简单的C ++输入从文件...如何？ [英] Simple C++ input from file...how to?
问题描述
我有一个档案：
P 0.5 0.6 0.3
30 300
80 150
160 400
200 150
250 300
T
r 45 0 0
s 0.5 1.5 0 0
t 200 –150
.
.
.
当我阅读'P'时，我知道3个浮标会跟随。其后将是有限数量的X和Y坐标。数字将变化，直到达到一个T，我必须认识。然后可能有一个'r'，'s'或't'后面跟一些值。
When I read in 'P' I know that 3 floats will follow. That will be followed by a finite number of X and Y coordinates. The number will vary until a 'T' is reached which I have to recognize. Then there could be an 'r', 's' or 't' followed by some values.
无论如何，我知道如何识别'P' 2浮动，但是我知道我必须有一个while循环的X和Y坐标，这将停止，当我到一个'T'。我不知道C ++让循环停止，并识别'T'，然后做别的事情。
Anyways I know how to recognize 'P' and then take in the 2 floats but then I know I have to have a while loop for the X and Y coordinates which will stop when I get to a 'T'. I do not know enough about C++ to make the loop stop and recognize the 'T' and then do something else.
一个例子来解释将不胜感激。提前感谢！
An example to explain would be appreciated. Thanks in advance!
推荐答案
我会告诉你我认为这是正确的C ++方式。首先定义一个类来表示你的第一行和做它的IO：
I'll show you what I think it's the proper C++ way of doing this. First define a class for representing your first line and for doing its IO:
struct FirstLine
{
double x, y, z;
friend std::istream & operator>>(std::istream & is, FirstLine & data)
{
std::string line, ignore;
std::getline(is, line);
std::istringstream iss(line);
iss >> ignore >> data.x >> data.y >> data.z;
assert(ignore == "P" && iss);
return is;
}
friend std::ostream & operator<<(std::ostream & os, FirstLine const & data)
{
return os << "P " << data.x << " " << data.y << " " << data.z;
}
};
我添加了一些基本的错误检查与assert，你可能会想要一个更强大的最终程序。
I've added some basic error checking with assert, you'll probably want something more robust in your final program.
现在一个类用于中间行：
Now a class for middle lines:
struct MiddleLine
{
double x, y;
friend std::istream & operator>>(std::istream & is, MiddleLine & data)
{
std::string line;
std::getline(is, line);
if(line == "T")
is.clear(std::ios::failbit);
else
{
int n = sscanf(line.c_str(), "%lf %lf", &data.x, &data.y);
assert(n == 2);
}
return is;
}
friend std::ostream & operator<<(std::ostream & os, MiddleLine const & data)
{
return os << data.x << " " << data.y;
}
};
当我们到达中间行的结束部分时，我们应该遇到一个T 。在这种情况下，我们提高流的失败位，这将告诉客户端没有更多的中间行可以读取。
When we reach the end of the section where the middle lines are we are supposed to encounter a "T". In that case we raise the fail bit of the stream, which will tell client that there are no more middle lines to read.
最后一行最后一行： / p>
Finally a class for the last lines:
struct LastLine
{
std::string identifier; // r, s or t
std::vector<double> values;
friend std::istream & operator>>(std::istream & is, LastLine & data)
{
std::string line;
std::getline(is, line);
std::istringstream iss(line);
iss >> data.identifier;
assert(data.identifier == "r" || data.identifier == "s"
|| data.identifier == "t");
std::copy(std::istream_iterator<double>(iss),
std::istream_iterator<double>(), std::back_inserter(data.values));
return is;
}
friend std::ostream & operator<<(std::ostream & os, LastLine const & data)
{
os << data.identifier << " ";
std::copy(data.values.begin(), data.values.end(),
std::ostream_iterator<double>(os, " "));
return os;
}
};
最后一行更复杂，因为我们不知道每个值中有多少个值，所以我们阅读尽可能多。
Last lines are more complicated becase we don't know how many values are in each, so we just read as many as we can.
这是棘手的部分。现在我们的主函数只读取一个第一行，然后是未知数量的中间行，最后是未知数量的最后一行：
That was the tricky part. Now our main function will simply read one first line, then an unknown number of middle lines, and finally an unknown number of last lines:
int main()
{
std::string const data = "P 0.5 0.6 0.3\n
"30 300\n"
"80 150\n"
"160 400\n"
"200 150\n"
"250 300\n"
"T\n"
"r 45 0 0\n"
"s 0.5 1.5 0 0\n"
"t 200 –150";
std::istringstream iss(data);
FirstLine first_line;
iss >> first_line;
std::vector<MiddleLine> middle_lines;
std::copy(std::istream_iterator<MiddleLine>(iss),
std::istream_iterator<MiddleLine>(),
std::back_inserter(middle_lines));
iss.clear();
std::vector<LastLine> last_lines;
std::copy(std::istream_iterator<LastLine>(iss),
std::istream_iterator<LastLine>(),
std::back_inserter(last_lines));
assert(iss.eof());
std::cout << first_line << "\n";
std::copy(middle_lines.begin(), middle_lines.end(),
std::ostream_iterator<MiddleLine>(std::cout, "\n"));
std::copy(last_lines.begin(), last_lines.end(),
std::ostream_iterator<LastLine>(std::cout, "\n"));
return 0;
}
这是您将获得的输出::
This is the output you'll get::
P 0.5 0.6 0.3
30 300
80 150
160 400
200 150
250 300
r 45 0 0
s 45 0 0 0.5 1.5 0 0
t 45 0 0 0.5 1.5 0 0 200
我使用一个字符串作为我的数据源，但你可能想从一个文件中读取。
I've used a string as the source of my data but you'll probably want to read from a file.
这样，你可以看到我没有写一个循环。
And that's all, you can see that I didn't write a single loop.
这是代码在键盘。
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