Creating array after getting variable in other function - c++

Question

A text file looks like this:
3
String
String2
String3
I must create a function to read all the strings from text file, save them to array and then display it in main function. For example
void reading(int & count, string strings[]) {
ifstream fd "text.txt";
fd >> count;
for (int i=0; i<count; i++)
fd >> strings[i];
fd.close();
And main function:
int main() {
int count=0;
string strings[100];
reading(count, strings);
for (int i=0; i<count; i++)
cout << strings[i] << endl;
The number of strings is written in first line of text file. How can I create an array of exactly that number? I need it to be able to access it in main/other functions. (For example function for writing into another text file).

Answer 1

In case there is a super important reason to do that with an array, then use std::new, like this:
#include <iostream>
#include <string>
#include <vector>
#include <fstream>
using namespace std;
int reading(string** str_array, int& size, string filename) {
ifstream infile(filename.c_str());
if(!infile) {
cerr << "Unable to access file!\n";
return -1;
}
int n = 0;
infile >> size;
try{
*str_array = new string[size];
string str;
for (; n < size && infile; ++n) {
infile >> str;
(*str_array)[n] = str;
}
if (n != size)
cerr << "ERROR, read less than " << size << " strings!!\n\n";
} catch(bad_alloc& exc) {
return -2;
}
return 0;
}
int main() {
string* str_array = NULL;
int size;
if(reading(&str_array, size, "test.txt")) {
cerr << "Din't read file, exiting...\n";
return -1;
}
for(int i = 0; i < size; ++i)
cout << str_array[i] << endl;
delete [] str_array; // DO NOT FORGET TO FREE YOUR MEMORY
str_array = NULL;
return 0;
}
Output:
C02QT2UBFVH6-lm:~ gsamaras$ ./a.out
String
String2
String3
However, you are in c++ and you are not using an std::vector for this?
Look how simple it is with that:
#include <iostream>
#include <vector>
#include <fstream>
using namespace std;
int reading(vector<string>& v, string filename) {
ifstream infile(filename.c_str());
if(!infile) {
cerr << "Unable to access file!\n";
return -1;
}
int N = -1, n = 0;
infile >> N;
string str;
for (; n < N && infile; ++n) {
infile >> str;
v.push_back(str);
}
if (n != N)
cerr << "ERROR, read less than " << N << " strings!!\n\n";
return 0;
}
int main() {
vector<string> v;
if(reading(v, "test.txt")) {
cerr << "Din't read file, exiting...\n";
return -1;
}
for(size_t i = 0; i < v.size(); ++i)
cout << v[i] << "\n";
return 0;
}
Output:
C02QT2UBFVH6-lm:~ gsamaras$ ./a.out
String
String2
String3
Edit:
We have to pass a pointer to what we want to modify (that is, the string*), otherwise the changes we apply won't take place. Test it yourself, pass a string* as a parameter instead of string**, modify the body of the function and she what happens.
To get the idea, imagine we want to write to the pointer, the new address, which new gave us and holds the memory requested. We do write that address inside the function, but when the function terminates, we want the changes to be persistent. See my Functions in C as an example.

Answer 2

Allocate an array on the heap, like this:
std::string* stringArr = new std::string[(place amout of strings here)];
and don't forget to delete it at the end of main()
delete stringArr[];
Variables / arrays on the heap are dynamic so the size doesn't have to be fixed!
std::string* → This is a pointer that points to the address of the beginning of this array in your memory. (Just to let your computer know where it is)
stringArr → the name of the array
new → allocates new memory
std::string[size] → says how much to allocate
I've seen some answers that were talking about "vectors". If you wanna use them you could have a look at this page for more info! →
Vector documentation

Answer 3

use a vector
#include <vector>
#include<fstream>
#include <iostream>
using namespace std;
void reading(int & count, vector<string> * strings) {
ifstream fd("text.txt");
fd >> count;
string T;
for (int i=0; i<count; i++)
{
fd >> T;
strings->push_back(T);
}
fd.close();
}
int main() {
int count=0;
vector<string> strings;
reading(count, &strings);
for (int i=0; i<count; i++)
cout << strings[i] << endl;
}