Returning a vector from a function - c++

Question

So I wrote a class in a header file called: main.h and I want to return the vector after calling the function. So here is the code for the header file:
//main.h
#include <iostream>
#include <vector>
#include <string>
template <typename T1>
class LSR {
public:
std::vector<T1> arr;
LSR(std::vector<T1> ARR) {
arr = ARR;
}
// template <typename T2>
void output() {
for (int i=0; i<arr.size(); i++) {
std::cout << arr[i] << std::endl;
}
}
template <typename T2>
int search(T2 to_find) {
for (int i=0; i<arr.size(); i++) {
if (arr[i] == to_find) { return i; }
}
return -1;
}
template <typename T3>
T1 replace(T3 to_find, T1 to_replace) {
int value = search(to_find);
if (value > -1) {
for (int x=0; x<arr.size(); x++) {
arr[x] = to_replace;
}
return std::vector<T1> arr; <-- error here
}
return NULL;
}
};
//main.cpp
#include "main.h"
int main() {
std::vector<std::string> board = {
"Hello", "there",
"how", "are", "you"
};
LSR<std::string> L1(board);
L1.replace("you", "yeah ya");
std::cout << board[4] << std::endl;
return 0;
}
And I have tried 2 things that I can think of but it didn't work:
just returning arr but that didn't work, it has the following errors:
expected primary-expression before 'arr'
expected ';' before 'arr'
Returning std::vector<T1> only, with the following error:
expected primary-expression before ';' token
I am unsure what I need to do to solve this problem, as I ran out of ideas.

Answer 1

replace() can't return the entire arr. For one thing, the syntax you are using is wrong, return std::vector<T1> arr; would need to be return arr; instead. But replace() returns a T1, and arr is not a T1 but a std::vector<T1> instead, so return'ing arr won't work.
In any case, there is no need to have replace() return anything at all, since it is modifying the contents of this.
Also, replace() is not actually replacing the found element correctly, it is replacing all elements in the arr if any of them match to_find.
You need to get rid of the loop altogether if you want to replace only the 1st matching element, eg:
template <typename T3>
void replace(const T3 &to_find, const T1 &to_replace) {
int value = search(to_find);
if (value > -1) {
arr[value] = to_replace;
}
}
Otherwise, get rid of the search() call if you want to replace all matching elements, eg:
template <typename T3>
void replace(const T3 &to_find, const T1 &to_replace) {
for (size_t x = 0; x < arr.size(); ++x) {
if (arr[i] == to_find) {
arr[i] = to_replace;
}
}
}
That being said, there is no reason to have all of the class methods use their own template parameters, since they all operate on the same type as the main template parameter of the class, so just use that one template by itself for everything, eg:
#include <iostream>
#include <vector>
template <typename T>
class LSR {
public:
std::vector<T> arr;
LSR(const std::vector<T> &ARR) {
arr = ARR;
}
void output() const {
for (size_t i = 0; i < arr.size(); ++i) {
std::cout << arr[i] << std::endl;
}
}
int search(const T &to_find) const {
for (size_t i = 0; i < arr.size(); ++i) {
if (arr[i] == to_find) { return i; }
}
return -1;
}
void replace(const T &to_find, const T &to_replace) {
// see further above...
}
};
Also, the class constructor makes a copy of the board array, so when main() outputs board[4], it is not going to see the result of replace(). main() would have to access the modified arr instead, eg:
#include <iostream>
#include <string>
#include "main.h"
int main() {
std::vector<std::string> board = {
"Hello", "there",
"how", "are", "you"
};
LSR<std::string> L1(board);
L1.replace("you", "yeah ya");
std::cout << L1.arr[4] << std::endl;
return 0;
}
Otherwise, change the class to keep a reference to the board vector, instead of making a copy of it, then board[4] will get modified as expected, eg:
#include <iostream>
#include <vector>
template <typename T>
class LSR {
public:
std::vector<T> &arr;
LSR(std::vector<T> &ARR) : arr(ARR) {}
...
};
#include <iostream>
#include <string>
#include "main.h"
int main() {
std::vector<std::string> board = {
"Hello", "there",
"how", "are", "you"
};
LSR<std::string> L1(board);
L1.replace("you", "yeah ya");
std::cout << board[4] << std::endl;
return 0;
}