如何从C ++中的文本文件中获取特定行？

Question

I need some help with C++

I am trying to create a program which contains excersises to practice the different German cases.

Hard-coding all questions and respective answers seems like an awful lot of work, and super inefficient.

What I want my program to do, is: grab a random line from file X, and grab the same line number from file Y. (This seems like the easiest way to get both questions and answers from external files.) To me, it seems the most logical to get a random number, and use that as a line number. But, that's about how far I got...

I know basic C++, but am very eager to learn.

Can anyone please explain to me how to pull this off, including all necessary command?

Answer 1

First, I would recommend that you store questions and answers in the same text file, probably by alternating between a question line and then an answer line. This will make correcting mistakes, adding/removing questions, and general maintenance of your data easier.

But if you want to keep them in separate files, the following code snippet will read your text file in and store the questions in an array (an stl vector) which you can then index or iterate any way you'd like:

#include <iostream>
#include <fstream>
#include <string>
#include <vector>

int main()
{
        std::ifstream file("questions.txt");
        std::string line;
        std::vector<std::string> questions;
        while (std::getline(file, line))
        {
            questions.push_back( line );
        }
        // Now do something interesting with your questions. You can index them
        // like this: questions[5], or questions[random_index]
}

Answer 2

There are two ways of doing this:

1. If you are planning on getting question/answer pairs you would be best to just read the who file line by line and store all the lines. Then you just look it up in the array.

2. If for some reason you only want to get one line at a time you'll have to read lines and count until you've gotten to the line you want.

Answer 3

you may have a keyword for each line, like an id.

that id can be paired to both questions, and answers if you have multiple files. or just pair the question, with the answer same order, or even same file.

Answer 4

You are constructing a database.
You should use a database.

The problem is that the question and answers are variable length records, which make positioning difficult. If all the records were the same length, you could position to a random record much faster.

In order to find a text line, you will need to read past all the other newlines (since they are not in the same column in every line). This is fine if you only need to search once, but very slow to search many times. Now comes the reason for the database.

To make finding the questions and answers faster, create an index file or table. (Starting to smell like a database). The index file will contain records of the form [question #, file position] where file position is the position in the question's file that the question starts on.

You would load this file into memory and use it to index into the "questions" file. By storing the index contents into a file, you won't have to construct it from scratch each time your program starts; only when the question's file changes.