Know if value is in the unparalleled equality array!

Question

I have the following code in C++:

#include <iostream>
using namespace std;

    int main (void){

           int e[5] = {10,20,30,40,50};       

           for (int i = 0; i <= 99; i++){

                if ( i in e ) cout << e[i] ;

           }
    }

I have an array with 5 numbered elements from 0 to 4 I have a bond for with 100 elements numbered from 0 to 99

In each loop of the for if for the number in question, there is array e[5], a combination.

In other words, to know if it exists in the array (IN) a combination

In PHP, just do

if ( i in e ) echo e[i] ;

But I don’t know how to do it in C++.

Answer 1

First, apart from the cout this code is C and not C++, almost everyone learns to do C++ the wrong way. And if doing it in C++ would be as simple as PHP, it would just use a ready-made function like find() for example. But if you want it the hard way would be:

#include <iostream>
using namespace std;
 
int main () {
    int e[5] = {10, 20, 30, 40, 50};       
    for (int i = 0; i < 100; i++) {
        for (int j = 0; j < 5; j++) {
            if (e[j] == i) {
                cout << i;
                break;
            }
        }
    }
}

Behold working in the ideone. And in the repl it.. Also put on the Github for future reference.

Or if that’s right (and then the way you do it in PHP is bad) you can do it in a smarter and more performative way:

#include <iostream>
using namespace std;
 
int main () {
    int e[5] = {10, 20, 30, 40, 50};       
    for (int i = 0; i < 5; i++) if (e[i] >= 0 && e[i] < 100) cout << i;
}

Behold working in the ideone. And in the repl it.. Also put on the Github for future reference.

Answer 2

Complementing what @Maniero has already said, you can use std::find which is part of the library algorithms to search for the element. O find receives 3 values, the initial, final element, and the value to be searched.

Transposing this into your code would look like this:

#include <iostream>
#include <algorithm> // inclusão do header associado
using namespace std;

int main (void){

       int e[5] = {10,20,30,40,50};       

       for (int i = 0; i <= 99; i++){
            if (find(e, e + 5, i) != e + 5) cout << i << " "; 
            //   ^---^----^----^---- utilização do find
       }
}

See this example in Ideone

In this example the initial value was defined by the pointer to the first element with e, and the end set also with a pointer, but advancing the amount of elements sufficient to stay after the last, the e + 5.

Like the find returns a pointer to the found element, or to the end if there is no such element, you must compare whether that pointer is at the end.

A more natural use of this language is to store the search result in a pointer/iterator and then use it when you want to access the found element:

for (int i = 0; i <= 99; i++){
    int *pesq = find(e, e + 5, i);
    if (pesq != e + 5) cout << *pesq << " ";
    //                           ^--- imprime o valor encontrado com base no ponteiro devolvido

}

See this example also in Ideone

As indicated by @Marioferoldi in comment, you can also use std::begin and std::end to get the start and end of a normal array. This ends up making the code more flexible as it defines only the size in one location:

int e[5] = {10,20,30,40,50};       

for (int i = 0; i <= 99; i++){
    int *pesq = find(begin(e), end(e), i);
    //                 ^--------^
    if (pesq != end(e)) cout << *pesq << " ";
    //           ^---
}

Check it out at Ideone

Another possible and common solution in the world of C++ and not C, is to use a vector to store the integers, which slightly changes the way the find has to be used:

#include <iostream>
#include <algorithm>
#include <vector> //vector também necessário agora

using namespace std;

int main (void){

       vector<int> e = {10,20,30,40,50};       

       for (int i = 0; i <= 99; i++){
            if (find(e.begin(), e.end(), i) != e.end()) cout << i << " "; 
       }
}

Example in Ideone

Now the beginning and end of the research was built based on the method begin and the end of vector which return iterators to the corresponding elements.

Note: the code examples given in this reply have been adapted from documentation