c++使用指针以相反的顺序显示句子

Question

我正在尝试构建一个程序，从用户那里获取简短的句子并以相反的顺序显示它们。不幸的是，我刚开始使用 c++，我需要知道这样做。例如:如果用户输入:

I like the red color
blue is also nice
and green is lovely
but I don't like orange

输出:

but I don't like orange
and green is lovely
blue is also nice
I like the red color

提前致谢!

#include<iostream>
#include<string>
using namespace std;

const int SIZE= 500;

int main()
{

   const int SIZE = 500;
   char mystring[SIZE];
int i;
for(i=0; i<SIZE; i++)
{

    cout<<"Enter a string: ";
    cin.getline(mystring, SIZE);
    } while (mystring != 0);

       char * reverse= new char[strlen(mystring) + 1];
        char *p1 = mystring+ strlen(mystring);
        char *p2 = reverse;


        while(p1 != mystring)
        {
        p1--;
        *p2= *p1;
        p2++;
    }

    *p2 = '\0';
    cout << reverse<<endl;


   system("PAUSE");
   return 0;
}

Answer 1

您打算采用以下算法来解决这个问题:

1. 将文件加载到缓冲区中，以空字符终止。
2. 将指针 p 定位到最后一个缓冲区槽的位置。
3. 当 p 没有指向缓冲区的开始时，执行以下操作:
   • 如果字符是换行符('\n')则
     1. 将字符串 过去 换行符 (p+1) 发送到标准输出。
     2. 用空字符覆盖p指向的换行符。
   • 将 p 递减一个字符位置。
4. 上面的循环结束后还剩下一行:第一行。将它发送到标准输出，你就完成了。

或者我被引导相信。需要考虑的重要事项如下:

1. 该算法是否适用于空文件？
2. 该算法是否适用于仅包含换行符的文件？
3. 该算法是否适用于没有尾随换行符的多行文件？
4. 该算法是否适用于没有尾随换行符的单行文件？
5. 该算法是否适用于带有尾随换行符的多行文件？
6. 该算法是否适用于带有尾随换行符的单行文件？

话虽如此，这是一个潜在的候选人:

#include <iostream>
#include <fstream>
#include <iterator>
#include <vector>
using namespace std;

int main(int argc, char *argv[])
{
    // assume the file to reverse-print is the first
    //  command-line parameter. if we don't have one
    //  we need to leave now.
    if (argc < 2)
        return EXIT_FAILURE;

    // will hold our file data
    std::vector<char> data;

    // open file, turning off white-space skipping
    ifstream inf(argv[1]);
    inf.seekg(0, inf.end);
    size_t len = inf.tellg();
    inf.seekg(0, inf.beg);

    // resize buffer to hold (len+1) chars
    data.resize(len+1);
    inf.read(&data[0], len);
    data[len] = 0; // terminator

    // walk the buffer backwards. at each newline, send
    //  everything *past* it to stdout, then overwrite the
    //  newline char with a nullchar (0), and continue on.
    char *start = &data[0];
    char *p = start + (data.size()-1);
    for (;p != start; --p)
    {
        if (*p == '\n')
        {
            if (*(p+1))
                cout << (p+1) << endl;
            *p = 0;
        }
    }

    // last line (the first line)
    cout << p << endl;

    return EXIT_SUCCESS;
}

输入

I like the red color
blue is also nice
and green is lovely
but I don't like orange

输出

but I don't like orange
and green is lovely
blue is also nice
I like the red color

---

一种相当简单的方法

有很多更简单的方法可以做到这一点，我会在评论中解释每个步骤。您很可能不能使用这样的东西，但重要的是您了解什么时候可以使用:

#include <iostream>
#include <fstream>
#include <iterator>
#include <vector>
using namespace std;

int main(int argc, char *argv[])
{
    // assume the file to reverse-print is the first
    //  command-line parameter. if we don't have one
    //  we need to leave now.
    if (argc < 2)
        return EXIT_FAILURE;

    // collection that will hold our lines of text
    vector<string> lines;

    // read lines one at a time until none are returned
    //  pushing each line in to our vector.
    ifstream inf(argv[1]);
    string line;
    while (getline(inf, line))
        lines.push_back(line);
    inf.close();

    // a LOT happens in the next single line of code, and
    //  I will try to describe each step along the way.
    //
    // we use std::copy() to copy all "items" from
    //   a beginning and ending iterator pair. the
    //   target of the copy is another iterator.
    //
    //  our target iterator for our formatted ouput
    //   is a special iterator class designed to
    //   perform an output-stream insertion operation
    //   (thats the << operator) to the stream it is
    //   constructed with (in our case cout) using each
    //   item we give it from our copy-iteration. to use
    //   this class the "copied" item must support the
    //   traditional insertion operator <<, which of
    //   course, std::string does. after each item is
    //   written, the provided suffix (in our case \n)
    //   is written as well. without this all the lines
    //   would be ganged together.
    //
    //  lastly, to glue this together (and the whole
    //   reason we're here), we use a pair of special
    //   iterators designed to  work just like the regular
    //   begin() and end() iterators you're familiar with,
    //   when traversing forward in a sequence, but these
    //   ones, rbegin() and rend(), move from the last
    //   item in the sequence to the first item, which is
    //   *exactly* what we need.

    copy(lines.rbegin(), lines.rend(), 
         ostream_iterator<string>(cout, "\n"));

    // and thats it.
    return EXIT_SUCCESS;
}

输入

I like the red color
blue is also nice
and green is lovely
but I don't like orange

输出

but I don't like orange
and green is lovely
blue is also nice
I like the red color

---

更新:合并用户输入

为第二个版本合并用户输入的示例是:

#include <iostream>
#include <iterator>
#include <vector>
using namespace std;

int main(int argc, char *argv[])
{
    // collection that will hold our lines of text
    vector<string> lines;
    do
    {   // prompt the user
        cout << "Sentance (<enter> to exit): ";
        string line;
        if (!getline(cin, line) || line.empty())
            break;
        lines.push_back(line);
    } while (true);

    // send back to output using reverse iterators
    //  to switch line order.
    copy(lines.rbegin(), lines.rend(),
         ostream_iterator<string>(cout, "\n"));

    return EXIT_SUCCESS;
}