iterating through file fail

Question

I want to iterate through a file and print all the string. example, the file contain:

a
1
2
10
a1
1
b

will output:

a
a1
b

I write my code like this:

int main(){
  ifstream stream;
  stream.open("example.txt");
  string temp;
  while (getline(stream, temp)){
    cout<<temp<<endl;
    int n;
    while(stream>>n){}
  }
}

this program online print "a" and "a1". and suggestion?

Answer 1

What happens is the following: once your program hits this line,

while(stream>>n){}

it attempts reading the following tokens from the file as integers. 1 succeeds. 2 succeeds. 10 succeeds. a1 fails.

stream is now in a fail state (stream.fail() == true). The next statement your code executes is

while (getline(stream, temp)){

This will read the next token (a1) but since stream is in a fail state, the result of getline will be interpreted as false, and the loop will be interrupted.

That’s why your program ends prematurely. However, it shouldn’t even print a1. Indeed, it only prints a on my machine.

You need to clear the fail state after eating the integer tokens inside the loop. Add the following statement at the end:

stream.clear();

This resets the fail state (stream.fail() == false) and the outer loop will continue to run.

Answer 2

What about this?

std::copy_if(std::istream_iterator<string>(stream), std::istream_iterator<string>(),
             std::ostream_iterator<string>(std::cout),
             [](std::string const& rhs) { return !std::isdigit(rhs[0]); })