How to read a delimited text file and load the data into arrays in C++

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Hello, I am stuck trying to convert a delimited text file with five columns of data structured like this:

col1 col2 col3 col4 col5
#1   #1    #1   #1   #1
#2   #2    #2   #2   #2
#3   #3    #3   #3   #3
...

I can print out each line of the text file easily using the code below (which I found on the internet). The problem is that each line is interpreted as a string, and I need each number in each column to be a float variable in an array. Does anyone know how to do this?

What I have tried:

C++

ifstream myfile ("cmb.txt");
  if (myfile.is_open())
  {
    while(getline(myfile, line)) {
      cout << line << endl;
    }
    myfile.close();
  }

Posted 23-Apr-24 6:22am

juliet6

Updated 23-Apr-24 6:53am

PIEBALDconsult

v2

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PIEBALDconsult 23-Apr-24 12:55pm

fscanf ?
strtok and atof ?

2 solutions

Solution 2

Take a look at std::basic_istream<CharT,Traits>::operator>> - cppreference.com[^]
Look at the example code, which shows you how to read a value from a file into a variable. This assumes that your data (#1, #2, #3 etc) are all normally formatted numbers, separated by white space (space, tab, etc), otherwise you'll have to massage your data on read. There's also an assumption here that you know how many rows and columns you have, so can arrange to pre-size (either dynamically using new or a smart pointer, or statically with something like double data[100][100]
If you don't know the rows and columns ahead of time, you might want to consider using a vector of vectors (e.g. vector<vector<double>>. In this case you'd use getline() to read in a line of text from the file, parse out the columns (see std::basic_stringstream - cppreference.com[^] on how you might do that) to construct the row vector, and then push_back() the row vector into the enclosing matrix.

Posted 23-Apr-24 7:51am

k5054

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CPallini · Accepted Answer · 2024-04-23T07:31:00

Try

C++

#include <fstream>
#include <iostream>
#include <array>
#include <vector>
using namespace std;


using Row = array<double, 5>;

int main()
{
  vector < Row > v;

  ifstream ifs("cmb.txt");

  if (ifs)
  {
    string line;
    getline(ifs, line); // read and discard the header
  }
  for (;;)
  {
    Row row;
    for (auto & d : row)
    {
      ifs >> d; // read all the items of the row
    }
    if ( !ifs) break;
    v.push_back(row); // append the row to the vector
  }

  // print a sample item
  cout << "v[2][2] = " << v[2][2] << "\n";
}