How can I solve this problem, i need to export a line from a text file?

Question

I have an input file like this:

   Virtual   (A) (A) (A) (A) (A) (A) (A) (A) (A) (A) (A) (A)

The electronic state is 1-A.

Alpha  occ. eigenvalues --   -0.26426  -0.26166  -0.25915  -0.25885  
Alpha  occ. eigenvalues --   -0.25284  -0.25172  -0.24273  -0.23559  
Alpha  occ. eigenvalues --   -0.20078  -0.19615  -0.17676  -0.10810  
Alpha virt. eigenvalues --   -0.07062  -0.06520  -0.05969  -0.01767  
Alpha virt. eigenvalues --   -0.01604  -0.00951  -0.00428   0.00041

I would like to export the first line obtaining first 11 characters " Alpha virt.". How should I do? I code by C++ language as below code, but it cant finish while loop functio. I dont know why, I am a fresher. Please help me. Thank you so much. My C++ code:

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

#define FILENAME "filelog.txt"

using namespace std;

int main(void) {
    char* line_buf = NULL;
    size_t line_buf_size = 0;
    int line_count = 0;
    string s;
    std::string dongsosanh = " Alpha virt.";
    FILE* fp = fopen(FILENAME, "r");
    getline(&line_buf, &line_buf_size, fp);
    std::string STRR(line_buf, 11);
    do {
        line_count++;
        getline(&line_buf, &line_buf_size, fp);
    } while(STRR.compare(dongsosanh) != 0);
    std::cout << STRR << endl;
    return 0;
}

Thank you so much.

Indentation *please!* It makes your code so much easier to read. — Some programmer dude, Mar 22 '19 at 13:22
As a hint about your problem: You never modify `STRR` in your loop. You would be able to see it easily if you [learned how to debug your programs](https://ericlippert.com/2014/03/05/how-to-debug-small-programs/). — Some programmer dude, Mar 22 '19 at 13:24
You don’t need any code, you can just use `awk`, `grep` or `FINDSTR`. — Mark Setchell, Mar 22 '19 at 13:27
@Someprogrammerdude so I must define STRR again? I am sorry for my fool question, but i am not an IT-er so I dont really know this. — Lê Quang Trung, Mar 22 '19 at 13:27
I'm sorry for maybe sounding harsh, but if you want to use a programming language you need to learn it first. It's just like with a spoken language, to be able to use it, even for simple things, one need to learn it first. [Here's a list of good books](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list/388282#388282), please get a couple to read. — Some programmer dude, Mar 22 '19 at 13:36
But it's unclear what you want. `Alpha virt` isn't in the first line so your loop will exit after having read the first line. — Ted Lyngmo, Mar 22 '19 at 13:36
`grep -m1 '^Alpha virt\.' filelog.txt` should do it on Linux/macOS. — Mark Setchell, Mar 22 '19 at 16:07
@Someprogrammerdude I will take a look. Thank for your sharing. — Lê Quang Trung, Mar 23 '19 at 01:54

Scamtex · Answer 1 · 2019-03-23T08:28:11.123

0

You could just do this:

std::ifstream input(FILENAME);
std::string line;

while(std::getline(input, line)) {
    if(line.substr(0, 11) == "Alpha virt.") {
        std::cout << line << endl;
        return 0;
    }
}

EDIT: added the return statement to make sure only the first line starting with 'Alpha virt.' is printed.

edited Mar 23 '19 at 08:28

answered Mar 22 '19 at 15:16

Scamtex

181
1
9

I tried but it printed all line containing "Alpha virt.". I need just the fisrt line that contain this one. Thank you so much. – Lê Quang Trung Mar 23 '19 at 01:53

score 0 · Answer 2 · answered Mar 22 '19 at 15:59

Many problems with your program:

line_buf - does not have memory allocated, undef behaviour

line_count - is 0, nothing will be red

You are not closing file at the end.

" Alpha virt." - this line will never be found, it has space at the begining.

STRR is never updated after line has been red, endless loop

Working solution:

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

#define FILENAME "filelog.txt"

using namespace std;

int main(void) {
    const std::string dongsosanh = "Alpha virt.";
    char* line_buf = new char[100];
    size_t line_buf_size = 100;
    int line_count = 0;
    string s;
    FILE* fp = fopen(FILENAME, "r");
    do {
        line_count++;
        getline(&line_buf, &line_buf_size, fp);
        std::cout << line_buf;
    } while(dongsosanh.compare(0, 11, line_buf, 11));
    free(line_buf);
    fclose(fp);
    return 0;
}

This is to show how it works in your case, but you should use vector instead of char* line_buf.

This is all I need, Thank you so much bro. – Lê Quang Trung Mar 23 '19 at 01:52 — Lê Quang Trung, Mar 23 '19 at 01:52

How can I solve this problem, i need to export a line from a text file?

2 Answers2