使用C ++中的字符串标题的getline（）函数，第一个字符未存储在字符串对象中

Question

string nums;

int main() {
  int cases;
  scanf("%d", &cases);
  while (cases--) {
    cin.ignore();
    getline(cin, nums);
    cout << nums << endl;
  }
}

input example

3

1 2 1 2 1

2 3 4 1 2 5 10 50 3 50

3 5 2 7 1 7 5 2 8 9 1 25 15 8 3 1 38 45 8 1

I expect right result below

1 2 1 2 1

2 3 4 1 2 5 10 50 3 50

3 5 2 7 1 7 5 2 8 9 1 25 15 8 3 1 38 45 8 1

However, the output is that

1 2 1 2 1

 3 4 1 2 5 10 50 3 50

 5 2 7 1 7 5 2 8 9 1 25 15 8 3 1 38 45 8 1

I don't know what the reason is. I clearly use cin.ignore() to flush the buffer. Why is the first char removed ?

Answer 1

Just put the line cin.ignore(); outside the while loop:

Following is corrected code. See it working here :

string nums;

int main() 
{
    int cases;
    scanf("%d", &cases);//Better if you use `cin>>cases;` here, just for sake of C++.
    cin.ignore();
    while (cases--) 
    {
        getline(cin, nums);
        cout << nums <<endl;
    }
    return 0;
}

Answer 2

You should initialize your cases to 0 just in case a user enters an invalid input for an integer type like a character or something. You should prefer to use std::cin for user input in C++ like I stated in the comments section. You can skip the \\n newline character after your initial input by calling get() . It's serving the same purpose as you were trying to achieve with ignore() . while(cases--) is a bit weird to look at but I get what you're going for. You can declare your string nums inside the loop since you're overwriting it every iteration anyway. The reason you do not need to use std::cin.ignore() in this code is because std::getline reads everything including the newline character from console input. This code should do exactly what you want.

#include <iostream>
#include <string>

int main()
{
    int cases(0);  ///< initialize in case user enters text
    (std::cin >> cases).get(); ///< use std::cin in C++; get() to skip `\n`
    while (cases--) ///< is that really what you want?
    {
        std::string nums; ///< nums only needed locally
        std::getline(std::cin, nums); ///< reads whole line + '\n'
        std::cout << nums << std::endl;
    }
    return 0;
}