变量已自动分配

Question

I was making a project that gets students' names, roll numbers & marks and returns percentage or grade according to need of user. I made a function using for loop to assign details(name, etc.) to variable but during change of for loop's argument, program was assigning ""(null) to the first variable in execution part. Am I missing any code here?

#include <iostream>
#include <stdio.h>

using namespace std;

class Data 
{
    public: 
    int Rno[3], phy[3], mat[3], chem[3];
    char name[3][100];

    void getInfo()
    {
        for(int i = 0; i < 3; i++)
        {
            cout << "Enter your name: ";
            gets(name[i]);

            cout << "Enter your Roll number: ";
            cin >> Rno[i];

            cout << "Enter your Physics marks: ";
            cin >> phy[i];

            cout << "Enter your Maths marks: ";
            cin >> mat[i];

            cout << "Enter your Chemistry marks: ";
            cin >> chem[i];
       }
    };
};

int main()
{
    Data d;

    d.getInfo();
    puts(d.name[1]);
}

Answer 1

Problem is with not flushing cout . In your case inside the loop cout must be flushed before calling gets .

cout << "Enter your name: "<<endl;  // This cout should be flushed properly **with** new line character
gets(name[i]);

From @ProXicT comment

cout << "Enter your name: "<<flush;  // This cout should be flushed properly **without** new line character
    gets(name[i]);

Answer 2

First an explanation. Then some options to address the problem.

The problem is that you are mixing line-oriented input ( gets() ) with formatted input ( cin >> .... ) on the same input device/stream.

Such functions treat whitespace - particularly newlines - differently. And, because of that, you can get unexpected interactions. In this case, cin >> chem[i] will read an integral value, stop when it encounters a newline, leave the newline character waiting to be read. Then gets() - in the next iteration of the loop - will encounter the newline, and return immediately (hence the "null", to use your description).

OK; that's the explanation. What to do about it?

The solution is not to mix such styles of input. One option consistently use line oriented input to read EVERYTHING as a string, and then separately parse the string.

Some people will suggest approaches that involve discarding (or "flushing") the offending newlines. That approach doesn't work well if your input is complex (eg lots of different statements reading from the same stream in different ways) because it is necessary to ensure EVERY stray newline is discarded. It is also error-prone - you can unintentionally discard input your program needs to read.

Also avoid using gets() like the plague. It has been removed from recent C standards for a reason - it is inherently unsafe.

There are various ways using C++ streams (like cin ) to safely read strings and other types. For example, if some_str is of type std::string , then std::getline(std::cin, some_str) can be used to read a complete line of input. std::getline() also has the advantage that it can read a string of arbitrary size. But, still, avoid using std::getline(cin, some_str) and streaming input ( cin >> ... ) on the same stream - because they STILL handle newlines differently.