c ++读取未定义的行数eof() [英] c++ reading undefined number of lines with eof()
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问题描述
我使用eof()处理一个问题。
使用
I'm dealing with a problem using eof(). using
string name;
int number, n=0;
while(!in.eof())
{
in >> name >> number;
//part of code that puts into object array
n++;
}
对我来说听起来很正常,只要文件中没有更多文本。
但是我得到的是n是4200317.当我查看数组条目,我看到第一个ats文件中的那些,其他是0.
sounds normal to me as it whenever there are no more text in the file. But what I get is n being 4200317. When I view the array entries, I see the first ones ats the ones in the file and other being 0s.
什么可能是问题,我该如何解决它?
What could be the problem and how should I solve it? Maybe there's an alternative to this reading problem (having undefined number of lines)
推荐答案
正确的方式:
string name;
int number;
int n = 0;
while(in >> name >> number)
{
// The loop will only be entered if the name and number are correctly
// read from the input stream. If either fail then the state of the
// stream is set to bad and then the while loop will not be entered.
// This works because the result of the >> operator is the std::istream
// When an istream is used in a boolean context its is converted into
// a type that can be used in a boolean context using the isgood() to
// check its state. If the state is good it will be converted to an objet
// that can be considered to be true.
//part of code that puts into object array
n++;
}
为什么您的代码失败:
string name;
int number, n=0;
while(!in.eof())
{
// If you are on the last line of the file.
// This will read the last line. BUT it will not read past
// the end of file. So it will read the last line leaving no
// more data but it will NOT set the EOF flag.
// Thus it will reenter the loop one last time
// This last time it will fail to read any data and set the EOF flag
// But you are now in the loop so it will still processes all the
// commands that happen after this.
in >> name >> number;
// To prevent anything bad.
// You must check the state of the stream after using it:
if (!in)
{
break; // or fix as appropriate.
}
// Only do work if the read worked correctly.
n++;
}
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