Extracting data from a CSV file in C++ - c++

Question

The content of my test csv file looks as follows:
*test.csv*
name;age;weight;height;test
Bla;32;1.2;4.3;True
Foo;43;2.2;5.3;False
Bar;None;3.8;2.4;True
Ufo;32;1.5;5.4;True
I load the test.csv file with the following C++ program that prints the file's content on the screen:
#include <iostream>
#include <vector>
#include <string>
#include <iomanip>
#include <fstream>
#include <sstream>
void readCSV(std::vector<std::vector<std::string> > &data, std::string filename);
void printCSV(const std::vector<std::vector<std::string>> &data);
int main(int argc, char** argv) {
std::string file_path = "./test.csv";
std::vector<std::vector<std::string> > data;
readCSV(data, file_path);
printCSV(data);
return 0;
}
void readCSV(std::vector<std::vector<std::string> > &data, std::string filename) {
char delimiter = ';';
std::string line;
std::string item;
std::ifstream file(filename);
while (std::getline(file, line)) {
std::vector<std::string> row;
std::stringstream string_stream(line);
while (std::getline(string_stream, item, delimiter)) {
row.push_back(item);
}
data.push_back(row);
}
file.close();
}
void printCSV(const std::vector<std::vector<std::string> > &data) {
for (std::vector<std::string> row: data) {
for (std::string item: row) {
std::cout << item << ' ';
}
std::cout << std::endl;
}
}
I have the following questions:
How can I load only rows where, for example, age == 32?
How can I load, for example, only the name, weight columns?
How can I exclude rows that contain None?
How can I skip the first row of the document?
Does it make more sense to extract the desired information after I loaded the entire csv file (if memory is not a problem)? If possible, I want to use only the STL.
Any assistance you can provide would be greatly appreciated

Answer 1

you can try some csv libraries, but if you want to do this with custom code then
Inside printCSV you ask the cin to enter column names
Maintain it in a variable
In this code for (std::vector<std::string> row: data)
Check the item again each of those input when first the loop runs
then inside the second loop keep an index, accordingly you skip the column number
Example code to print only two columns
void printCSV(const std::vector<std::vector<std::string> > &data) {
int col = 0;
std::vector<std::string> column_filter;
std::vector<int> column_index;
column_filter.push_back("name");
column_filter.push_back("weight");
int row1 =0;
for (std::vector<std::string> row: data) {
col = 0;
if(row1==0) {
int col1 = 0;
for (std::string item: row) {
for (std::string colname: column_filter){
if(item.compare(colname)==0) {
column_index.push_back(col1);
}
}
col1++;
}
}
for (std::string item: row) {
int found =0;
for (int index: column_index) {
if(index==col) found = 1;
}
if(found==1)
std::cout << item << ' ';
col++;
}
std::cout << std::endl;
row1++;
}
}
Output
name weight
Bla 1.2
Foo 2.2
Bar 3.8
Ufo 1.5

Answer 2

Before you close. Here all answers in one file. But I will then explain in your single questions then later.
#include <iostream>
#include <vector>
#include <string>
#include <iomanip>
#include <fstream>
#include <regex>
#include <algorithm>
#include <iterator>
// Save typing Work
using Rec = std::vector<std::string>;
std::regex delimiter{ ";" };
// Proxy class for easier input and output
struct Record {
// Our data for one record
Rec data{};
// Overwrite extractor operator
friend std::istream& operator >> (std::istream& is, Record& r) {
// Read on complete line from the input stream, and check, if the read was successfull
if (std::string line{}; std::getline(is, line)) {
// If there is something in our data vector already, delete it
r.data.clear();
// Now, in one statement, split the string into tokens and copy the result into our data vector
std::copy(std::sregex_token_iterator(line.begin(), line.end(), delimiter, -1), {}, std::back_inserter(r.data));
}
return is;
}
// Overwrite inserter for easier output
friend std::ostream& operator << (std::ostream& os, const Record& r) {
std::copy(r.data.begin(), r.data.end(), std::ostream_iterator<std::string>(os,"\t"));
return os;
}
};
// Proxy for the complete CSV file
struct Roster {
// The header
Rec header{};
// All records of the CSV file
std::vector<Record> records{};
friend std::istream& operator >> (std::istream& is, Roster& r) {
// Read on complete line from the input stream, and check, if the read was successfull
if (std::string line{}; std::getline(is, line)) {
// So, we just have read the header
// Now, in one statement, split the string into tokens and copy the result into the header
r.header.clear();
std::copy(std::sregex_token_iterator(line.begin(), line.end(), delimiter, -1), {}, std::back_inserter(r.header));
// Now, in one statement, read all lines, split the string into tokens and copy the result into our record vector
r.records.clear();
std::copy(std::istream_iterator<Record>(is), {}, std::back_inserter(r.records));
}
return is;
}
// Overwrite inserter for easier output
friend std::ostream& operator << (std::ostream& os, const Roster& r) {
std::copy(r.records.begin(), r.records.end(), std::ostream_iterator<Record>(os, "\n"));
return os;
}
};
int main() {
// Open CSV file and check, if it could be opened
if (std::ifstream csvFileStream("r:\\test.csv"); csvFileStream) {
Roster roster{};
// Read the complete CSV file
csvFileStream >> roster;
// Show all read data on std::cout
std::cout << roster;
// All records with age ==32
std::cout << "\n\nAge 32\n";
std::vector<Record> age32{};
std::copy_if(roster.records.begin(), roster.records.end(), std::back_inserter(age32), [](const Record& r) { return r.data[1] == "32"; });
for (const Record& r : age32) std::cout << r << "\n";
// Or
std::cout << "\n\nAge 32 Option 2\n";
csvFileStream.clear(); csvFileStream.seekg(std::ios::beg); age32.clear();
std::copy_if(std::istream_iterator<Record>(csvFileStream), {}, std::back_inserter(age32), [](const Record& r) { return r.data[1] == "32"; });
for (const Record& r : age32) std::cout << r << "\n";
// Get Name and weight columns
std::cout << "\n\nweight and columns\n";
std::vector<std::vector<std::string>> nameAndWeight{};
std::transform(roster.records.begin(), roster.records.end(), std::back_inserter(nameAndWeight),
[](const Record& r) { std::vector<std::string>rec{ r.data[0], r.data[2] } ; return rec; });
for (const std::vector<std::string>& r : nameAndWeight) std::cout << r[0] << "\t" << r[1] << "\n";
// Everything but none
std::cout << "\n\nEverything but none\n";
std::vector<Record> notNone{};
std::copy_if(roster.records.begin(), roster.records.end(), std::back_inserter(notNone), [](const Record& r) { return r.data[1] != "None"; });
for (const Record& r : notNone) std::cout << r << "\n";
}
else {
std::cerr << "\n*** Error: Could not open source file\n";
}
return 0;
}