I have a .csv file with integers in two columns and many rows. It looks like this:
A B
1 584 146
2 586 167
. . .
. . .
I want to create a 2D array of integers from this .csv file in C++. I've searched dozens of websites but all of the answers I could get was with strings.
This is how the raw data looks like:
584,146
586,167
588,189
The easy way to approach the problem is to recognize that you can read the entire line, and then iterate over the line as a stringstream using getline with a ',' as the delimiter to separate the values.
(stringstream is required, you can't simply separate the values while reading the line itself with getline -- there would be no obvious end of read at the end of the line -- you would simply read the next value beginning on the next line)
However, if you read the line first, create a stream, and then parse the stream, the read stops at the end of the stream providing you a way to detect the last value in a row, e.g. using a vector to store the values converted to int with std::stoi, you could fill each row vector as follows:
std::string line, val; /* string for line & value */
...
while (std::getline (f, line)) { /* read each line */
std::vector<int> v; /* row vector v */
std::stringstream s (line); /* stringstream line */
while (getline (s, val, ',')) /* get each value (',' delimited) */
v.push_back (std::stoi (val)); /* add to row vector */
For your "2D" array use, you actually want a vector of row vectors or a std::vector<std::vector<int>>. So declaring your array as:
std::vector<std::vector<int>> array; /* vector of vector<int> */
you can complete your read of row into the array simply by pushing each row vector v back into your array, e.g. (with declarations and loop in context)
std::string line, val; /* string for line & value */
std::vector<std::vector<int>> array; /* vector of vector<int> */
while (std::getline (f, line)) { /* read each line */
std::vector<int> v; /* row vector v */
std::stringstream s (line); /* stringstream line */
while (getline (s, val, ',')) /* get each value (',' delimited) */
v.push_back (std::stoi (val)); /* add to row vector */
array.push_back (v); /* add row vector to array */
}
All that remains is accessing each value so you can make use of them. The range-based loop provides just what you need (actually a nested pair of range based loops, one to iterate over the vectors, the second to iterate over values. For example, you could do:
for (auto& row : array) { /* iterate over rows */
for (auto& val : row) /* iterate over vals */
std::cout << val << " "; /* output value */
std::cout << "\n"; /* tidy up with '\n' */
}
Putting all the pieces of the puzzle together, you could do something similar to:
#include <iostream>
#include <fstream>
#include <sstream>
#include <vector>
int main (int argc, char **argv) {
std::ifstream f;
if (argc > 1) { /* if argument given */
f.open (argv[1]); /* open file with filename as argument */
if (! f.is_open()) { /* validate file open for reading */
std::cerr << "error: file open failed '" << argv[1] << "'.\n";
return 1;
}
}
else { /* no argument given, error show usage */
std::cerr << "error: insufficient input. <filename> required.\n";
return 1;
}
std::string line, val; /* string for line & value */
std::vector<std::vector<int>> array; /* vector of vector<int> */
while (std::getline (f, line)) { /* read each line */
std::vector<int> v; /* row vector v */
std::stringstream s (line); /* stringstream line */
while (getline (s, val, ',')) /* get each value (',' delimited) */
v.push_back (std::stoi (val)); /* add to row vector */
array.push_back (v); /* add row vector to array */
}
for (auto& row : array) { /* iterate over rows */
for (auto& val : row) /* iterate over vals */
std::cout << val << " "; /* output value */
std::cout << "\n"; /* tidy up with '\n' */
}
}
Example Input File
Which given your input file of:
$ cat dat/nums.csv
584,146
586,167
588,189
Example Use/Output
It would be parsed and stored as a vector of integer vectors (providing plain-old 2D array access).
$ /bin/vect2d_strstream dat/nums.csv
584 146
586 167
588 189
Look things over and let me know if you still have questions.
You should start with reading each line with std::getline, store the line in a std::stringstream, then read the individual tokens using the overloaded std::getline that takes a delimiter.
You can then parse these tokens to integers using std::stoi, and store them in a std::vector< std::pair<int, int> >.
A good explanation with a working code sample can be found here.
