为什么我的代码无法在屏幕上打印任何内容？ （C ++）

Question

I am working with a file that contains a set of data:

x values    y values
20.00       0
20.02       15
20.04       27
20.06       39
20.08       54
20.10       65
20.12       75

The program is supposed to calculate the area under curve. In this part I am to do so for the first 6 xy sets. My algorithm for finding the area is the sum of each trapezoid , which for these constraints, should only be 5 trapezoids. The each of each trapezoid is defined by: A = 0.5(base-base_0) * (height + height_0).

This is my code:

int main()
{
    ifstream infile;
    double AreaCurve = 0.0, NumberTraps = 0.0, height, height_0, base, base_0, AreaTrap;

    infile.open("xydata.dat");

    cin.ignore(20, '2');
    while (NumberTraps < 6)
    {
        infile >> base_0;
        infile >> height_0;
        infile >> base;
        infile >> height;
        AreaTrap = 0.5 * (base - base_0) * (height + height_0);
        AreaCurve += AreaTrap;
        NumberTraps++;
    }

    cout << "The area under the curve is: " << setprecision(4) << AreaCurve << endl;

    infile.close();

    return 0;
}

When I compile the program nothing prints to the screen. I am not sure why this is happening but I believe it may have to do with an error in the line of my code containing the cin.ignore function (I can't edit the data, and I need to skip the line that reads: "x values\\t\\ty values")

Answer 1

Remove the line cin.ignore(20, '2'); .

This line will make your program wait for user input until they either enter twenty 2 s, or enter a different character.

You shouldn't be reading any user input in this program, your input data comes from infile .

You will need to add code to ignore the first line of infile . A simple way to do that is string s; getline(infile, s); string s; getline(infile, s);

Your program's main loop has a logic bug as well. You are reading in 2 lines at a time and considering that trapezoid, but then you are ignoring the trapezoid just after that one. (So you only count about half of the trapezoids).

Finally you should check for input success before processing that trapezoid:

if ( !infile )
    break;

Answer 2

To ignore the first line of the infile stream (regardless of its length) you need:

infile.ignore( numeric_limits<streamsize>::max(), '\n' ) ;

Note, this needs the <limits> header.

You do not need to ignore to the first '2'. For starters the first x value would always have to start with '2', but more usefully std::istream::operator>> (double&); skips white-space including line ends in any case.

There are other issues with this code, such as attempting to read more values that the file contains, an incorrect integration algorithm, and having no error checking for a valid stream. Consider:

infile.ignore( numeric_limits<streamsize>::max(), '\n' ) ;

infile >> base_0;
infile >> height_0;

while( infile.good() )
{
    infile >> base;
    infile >> height;

    if( infile.good() )
    {
        AreaTrap = 0.5 * (base - base_0) * (height + height_0);
        AreaCurve += AreaTrap;

        base_0 = base ;
        height_0 = height ;
    }
}