为什么简单的乘法会导致一些乱码？

Question

我尝试设计一个程序来实现两个大整数之间的乘法（使用 C++）。但是在我完成之后，我发现如果我通过命令参数输入两个整数，结果有时会很奇怪，有时会正确。请帮我找出原因并告诉我如何解决它。谢谢（实现两个大整数相乘功能的主要函数是mul()）。

#include <iostream>
#include <cstring>
#include <stdlib.h>
using namespace std;

void mul(char *c1, char *c2, int len1, int len2);

void printArr(char *c1, char *c2, int len1, int len2);

int main(int argc, char **argv)
{
    if (argv[1] != NULL)
    {
        char cArr1[500], cArr2[500];
        for (int i = 0; i < strlen(argv[1]); i++)
        {
            cArr1[i] = argv[1][i];
        }
        for (int i = 0; i < strlen(argv[2]); i++)
        {
            cArr2[i] = argv[2][i];
        }
        int len1 = strlen(cArr1);
        int len2 = strlen(cArr2);
        printArr(cArr1, cArr2, len1, len2);
        (len1 > len2) ? mul(cArr1, cArr2, len1, len2) : mul(cArr2, cArr1, len2, len1);
        exit(100);
    }
    while (true)
    {
        cout << "Please input two integers" << endl;
        char cArr1[500], cArr2[500];
        cin >> cArr1;
        if (cArr1[0] == 'q' && cArr1[1] == 'u' && cArr1[2] == 'i' && cArr1[3] == 't')
        {
            exit(1000);
        }
        cin >> cArr2;
        int parity = 0;
        int len1 = strlen(cArr1);
        int len2 = strlen(cArr2);
        printArr(cArr1, cArr2, len1, len2);
        if (cArr1[0] == '-')
        {
            for (int i = 1; i < len1; i++)
            {
                cArr1[i - 1] = cArr1[i];
            }
            parity++;
            len1--;
        }
        if (cArr2[0] == '-')
        {
            for (int i = 1; i < len2; i++)
            {
                cArr2[i - 1] = cArr2[i];
            }
            parity++;
            len2--;
        }
        bool isDigit = true;
        for (int i = 0; i < len1; i++)
        {
            if (!isdigit(cArr1[i]))
            {
                isDigit = false;
            }
        }
        for (int i = 0; i < len2; i++)
        {
            if (!isdigit(cArr2[i]))
            {
                isDigit = false;
            }
        }
        if (!isDigit)
        {
            cout << "\rInvalid input. Try again" << endl;
            continue;
        }
        if (parity % 2 != 0)
        {
            cout << "-";
        }
        (len1 > len2) ? mul(cArr1, cArr2, len1, len2) : mul(cArr2, cArr1, len2, len1);
    }
}

void mul(char *bigger, char *smaller, int bigLen, int smallLen)
{
    int *bigNum = new int[bigLen];
    int *smallNum = new int[smallLen];
    for (int i = 0; i < bigLen; i++)
    {
        bigNum[i] = bigger[bigLen - i - 1] - '0';
    }
    for (int i = 0; i < smallLen; i++)
    {
        smallNum[i] = smaller[smallLen - i - 1] - '0';
    }
    int res[30];
    for (int i = 0; i < 30; i++)
    {
        res[i] = 0;
    }
    for (int i = 0; i < smallLen; i++)
    {
        for (int j = 0; j < bigLen; j++)
        {
            res[i + j] += bigNum[j] * smallNum[i];
        }
    }
    for (int i = 0; i < bigLen + smallLen; i++)
    {
        int digit = res[i] % 10;
        int carry = res[i] / 10;
        res[i] = digit;
        res[i + 1] += carry;
    }
    bool null = false;
    for (int i = bigLen + smallLen - 1; i >= 0; i--)
    {
        if (res[i] != 0 && res[i + 1] == 0)
        {
            null = true;
        }
        if (null)
        {
            cout << res[i];
        }
    }
    cout << endl;
}

void printArr(char *c1, char *c2, int len1, int len2)
{
    for (int i = 0; i < len1; i++)
    {
        cout << c1[i];
    }
    cout << " * ";
    for (int i = 0; i < len2; i++)
    {
        cout << c2[i];
    }
    cout << " = ";
}

Answer 1

只需将您的 char 数组初始化为空数组：

char cArr1[500] = {};
char cArr2[500] = {};

然后，为了清楚起见，将参数的长度分配给两个整数，将它们强制转换，因为编译器可能会警告您 size_t 和 int 之间的不兼容。

int lenArg1 = 0;
int lenArg2 = 0;

lenArg1 = (int)strlen ( argv[1] );
lenArg2 = (int)strlen ( argv[2] );

然后，打印长度 len1 和 len2 仅用于调试目的：

int len1 = strlen ( cArr1 );
int len2 = strlen ( cArr2 );

cout << "len1 >> " + to_string(len1) <<endl;
cout << "len2 >> " + to_string(len2) <<endl;

事实上，正如@Kevin SUN 所提到的，您的参数读取步骤可能缺少空字符，但是，在运行一些测试之后，它似乎只需初始化 char 数组就可以正常工作。

另外，如 cmets 中所述，您需要增加为 res 数组保留的大小，我将其增加到 500

使用g++ -Wall main.cpp -o calc.exe 编译并运行：calc 10 100

如果没有初始化，您会遇到以下问题：

初始化后，输出正常：