (i = 0; i < length1; i++) {
tempResult[i + j][j] = (num1[i] * num2[j] + rem) % 10;
rem = (num1[i] * num2[j] + rem) / 10;
}
//fill 0s in the last part of tempResult
tempResult[length1 + j][j] = rem;
for (k = length1 + j + 1; k < lengthTempResult; k++)
tempResult[k][j] = 0;
}
destroy(num1);
destroy(num2);
//initialize result[]
for (i = 0; i < lengthTempResult; i++)
result[i] = 0;
//calculate result[]
int rem = 0;
for (i = 0; i < lengthTempResult; i++) {
for (j = 0; j < length2; j++)
result[i] += tempResult[i][j];
result[i] += rem;
rem = result[i] / 10;
result[i] = result[i] % 10;
}
//deallocate memory for tempResult
for (i = 0; i < lengthTempResult; i++)
destroy(tempResult[i]);
delete[] tempResult;
reverseArray(result, actualResult, lengthTempResult);
destroy(result);
*actualLength = lengthTempResult;
}
void main(void)
{
char s1[500];
char s2[500];
int *result;
int length;
cout << "A program to calculate the product of 2 big numbers" << endl
<< " (each has less than 500 digits)" << endl << endl;
cout << "Enter the first number ";
cin >> s1;
cout << "Enter the second number ";
cin >> s2;
if (s1[0] == '0' || s2[0] == '0') {
cout << "Input error" << endl;
return;
}
product(s1, s2, result, &length);
cout << endl << "The product is ";
print(result, length);
cout << endl;
cout << "It has " << length - (result[0] == 0) << " digits" << endl;
destroy(result);
}
