C++ 从 istream 读取直到换行（但不是空格）
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C++ read from istream until newline (but not whitespace)
提问by StephQ
I have a std::istream which refers to matrix data, something like:
我有一个 std::istream 指的是矩阵数据，例如：
0.0 1.0 2.0
3.0 4.0 5.0
Now, in order to assess the number of columns I would like to have some code like:
现在，为了评估列数，我想要一些代码，例如：
std::vector<double> vec;
double x;
while( (...something...) && (istream >> x) )
{
vec.push_back(x);
}
//Here vec should contain 0.0, 1.0 and 2.0
where the ...something... part evaluates to false after I read 2.0 and istream at the point should be at 3.0 so that the next
在我阅读 2.0 和 istream 之后，...something... 部分评估为 false 应该在 3.0 以便下一个
istream >> x;
should set x equal to 3.0.
应该设置 x 等于 3.0。
How would you achieve this result? I guess that the while condition
你将如何达到这个结果？我猜是 while 条件
Thank you very much in advance for your help!
非常感谢您的帮助！
采纳答案by tzaman
回答by Clifford
Read the lines into a std::string using std::getline(), then assign the string to a std::istringstreamobject, and extract the data from that rather than directly from istream.
使用std::getline()将行读入 std::string ，然后将该字符串分配给std::istringstream对象，并从中提取数据，而不是直接从 istream 中提取数据。
回答by David Rodríguez - dribeas
std::vector<double> vec;
{
std::string line;
std::getline( ifile, line );
std::istringstream is(line);
std::copy( std::istream_iterator<double>(is), std::istream_iterator<double>(),
std::back_inserter(vec) );
}
std::cout << "Input has " << vec.size() << " columns." << std::endl;
std::cout << "Read values are: ";
std::copy( vec.begin(), vec.end(),
std::ostream_iterator<double>( std::cout, " " ) );
std::cout << std::endl;
回答by MKroehnert
You can use std::istream::peek()to check if the next character is a newline.
See this entryin the cplusplus.com reference.
您可以使用std::istream::peek()来检查下一个字符是否为换行符。见该条目在cplusplus.com参考。
回答by Manohar Reddy Poreddy
I had similar problem
Input is as below:
我有类似的问题
输入如下：
1 2
3 4 5
The 1st two were N1 and N2
Then there is a newline
then elements 3 4 5, i dont know how many these will be.
前两个是 N1 和 N2
然后有一个换行符，
然后是元素 3 4 5，我不知道这些会有多少。
// read N1 & N2 using cin
int N1, N2;
cin >> N1;
cin >> N2;
// skip the new line which is after N2 (i.e; 2 value in 1st line)
cin.ignore(numeric_limits<streamsize>::max(), '\n');
// now read 3 4 5 elements
int ele;
// 2nd EOF condition may required,
// depending on if you dont have last new-line, and it is end of file.
while ((cin_.peek() != '\n') && (cin_.peek() != EOF)) {
cin >> ele;
// do something with ele
}
This worked perfect for me.
这对我来说很完美。
回答by Messa
Read the number, then read one character to see if it's newline.
读取数字，然后读取一个字符以查看它是否为换行符。
