Passing array values using pointer - c++

Question

I have two functions one is for file reading and another one for little sorting of numbers. With function read() I am reading file's each line and put each line into array. File looks like:
1
2
3
With function sort() I want to print only numbers with value greater than 1.
What is wrong: I got printed two arrays, but my sort array still printing all values, not only greater than 1.
My code:
#include <iostream>
#include <fstream>
#include <string>
#include <cstdlib>
using namespace std;
class UZD
{
private:
int numLines;
int *value;
public:
UZD();
int * read();
int sort();
};
// =========================================================
UZD::UZD()
{
}
// =========================================================
int * UZD::read(){
ifstream myfile("stu.txt");
int value[20];
string line[20];
int i=0;
while(!myfile.eof() && i < 20) {
getline(myfile,line[i]);
++i;
}
numLines = i;
for (i=0; i < numLines; ++i) {
value[i] = atoi(line[i].c_str());
cout << i << ". " << value[i]<<'\n';
}
return value;
}
// =========================================================
int UZD::sort(){
int *p;
p = read();
int i;
if(p[i] > 1) {
cout << p <<'\n';
}
}
// =========================================================
int main() {
UZD wow;
wow.read();
wow.sort();
}

Answer 1

There are many issues in your code, the most obvious one is "return value" in the read() method. Value is a local array and will be gone once out of scope of read(). Also the design seems faulty. You are calling read() twice, once from the main() and again internally from sort().
I have written a working code, using vectors. Probably this is what you are expecting:
#include <iostream>
#include <fstream>
#include <string>
#include <cstdlib>
#include <vector>
using namespace std;
class UZD
{
private:
int numLines;
vector<int> value;
public:
UZD();
vector<int> & read();
void sort();
};
// =========================================================
UZD::UZD()
{
}
// =========================================================
vector<int> & UZD::read(){
ifstream myfile("stu.txt");
vector<string> line(20);
int i=0;
while(!myfile.eof() && i < 20) {
getline(myfile,line[i]);
++i;
}
numLines = i;
cout << "After reading file: " << endl;
for (i=0; i < numLines; ++i) {
value.push_back(atoi(line[i].c_str()));
cout << i << ". " << value[i]<<'\n';
}
return value;
}
// =========================================================
void UZD::sort(){
cout << "Inside sort()" << endl;
for(int i=0; i<value.size(); ++i){
if(value[i] > 1)
cout << value[i] << endl;
}
}
// =========================================================
int main() {
UZD wow;
wow.read();
wow.sort();
return 0;
}
I have kept the variable names same for clarity. Let me know if you don't get anything.

Answer 2

There are a lot of issues in your program; just to mention some of them:
In sort, you use variable i uninitialized, which is undefined behaviour (probably crashes); You write while(!myfile.eof()..., which is usually considered wrong (see this SO answer; The use of atoi is not recommended, as it is not safe if the parameter does not represent a number; You return a pointer to a local variable, which is destroyed one going out of scope; You declare member variables, but pass through local ones (e.g. value)...
See the following code which demonstrates the usage of int* and a vector<int>; hope it helps.
class UZD
{
private:
int numLines;
int *value = nullptr;
public:
~UZD() {
if (value)
delete value;
};
void read();
void print();
};
// =========================================================
void UZD::read(){
ifstream myfile("stu.txt");
value = new int[20];
int val;
numLines = 0;
while(numLines < 20 && myfile >> val) {
value[numLines] = val;
numLines++;
}
}
// =========================================================
void UZD::print(){
for (int i=0; i<numLines; i++)
cout << value[i] << endl;
}
class UZD_vector
{
private:
vector<int> values;
public:
void read();
void print();
};
// =========================================================
void UZD_vector::read(){
ifstream myfile("stu.txt");
int val;
while(myfile >> val) {
values.push_back(val);
}
}
// =========================================================
void UZD_vector::print(){
for (auto val : values)
cout << val << endl;
}
// =========================================================
int main() {
cout << "with int[]:" << endl;
UZD wow;
wow.read();
wow.print();
cout << "with vector:" << endl;
UZD wow_vector;
wow_vector.read();
wow_vector.print();
}

Answer 3

Here is your own code rectified, just in case you find vectors too difficult to learn (which should not be true, though)
#include <iostream>
#include <fstream>
#include <string>
#include <cstdlib>
using namespace std;
class UZD
{
private:
int numLines;
int *value;
int num;
public:
UZD();
void read();
void sort();
};
// =========================================================
UZD::UZD():num(20)
{}
// =========================================================
void UZD::read(){
ifstream myfile("stu.txt");
value = new int[num];
string line[num];
int i=0;
while(!myfile.eof() && i < num) {
getline(myfile,line[i]);
++i;
}
numLines = i;
for (i=0; i < numLines; ++i) {
value[i] = atoi(line[i].c_str());
cout << i << ". " << value[i]<<'\n';
}
}
// =========================================================
void UZD::sort(){
cout << "After sorting: " << endl;
for (int i = 0; i < num; ++i) {
if(value[i] > 1)
cout << value[i] << endl;
}
}
// =========================================================
int main() {
UZD wow;
wow.read();
wow.sort();
return 0;
}