当我调用另一个 function 读取文本文件中的下几行时，如何使 getline 不跳过文本文件中的一行

Question

I'm trying to make a program that will read input from a text file and call a function when it detects "1" in the text file. When the program detects "1" in the text file, it will call ReadLines(fstream & file). Readlines() will then read the next 4 lines in the program. The problem I'm having is that after Readlines() is called, the loop inside main doesn't read the next line in the text file. It skips over it and continues to read the file in the while loop created in main.

fstream file("paper.txt");
std::string str;

//Check if file is open
if (file.is_open()) {
    cout << "File is open" << endl;
}
else {
    cout << "File is not open" << endl;
    return 0;
}

//Get line from text file until it is at the end of file
while (std::getline(file, str)) {
    //Print the current line
    std::cout << str << endl;

    //If getline detects a "1", call ReadLines function
    if (str == "1") {
        cout << "---enter loop----" << endl;
        ReadLines(file);
    }
}

file.close();
return 0;

}

void ReadLines(fstream& file) {
int i = 1;
std::string str;

//Read the next 4 lines
while (std::getline(file, str) && i < 5) {
    std::cout << str << endl;
    i++;
}

cout << "--exit loop--" << endl;

}

Here are the contents of the text file

1
234
10
12.5
tacos
1
123
12
23.22
cake

As you can see, "1" is seen twice in the text file. The loop inside the ReadLines function seems to work fine, but after the loop goes back to the main loop, the main loop doesn't detect the second "1". It skips over it and doesn't call the ReadLines function.

Answer 1

Your while conditional in ReadLines is executed 5 times. Once when i == 1 , i == 2 , ..., and i == 5 . On the last execution it finally evaluates to false , but only after getline is evaluated (executed) does i < 5 get evaluated as false . You don't enter the loop body, so the line that was read is discarded.

Swap the order of conditional statements around your && so that i < 5 evaluates first, short circuits and doesn't execute getline when i == 5 .