来自文件的简单 C++ 输入......如何? [英] Simple C++ input from file...how to?
问题描述
我有一个文件:
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
.
.
.
当我读到P"时，我知道接下来会出现 3 个浮点数.接下来是有限数量的 X 和 Y 坐标.数字会有所不同，直到达到我必须识别的T".然后可能有一个 'r'、's' 或 't' 后跟一些值.
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"，然后接收 2 个浮点数，但后来我知道我必须为 X 和 Y 坐标设置一个 while 循环，当我到达T"时该循环将停止.我对 C++ 的了解不够，无法让循环停止并识别T"，然后再做其他事情.
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.
一个解释的例子将不胜感激.提前致谢！
An example to explain would be appreciated. Thanks in advance!
推荐答案
我将向您展示我认为正确的 C++ 方法.首先定义一个类来表示您的第一行并执行其 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;
}
};
我已经使用断言添加了一些基本的错误检查，您可能希望在最终程序中使用更强大的功能.
I've added some basic error checking with assert, you'll probably want something more robust in your final program.
现在是中间线的类:
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;
}
};
当我们到达中间线所在部分的末尾时，我们应该会遇到一个T".在这种情况下，我们提高流的失败位，它会告诉客户端没有更多的中间行要读取.
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.
最后几行的类:
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;
}
};
最后几行比较复杂，因为我们不知道每一行有多少个值，所以我们尽可能多地读取.
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.
那是棘手的部分.现在我们的 main 函数将简单地读取第一行，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
"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";
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 << "
";
std::copy(middle_lines.begin(), middle_lines.end(),
std::ostream_iterator<MiddleLine>(std::cout, "
"));
std::copy(last_lines.begin(), last_lines.end(),
std::ostream_iterator<LastLine>(std::cout, "
"));
return 0;
}
这是你将得到的输出::
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
我使用了一个字符串作为我的数据源，但您可能想要从文件中读取.
I've used a string as the source of my data but you'll probably want to read from a file.
仅此而已，您可以看到我没有编写一个循环.
And that's all, you can see that I didn't write a single loop.
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