无法从输入文件正确读取数据

Question

please see below c++ function. The point of it is to store variables in an input file ("is") to global arrays. I go through with the debugger with a watch on the "temp" variable (the first if statement seems to work fine), and after reading the first line of the input file, the variable temp no longer updates.

The file is in a specific format so it should be reading an int, although I did eventually put a char at the end of the KEYFRAME if statement to see if it was reading the endline character (it wasn't).

What are some possible reasons for this? Thank you so much!

 void readFile(istream& is){
        string next;
        int j = 0;
        int i = 0;
    while (is){
    for (int i = 0; i < F; i++){ 
            is >> next;
            if (next == "OBJECT")
            {

                int num;
                is >> num;
                string name;
                is >> name;
                objects[j].objNum = num;
                objects[j].filename = name;
                j++;
            }
            else if (next == "KEYFRAME"){
                int k;
                int temp;
                is >> k;
                int time;
                is >> time;
                objects[k].myData[time].setObjNumber(k);
                objects[k].myData[time].setTime(time);
                is >> temp;
                objects[k].myData[time].setPosition('x', temp) ;
                is >> temp;
                objects[k].myData[time].setPosition('y', temp);
                is >> temp;
                objects[k].myData[time].setPosition('z', temp);
                is >> temp;
                objects[k].myData[time].setRotation('x', temp);
                is >> temp;
                objects[k].myData[time].setRotation('y', temp);
                is >> temp;
                objects[k].myData[time].setRotation('z', temp);
                is >> temp;
                objects[k].myData[time].setScaling('x', temp);
                is >> temp;
                objects[k].myData[time].setScaling('y', temp);
                is >> temp;
                objects[k].myData[time].setScaling('z', temp);
                char get;
                is >> get;
            }
            else {
                cout << "Error reading input file";
                return;
            }

        }
    }
}

Answer 1

The most common reasons why std::istream 's operator>> does not update the variable can be demonstrated using the following simple example:

#include <sstream>
#include <iostream>

int main() {

    std::string sample_input="1 2 3 A 4 5";

    std::istringstream i(sample_input);

    int a=0, b=0, c=0, d=0;
    std::string e;
    int f=0;

    i >> a >> b >> c >> d >> e >> f;

    std::cout << a << std::endl;
    std::cout << b << std::endl;
    std::cout << c << std::endl;
    std::cout << d << std::endl;
    std::cout << e << std::endl;
    std::cout << f << std::endl;
}

The resulting output from this program is:

    1
    2
    3
    0

    0

This example forces a conversion error on the fourth parameter. Once operator>> encounters a formatting conversion error, an error bit is set on the stream, which prevents further conversions.

When using operator>> , it's necessary to check for conversion errors after every input format conversion, and reset the stream's state, if necessary.

So, your answer is that you have an input conversion error someplace. I generally avoid using operator>> . It's fine for simple situations where the input is known, and there is no possibility of bad input. But as soon as you get into a situation where you might need to potentially handle bad input, using operator>> becomes painful, and its much better to take some other approach for parsing stream-based input.