好的，在没有阅读多项式之后，我首先尝试了一种基本的方法。
所以我有类polinom功能读取和打印：
#ifndef _polinom_h
#define _polinom_h
#include <iostream>
#include <list>
#include <cstdlib>
#include <conio.h>
using namespace std;
class polinom
{
class term
{
public:
double coef;
int pow;
term(){
coef = 0;
pow = 0;
}
};
list<term> poly;
list<term>::iterator i;
public:
void read(int id)
{
term t;
double coef = 1;
int pow = 0;
int nr_term = 1;
cout << "P" << id << ":\n";
while (coef != 0) {
cout << "Term" << nr_term << ": ";
cout << "coef = ";
cin >> coef;
if (coef == 0) break;
cout << " grade = ";
cin >> pow;
t.coef = coef;
t.pow = pow;
if (t.coef != 0) poly.push_back(t);
nr_term++;
}
}
void print(char var)
{
for (i=poly.begin() ; i != poly.end(); i++ ) { //going through the entire list to retrieve the terms and print them
if (poly.size() < 2) {
if (i->pow == 0) //if the last term's power is 0 we print only it's coefficient
cout << i->coef;
else if (i->pow == 1) {
if (i->coef == 1)
cout << var;
else if (i->coef == -1)
cout << "-" << var;
else
cout << i->coef << var;
}
else
cout << i->coef << var << "^" << i->pow; //otherwise we print both
}
else {
if (i == poly.end()) { // if we reached the last term
if (i->pow == 0) //if the last term's power is 0 we print only it's coefficient
cout << i->coef;
else if (i->pow == 1)
cout << i->coef << var;
else
cout << i->coef << var << "^" << i->pow; //otherwise we print both
}
else {
if (i->coef > 0) {
if (i->pow == 1)//if the coef value is positive
cout << i->coef << var << " + "; //we also add the '+' sign
else
cout << cout << i->coef << var << "^" << i->pow << " + ";
}
else {
if (i->pow == 1)//if the coef value is positive
cout << i->coef << var << " + "; //we also add the '+' sign
else
cout << cout << i->coef << var << "^" << i->pow << " + ";
}
}
}
}
}
};
#endif
嗯，当只读一个词时它起作用，但是当读取更多时，打印系数是一些随机值，并且在最后一个词之后它打印'+'或' - '当它不应该。
所以知道什么是错的？
谢谢！
最终更新
好的，我通过修改Bill的代码使其完美运行，所以非常感谢Bill和其他评论或回答的人！
这是最终的打印功能：
void print(char var)
{
list<term>::iterator endCheckIter;
for (i=poly.begin() ; i != poly.end(); i++ )
{
//going through the entire list to retrieve the terms and print them
endCheckIter = i;
++endCheckIter;
if (i->pow == 0)
cout << i->coef;
else if (i->pow == 1)
cout << i->coef << var;
else
cout << i->coef << var << "^" << i->pow;
if (endCheckIter != poly.end()) {
if (endCheckIter->coef > 0)
cout << " + ";
else {
cout << " - ";
endCheckIter->coef *= -1;
}
}
}
}
答案 0 :(得分：1)
if (i == poly.end()) { // if we reached the last term
此评论显示您的错误。对于任何给定的项目集合，items.end()将返回最后一项之后的条目。
例如，假设我有一个5项std :: vector：
[0] [1] [2] [3] [4]
然后begin（）指向：
[0] [1] [2] [3] [4]
/\
结束（）指向：
[0] [1] [2] [3] [4] []
/\
你的for循环，它看起来像：
for (i=poly.begin() ; i != poly.end(); i++ )
请注意，在使用iter之前，会比较i到poly.end()。只要i == poly.end()，你就完成了。
if (i == poly.end()) {内的代码永远不会被执行，因为这永远不会成真。
您可以使用以下方法测试结尾：
// get access to the advance function
#include <iterator>
....
std::list<term>::iterator endCheckIter = i;
std::advance(endCheckIter, 1);
if (endCheckIter == poly.end())
{
...
}
但更简单的方法可能是：
std::list<term>::iterator endCheckIter = i;
++endCheckIter;
if (endCheckIter == poly.end())
{
...
}
编辑： 我不确定你为什么要垃圾。添加缺少的大括号并处理非结尾案例，一切都在这里工作：
void print(char var)
{
list<term>::iterator endCheckIter;
for (i=poly.begin() ; i != poly.end(); i++ )
{ // <- MISSING BRACE
//going through the entire list to retrieve the terms and print them
endCheckIter = i;
++endCheckIter;
cout << i->coef << var << "^" << i->pow; // <- MISSING OUTPUT
if (endCheckIter != poly.end()) {
if (i->coef > 0)
cout << " + ";
else
cout << " - ";
}
} // <- MISSING BRACE
}
答案 1 :(得分：1)
好的，既然弗拉德已经决定他如何去做这件事，我就是这样做的：
#ifndef _polinom_h
#define _polinom_h
#include <iostream>
#include <list>
#include <cstdlib>
#include <cmath>
#include "infix_iterator.h"
using namespace std;
char var;
class polinom {
class term {
double coef;
int power;
ostream &write(ostream &os) const {
// At least to me, the logic is easier to follow if we
// handle one piece at a time.
// It may be longer, but I think it's easier to understand.
// First, if the coefficient is negative, subtract the term instead of adding it.
if (coef < 0)
// backspace over the "+ " and print '- ' in its place.
os << "\b\b- ";
// Then print the absolute value of the coefficient (if needed).
if (fabs(coef) != 1)
os << fabs(coef);
// Then print the var (if needed)
if (power != 0)
os << var;
// then print the power (if needed)
if (abs(power) > 1)
os << "^" << power;
// And we're done.
return os;
}
// support inserting a term into a stream.
friend std::ostream &operator<<(std::ostream &os, term const &t) {
return t.write(os);
}
public:
term(double c=0.0, int p=0) : coef(c), power(p) {}
bool read(std::ostream &os, std::istream &is, int num) {
// This is only slightly modified from the originally posted question
os << "\nTerm " << num << ": coef = ";
is >> coef;
if (coef == 0.0)
return false;
if (coef != 0.0) {
os << " grade = ";
is >> power;
}
return true;
}
bool operator<(term const &other) const {
// order by descending powers.
return other.power < power;
}
};
list<term> poly;
public:
void read(int id) {
term t;
int nr_term = 1;
std::cout << "P: " << id;
// Read and save individual terms:
while (t.read(std::cout, std::cin, nr_term++))
poly.push_back(t);
}
void write(char var) {
// sort the polynomial so the highest powers come first.
poly.sort();
// save the variable name for later use.
::var = var;
// Print out all the terms:
std::copy(poly.begin(), poly.end(), infix_ostream_iterator<term>(std::cout, " + "));
}
};
#endif
使用它非常简单：
#include "polynom.h"
int main() {
polinom p;
p.read(1);
p.write('x');
return 0;
}
答案 2 :(得分：0)
void print(char var)
{
for (list<term>::const_iterator i = poly.begin(), e = poly.end(); i != e; ++i) {
if (i != poly.begin() || i->coef < 0) {
cout << (i->coef > 0 ? '+' : '-');
}
if (abs(i->coef) != 1) {
cout << abs(i->coef);
}
if (i->pow == 0) {
if (abs(i->coef) == 1) {
cout << 1;
}
} else {
cout << var;
if (i->pow != 1) {
cout << '^' << i->pow;
}
}
}
}