How to read from a file char by char in C++?

Question

I'm trying to read from a file, but the only thing I get on working is using getline().

The problem is that reading a whole line doesnt to the job for me.

My input file looks like this:

abc 10 20
bbb 10        30
ddd 40 20

when the first word in each line should be saved as a string, and both number afterwards as ints. The delimiter between the "words" in each line can be either a SPACE or a TAB.

So is the only solution is reading char by char? Or is there another solution?

Answer 1

Supposedly you want something like this:

std::string s;
int         v0, v1;
while (in >> s >> v0 >> v1) {
    std::cout << "do something with s='" << s << "' v0=" << v0 << " v1=" << v1 << "\n";
}

This doesn't make sure that the values are all on one line, however. If you want to arrange for this you probably want to read a line using std::getline() and then split this line up as above using an std::istringstream .

Answer 2

您可以使用getline()并使函数返回从getline()接收的字符串中的每个连续字符。

Answer 3

For what it's worth, I agree with @Dietmar's answer -- but I'd probably go a bit further. From the looks of things, each line of input represents some sort of logical record. I'd probably create a class to represent that record type, and provide an overload of operator>> for that class:

class my_data { 
    std::string name;
    int val1, val2;

    friend std::istream &operator>>(std::istream &is, my_data &m) { 
        std::string temp;
        std::getline(is, temp);
        std::istringstream buffer(temp);
        buffer >> m.name >> m.val1 >> m.val2;
        return is;
    }
};

You might want to do a little extra logic to propagate a failed conversion in the stringstream out to the istream where you read the raw data.

In any case, with this in place, you can (for example) initialize a vector of objects directly from the stream:

std::vector<my_data> whatever(
    (std::istream_iterator<my_data>(some_stream)),
    (std::istream_iterator<my_data>());

Answer 4

I'm not entirely sure what you are asking for.I suppose you want to read a text file and save a string and two ints (at each line) and printing each in a new line.If so try this:

#include <iostream>
#include <string>
#include <fstream>
using namespace std;

int main()
{
    string str;
    int a,b;
    ifstream file("test.txt");
    if(file.is_open() == false)
        {
        cout << "Error: File can't be loaded" << endl;
        exit(1);
        }
    while(1)    
    {
        if(!file)
        break; 
        file >> str;
        file >> a;
        file >> b;
        cout << str << endl;
        cout << a << endl;
        cout << b << endl;
    }
    file.close(); // Close the file to prevent memory leaks
    return 0;
} 

Answer 5

To read a file char-by-char, while preserving input text formatting, you can use the following:

if (in.is_open())
    char c;
    while (in.get(c)) {
        std::cout << c;
    }
}

where in is an input stream of type std::ifstream . You can open such a file, like so: std::ifstream in('myFile.txt');

If you don't mind formatting and would rather print all in one line, then you can follow Dietmar Kühl's advice.

Answer 6

The idea from @Dietmar of reading with operator>> for each single value is good, but you still have this problem with the endline.

However, you don't have to store the whole line in a temporary string, you can do it streamed and more efficiently with std::istream::ignore():

bool read_it(std::istream& in, std::string& s, int& a, int& b)
{
  if (in >> s >> a >> b) // read the values
    in.ignore(std::numeric_limits<std::streamsize>::max(), '\n'); // read the newline      
  return in;
}

Answer 7

使用fscanf， http： //www.cplusplus.com/reference/clibrary/cstdio/fscanf/

fscanf(stream, "%s %d %d", &s, &a, &b);