What does for(auto &it: a) mean?

Question

I'm a newbie and learning how to use the c++ language in a more flexible way.

In a contest problem I have seen someone write code like this:

#include <bits/stdc++.h>
using namespace std;

int main() {
#ifdef _DEBUG
    freopen("input.txt", "r", stdin);
//  freopen("output.txt", "w", stdout);
#endif
    
    int t;
    cin >> t;
    while (t--) {
        int n;
        cin >> n;
        vector<int> a(n);
        for (auto &it : a) cin >> it;
        sort(a.begin(), a.end());
        bool ok = true;
        for (int i = 1; i < n; ++i) {
            ok &= (a[i] - a[i - 1] <= 1);
        }
        if (ok) cout << "YES" << endl;
        else cout << "NO" << endl;
    }
    return 0;
}

I'm assuming that this line vector<int> a(n); declares a vector and allocates space.

My doubts are:

1. I'm confused on how this for (auto &it : a) cin >> it; works.

2. I also don't understand the expression ok &= (a[i] - a[i - 1] <= 1);.

Answer 1

1.

for (auto &it : a) cin >> it 

means you are taking the input of vector a.

It's similar to:

for(int i = 0 ; i < n; i++){
    cin>>a[i]; 
}

The auto keyword specifies that the type of the variable that is being declared will be automatically deducted from its initializer. So here it takes the references of a[i] and works instead of a[i] as it.

2.

(a[i] - a[i - 1] <= 1)

results boolean 1 (if condition is true) or 0 (if condition is false).

ok &= (a[i] - a[i - 1] <= 1) 

means ok = ok & 1 or ok = ok & 0, depending on the condition true or false.

Answer 2

The syntax:

for (auto& it : a)

is called a range-based loop.

It's used to manipulate each element given in a array or vector and the type of the element will be equivalent to the type of the a.

OTOH, the & states that anything modified to any of the element it in a will be originally replaced and no copy will be made for temporary use (i.e. by reference).

In short words, if an element has value 3 and if you change it to 5, the original array or vector will get affected.

---

ok &= (a[i] - a[i - 1] <= 1);

is a equivalent to isn't a reference to operator, it's bitwise in this context:

ok = ok & (a[i] - a[i - 1] <= 1);

Since the type of ok is a bool, only true (1) or false (0) could be held.

Answer 3

you have a vector<T> x then you want to iteate over every element

you could do in the old "c" style something like:

for (int i=0; i<x.size(); ++i)

but you could also use a range-based loop doing:

for (T i:x)

with the consequence that i is a copy taken from the element in the vector so if you need instead the reference you do:

for (T& i:x)

AND here the answer to your code, since c++11 you can infer the type of i auto-matically ergo the syntax

for (auto& i : x)