使用c字符串从多项式c ++提取指数 [英] extracting exponent from polynomial c++ using c string

问题描述

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 提取系数。我应用相同的概念来找到指数，但我不知道如何使用 strtok 在分隔符之后提取字符串或跳过第一个字符， strtok 是我知道的唯一提取工具。

I'm trying to extract the value of the coeffecients and the exponent from a polynomial. I have already succeeded in extracting the coeffients using strtok. I applied the same concept to find the exponent, but I don't know how to use strtok to extract the string AFTER the delimiters or skip the first character, and strtok is the only extracting tool I know.

这是主要函数

#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coeff（工作）的函数

This is the function to extract coeff (worked)

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（不工作）的函数

This is the function to extract exponents (not working)

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;
}

推荐答案

c $ c> strtok 是破坏性的。你部分似乎知道，当你做一个副本，以便能够在功能中使用它两次。但是 extractCoeff 返回到main后， string 指向的C字符串的内容已损坏，因此当您 $ extractExp 您传递一个严重截断的字符串的两个副本。

Your problem is that strtok is destructive. You partly seem to know that, as you make a copy to be able to use it twice in the functions. But after extractCoeff returns to main, the content of the C-string pointed to by string is damaged, so when you call extractExp you pass two copies of a badly truncated string.

> std :: string 用于处理字符串。使用 std :: string ，您可以使用成员函数 find ， find_first_of 和 find_first_not_of 找到子字符串，您正在寻找并使用 substr

In C++ you should use std::string for handling strings. With std::string you can use the member functions find, find_first_of and find_first_not_of to locate the substrings, you are looking for and use substr to extract them without destroying the original string.

您可以使用C函数在C字符串上做类似的事情，但这将是一个C问题。 （使用 cout 和C ++标题使得你的程序不能作为C程序有效，但其他一切都是纯C而不是惯用的C ++。）

You can do similar things on C strings, using C functions, but that would be a C question. (Using cout and C++ headers makes your program not valid as a C program, but everything else is pure C rather than idiomatic C++.)

顺便说一句， strtok 不是你应该学习的方式来解析字符串。它是破坏性的，它不能可重用，在某些平台上不是线程安全的。除非你有非常好的理由需要破坏性的就地解析替代品，不要使用它或其稍微更好的亲（在POSIX） strtok_r 。

And, by the way: strtok is not something you should learn as the way to parse strings. It is destructive, it can't be used reentrantly and on some platform is not thread-safe. Unless you have very good reasons to need destructive in-place parsing over alternatives, don't use it or its slightly better kin (in POSIX) strtok_r.

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