英文:
Why I can not get the same string after using `seekg`?
问题
I am tring to use seekg to move the pos and then read a line from the .txt.
然而，尽管 pos 相同，使用 seekp 后我无法获得相同的字符串（一个完整的行）。
The compiler is MinGW 11.2.0 64-bit on x86 windows 11
编译器是 MinGW 11.2.0 64 位，运行在 x86 Windows 11 上
Here is the test.txt
以下是 test.txt
Here is my code.
以下是我的代码。
And the output
以及输出
For supplement, here is the string s in memory.
此外，在内存中是字符串 s 的补充信息。
I don't know if it's the problem that there are \ts in the string.
我不知道是否因为字符串中存在 \t 而导致了问题。
So how can I solve this problem and get a completed line using seekg? It would be appreciated if someone can give me a hand.
那么，我该如何解决这个问题，使用 seekg 获取完整的一行呢？如果有人可以帮助我，将不胜感激。
英文:
I am tring to use seekg to move the pos and then read a line from the .txt.
However, I can not get the same string(a completed line) after using seekp, though the pos is same.
The compiler is MinGW 11.2.0 64-bit on x86 windows 11
Here is the test.txt
2 33.10 30.10 29.90 33.00 33.20 33.00 32.90 33.10 40.20 40.20 40.00 39.90 40.30 37.20 40.30 36.70
4 33.00 30.10 29.90 33.00 33.10 32.90 32.90 33.00 40.30 40.20 40.00 40.00 40.30 37.30 40.20 36.70
6 33.00 30.10 29.90 33.00 33.10 33.00 32.90 33.00 40.20 40.30 40.10 40.00 40.20 37.20 40.10 36.90
Here is my code.
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
int main(){
string file_path = "./test.txt";
string s;
// don't use seekg
fstream f(file_path);
getline(f, s);
int p = f.tellg();
cout << "the cur pos is:" << p << endl;
getline(f, s);
cout << s << endl;
getline(f, s);
cout << s << endl;
f.close();
// use seekg
fstream t(file_path);
t.seekg(p, ios::beg);
p = t.tellg();
cout << "the cur pos is:" << p << endl;
getline(t, s);
cout << s << endl;
}
And the output
the cur pos is:139
2 33.10 30.10 29.90 33.00 33.20 33.00 32.90 33.10 40.20 40.20 40.00 39.90 40.30 37.20 40.30 36.70
4 33.00 30.10 29.90 33.00 33.10 32.90 32.90 33.00 40.30 40.20 40.00 40.00 40.30 37.30 40.20 36.70
the cur pos is:139
2.90 33.10 40.20 40.20 40.00 39.90 40.30 37.20 40.30 36.70
For supplement, here is the string s in memory.
2\t33.10\t30.10\t29.90\t33.00\t33.20\t33.00\t32.90\t33.10\t40.20\t40.20\t40.00\t39.90\t40.30\t37.20\t40.30\t36.70
4\t33.00\t30.10\t29.90\t33.00\t33.10\t32.90\t32.90\t33.00\t40.30\t40.20\t40.00\t40.00\t40.30\t37.30\t40.20\t36.70
2.90\t33.10\t40.20\t40.20\t40.00\t39.90\t40.30\t37.20\t40.30\t36.70
I don't know if it's the problem that there are \ts in the string.
So how can I solve this problem and get a completed line using seekg? It would be appreciated if someone can give me a hand.
答案1
得分: 1
请记得指定打开模式，例如 ios::binary。
我认为我已经找到了问题所在。
当我打开文件时，我应该指定模式，例如 ios::binary。
如果没有这个规定，tellg() 返回的偏移量在某些情况下可能不准确。
所以我将
fstream f(file_path);
改为
fstream f(file_path, ios::in | ios::binary);
而 tellg() 的值变小了，这对我有效。
英文:
Remember to specify the open mode, e.g. ios::binary.
I think I have found the problem.
When I open the file, I should specific the mode, e.g., ios::binary.
Without that specification, the offset which tellg() returns may be not accurate under some conditions.
So I change
fstream f(file_path);
to
fstream f(file_path, ios::in | ios::binary);
And the value of tellg() gets smaller, which works for me.
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