How can the Lambda expression be modified to put string in text file?

[leelion]

Dear all,

I have a code, with that I can show the out put on the screen. But, I want to put the output in a text file. Frankly, I read about the Lambda expression, but it is hard for me to understand its function (as I am beginner).

  #include <algorithm>
#include <fstream>
#include <iostream>
#include <string>
using namespace std;
#include <vector>
// if the dimensions are constant, then you could use std::array instead of std::vector.
using Matrix = std::vector<std::vector<float>>;

void addRow(Matrix& M, const int rowNum, const int id, const float type, const float x, const float y,const float z) {
    M[rowNum][0] = id;
    M[rowNum][1] = type;
    M[rowNum][2] = x;
    M[rowNum][3] = y;
    M[rowNum][4] = z;
}

int main() {
    string line;
    float x0, y0, z0, x1, y1, z1, type;
    int NUMBER_line=0, id=0,NUMBER_line1=0,NUMBER_line3=0;
	
	ofstream secondoutput;					// the output text file
	secondoutput.open ("secondoutput.txt");	// the output text file
	
    ifstream myfile("1.lammpstrj");
    string namefile;
    if (myfile.is_open()) {
        for (int lineno = 0; getline(myfile, line) && lineno < 7; lineno++) {
            if (lineno == 2)  myfile >> NUMBER_line;
        }
        cout << " NUMBER_line: " << NUMBER_line << endl;

        Matrix Input0(NUMBER_line, std::vector<float>(5));
        for (float linenoq = 0; getline(myfile, line) && linenoq < NUMBER_line; linenoq++) {
            myfile >> id >> type >> x0 >> y0 >> z0;
            addRow(Input0, linenoq, id, type, x0, y0, z0);
			
        }


		for (int i=0; i <3; i++){
			for (int lineno = 0; getline(myfile, line) && lineno < 7; lineno++) {
				if (lineno == 2)  myfile >> NUMBER_line1;
			}
			
			
			Matrix Input1(NUMBER_line1, std::vector<float>(5));
			for (int linenoq = 0; getline(myfile, line) && linenoq < NUMBER_line1; linenoq++) {
				myfile >> id >> type >> x1 >> y1 >> z1;
				addRow(Input1, linenoq, id, type, x1, y1, z1);
				
			}
		
			Matrix Output(NUMBER_line, std::vector<float>(5));
			for (size_t row = 0; row < Output.size(); ++row) {
				for (size_t col = 0; col < Output[0].size(); ++col) {
					if (col < 2) {
						Output[row][col] = Input0[row][col];
					}
					else {
						Output[row][col] = Input1[row][col] - Input0[row][col] ;
					}
				}
			}
			
			std::for_each(Output.cbegin(), Output.cend(),
			[](auto const& row) {
				std::for_each(row.cbegin(), row.cend(),
					[](const float value) { 
						cout << value << " "; 
						secondoutput << value << " "; // the output text file
						});
				cout << endl;
				secondoutput << "\n" ; // the output text file
				 
			});
		}
		secondoutput.close();
	}
    else cout << "Unable to open file";
    return 0;
}

Could I ask to help me? I want to have a output text file like what the output writes on the screen.

Thanks in advance for any help

regards

[jonnin]

lambdas are 'tiny functions' and are typically used, for the most part, as parameters to functions that accept functions. A simple example of this is std::sort which can accept a user-defined comparison operator (this can be used to sort the data in decending instead of acending order, among other things). The comparison function is typically very small (1-2 lines in most cases) and is a good candidate for a lambda.

normal, named functions are a better way to do larger, reusable tasks, like writing to a file.

Why do you want to do this in a lambda?

[leelion]

Dear jonnin,
I am not a programmer, just have an output from a software and want to do a post-process on it. this code can help me, but I need to put its output to another file ("secondoutput.txt"). then, I can use the new file for obtaining another results.
If you could help me, it would be appreciated.

[leelion]

It seems the most important matters are about the the place where I put the "secondoutput << value << " ";" and "secondoutput << "\n" ;"

I don't know where they should be. I always put "secondoutput << value << " ";" in other codes, and they work well, and give me an output text file. But now, I don't know why they don't work.

[jonnin]

you can dump the output of a program to text directly. Is that helpful?
from the console, in both unix and windows, you can do it like this:
program > filename
unix can do that or it can skip writing a real file and just hand the data off to the next program directly (would feed cin statements I think, I don't do a lot of this).

that aside, what you have is correct, except stop trying to make a lambda from it. You don't need that here.

Just put the loops of the lambda right where you have the lambda as if normal code ...

[leelion]

yes really, that is enough for me.
I use cygwin, then I turn the C++ code to an .exe file.
you mean I just add "program > filename" at the end of my code?
then, after running the .exe file, it gives me a text file with the name of "filename".

[jonnin]

no, not in the code.
at your windows console where you run your program, do it there:
programname.exe > filename.txt
but you can leave the .exe off and .txt is just a suggestion so you can open it easier in a text editor

the operating system does the work for you.
are you using the unix shell or a windows shell? I do cygwin too but from a windows shell.

both unix and windows provide these:
> direct the output of a program to a text file.
< direct a text file as input to the program as if typed at the keyboard
>> same as > but appends instead of overwrites the file

and unix also provides pipes but I am rusty with them. Windows may too, now, since it has picked up a posix shell of sorts. I don't know. If you want to play with this stuff we can google around on it.

[leelion]

Dear jonnin,
so many thanks for your help.
It works for me. And now I can have an output, and it shows exactly what I tried to attain.