用C语言实现动态数组的大整数乘法

描述：

要求你用C语言实现两个数相乘，这两个数字的位数可能很大，而且还可能有不合法的输入。

具体做法：

首先，大整数乘法的主要部分要知道：

for (i = 0; i < lenA; i++)

for (j = 0; j < lenB; j++)

ans[i + j] += (int)(pA[i] - '0') * (int)(pB[j] - '0');

其次就是处理用户的输入了，这里我们给每个数字预分配1024个字节的空间，对于用户的输入我们采取逐个字符读入，直到碰到空格或者换行符。如果用户输入的字符多于1024，那么我们再调用

realloc

这个函数再次分配更多的空间。最后再扫一遍字符串验证字符串的合法性。大整数乘法就不多说了，先把两个数字反转过来，然后用上述代码计算出的

ans

数组，最后处理

ans

数组的进位和前导0。

代码：

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <malloc.h>
#include <ctype.h>

#define MAX_LINE 1024

typedef char *string;

typedef enum {false, true} bool;

void swap_char(char *pa, char *pb)
{
char tmp = *pa;
*pa = *pb;
*pb = tmp;
}

int main()
{
// freopen("1.txt", "r", stdin);

int i, j;
int n, size;
char c;
string a = NULL, b = NULL;

int *ans = NULL, lenAns = 0;

while (true) {

int la = 0, lb = 0;

bool isOk;
bool isFa, isFb;
string pA = NULL, pB = NULL;
int lenA, lenB;
int up;

size = MAX_LINE;
if ((a = (string)malloc((size) * sizeof(char))) == NULL)
exit(1);
if ((b = (string)malloc((size) * sizeof(char))) == NULL) {
free(a);
a = NULL;
exit(1);
}

while (!((c = getchar()) != ' ' && c != '\n'));

n = 0;
a[n++] = c;
while ((c = getchar()) != ' ' && c != '\n') {
a[n++] = c;
if (n >= size) {
if ((a = (string)realloc(a, (size * 2) * sizeof(char))) == NULL) {
free(a);
free(b);
a = b = NULL;
exit(1);
}
size  = _msize(a);
}
}
a
= 0;
la = n;
n = 0;

while (!((c = getchar()) !=
e14e
' ' && c != '\n'));

b[n++] = c;

size = MAX_LINE;
while ((c = getchar()) != ' ' && c != '\n') {
b[n++] = c;
if (n >= size)
if ((b = (string)realloc(b, (size * 2) * sizeof(char))) == NULL) {
free(a);
free(b);
a = b = NULL;
exit(1);
}
size  = _msize(b);
}
b
= 0;
lb = n;

isOk = true;

isFa = a[0] == '-' ? true : false;
isFb = b[0] == '-' ? true : false;

pA = isFa ? a + 1 : a;
pB = isFb ? b + 1 : b;

lenA = isFa ? la - 1 : la;
lenB = isFb ? lb - 1 : lb;

pA = pA[0] == '+' ? pA + 1 : pA;
lenA = pA[0] == '+' ? lenA - 1 : lenA;
pB = pB[0] == '+' ? pB + 1 : pB;
lenB = pB[0] == '+' ? lenB - 1 : lenB;

for (i = 0; i < lenA; i++)
if (!isdigit(pA[i])) {
puts("Please input again!");
isOk = false;
break;
}
if (!isOk) {
free(a);
free(b);
a = b = NULL;
continue;
}
for (i = 0; i < lenB; i++)
if (!isdigit(pB[i])) {
puts("Please input again!");
isOk = false;
break;
}
if (!isOk) {
free(a);
free(b);
a = b = NULL;
continue;
}
for (i = 0; i < lenA / 2; i++)
swap_char(pA + i, pA + lenA - i - 1);
for (i = 0; i < lenB / 2; i++)
swap_char(pB + i, pB + lenB - i - 1);

if ((ans = (int *)malloc((lenA + lenB + 10) * sizeof(int))) == NULL) {
free(a);
free(b);
a = b = NULL;
exit(1);
}
for (i = 0; i < lenA + lenB + 10; i++)
ans[i] = 0;

for (i = 0; i < lenA; i++)
for (j = 0; j < lenB; j++)
ans[i + j] += (int)(pA[i] - '0') * (int)(pB[j] - '0');

up = 0;
lenAns = lenA + lenB;
for (i = 0; i < lenAns; i++) {
int tmp = ans[i];
ans[i] = (ans[i] + up) % 10;
up = (tmp + up) / 10;
}
for (; lenAns > 0 && ans[lenAns - 1] == 0; lenAns--);

if (lenAns == 0) {
puts("0");
continue;
}
for (; up != 0; lenAns++)
ans[lenAns] = up % 10, up /= 10;
if (!((isFa && isFb) || (!isFa && !isFb)))
putchar('-');
for (i = lenAns - 1; i >= 0; i--)
printf("%d", ans[i]);
putchar('\n');

free(a);
free(b);
free(ans);
a = b = NULL;
ans = NULL;
}
return 0;
}