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我试图从多项式中提取系数和指数的值。我已经成功地使用strtok. 我应用相同的概念来查找指数,但我不知道如何strtok在分隔符之后提取字符串或跳过第一个字符,并且strtok是我知道的唯一提取工具。

这是主要功能

#include <iostream>
#include <cctype>
#include <cstring>
#include <cstdlib>
#include <string>
using namespace std;

void extractCoeff (char *str, char *copy);
void extractExp (char *str, char *copy);
int main()
{
    const int SIZE = 150; // size for string input

    char *string;
    string = new char[SIZE];
    cout << "Enter the polynomial\n"<<"minus sign must not have a blank with a coeff";
    cin.ignore();
    cin.getline(string, SIZE); // input string example: -4x^0 + x^1 + 4x^3 -3x^4

    char *copy1;
    copy1 = new char[SIZE];
    strcpy(copy1, string);  
    extractCoeff(string, copy1);

    cout << endl << endl;
    char *copy2;
    copy2 = new char[SIZE];
    strcpy(copy2, string); 
    extractExp(string, copy2);


    return 0;
}

这是提取系数的功能(有效)

void extractCoeff (char *str, char *copy)
{   
    char *p = strtok(str, " +"); // extract the first time
    char *search;
    int counter = 0;
    while (p) 
    {
        search = strstr(p, "x^");
        cout << "Token: " << p << endl;
        cout << "Search " << search << endl;
        p = strtok(NULL, " +");
        counter++;
    }

    cout << copy << endl;

    // find coeff
    int *coefficient;
    coefficient = new int[counter];

    p = strtok(copy, " +"); // extract the second time to find coeff
    int a = 0;
    while (p)
    {
        cout << "p: " << p << endl;
        long coeff;
        if (*p == 'x')
        {
           coeff = 1;
        }
        else if (*p == NULL)
        {
            coeff = 0;
        }
        else
        {
            char *endptr;
            coeff = strtol(p, &endptr, 10);
        }
        coefficient[a] = coeff;
        p = strtok(NULL, " +");
        a++;
    }

    for (int i = 0; i < counter; i++)
        cout << coefficient[i] << endl;
}

这是提取指数的功能(不起作用)

void extractCoeff (char *str, char *copy)
{   
    char *p = strtok(str, " +"); // extract the first time
    char *search;
    int counter = 0;
    while (p) 
    {
        search = strstr(p, "x^");
        cout << "Token: " << p << endl;
        cout << "Search " << search << endl;
        p = strtok(NULL, " +");
        counter++;
    }

    cout << copy << endl;

    // find coeff
    int *coefficient;
    coefficient = new int[counter];

    p = strtok(copy, " +"); // extract the second time to find coeff
    int a = 0;
    while (p)
    {
        cout << "p: " << p << endl;
        long coeff;
        if (*p == 'x')
        {
           coeff = 1;
        }
        else if (*p == NULL)
        {
            coeff = 0;
        }
        else
        {
            char *endptr;
            coeff = strtol(p, &endptr, 10);
        }
        coefficient[a] = coeff;
        p = strtok(NULL, " +");
        a++;
    }

    for (int i = 0; i < counter; i++)
        cout << coefficient[i] << endl;
}

void extractExp (char *str, char *copy)
{   
    char *p = strtok(str, " x^"); // extract the first time
    //char *search;
    int counter = 0;
    while (p) 
    {
        //search = strstr(p, "x^");
        //cout << "Token: " << p << endl;
        //cout << "Search " << search << endl;
        p = strtok(NULL, " x^");
        counter++;
    }

    cout << copy << endl;

    // find coeff
    int *exp;
    exp = new int[counter];

    p = strtok(copy, " x^"); // extract the third time
    int b = 0;
    while (p)
    {
        cout << "p2: " << p << endl;
        int expVal;
        if (*p == NULL)
        {
            expVal = 0;
        }
        else
        {
            char *endptr;
            expVal = strtol(p, &endptr, 10);
        }
        exp[b] = expVal;
        p = strtok(NULL, " x^");
        b++;
    }

    for (int i = 0; i < counter; i++)
        cout << exp[i] << endl;
}
4

1 回答 1

1

你的问题是strtok破坏性的。您似乎部分知道这一点,因为您制作了一个副本以便能够在函数中使用它两次。但是在extractCoeff返回到 main 之后,指向的 C 字符串的内容string被损坏了,所以当你调用时,extractExp你传递了一个被严重截断的字符串的两个副本。

在 C++ 中,您应该使用它std::string来处理字符串。您可以使用std::string成员函数findfind_first_offind_first_not_of定位子字符串,您正在寻找并使用substr它们来提取它们,而不会破坏原始字符串。

您可以使用 C 函数对 C 字符串执行类似的操作,但这将是一个 C 问题。(使用cout和 C++ 头文件会使您的程序作为 C 程序无效,但其他一切都是纯 C 而不是惯用的 C++。)

而且,顺便说一句:strtok不是你应该学习的解析字符串的方法。它具有破坏性,不能重入使用,并且在某些平台上不是线程安全的。除非您有充分的理由需要对替代方案进行破坏性的就地解析,否则不要使用它或其稍微更好的亲属(在 POSIX 中)strtok_r

于 2013-03-01T11:40:01.503 回答