这是在C ++中使用for循环的正确方法吗？

Question

I seen weird for loop syntax in C++. Please see following program.

#include <iostream>
using namespace std;

int main()
{
    int num, count = 0;
    int array[]= {1,1,2,3,4,1,3,2,9,8,7};
    cout<<"Which number would you like to know about how many times it appeared?"<<endl;
    cin>>num;

    for (int i:array)
    {
        if (i == num)
            ++count;
    }

    cout<<"Number "<<num<<" appeared "<<count<<" times in the array"<<endl;
    return 0;
}

It is successfully run on GCC Linux platform.

Reference link Here .

So, My question is, Is it the correct way to use for loop in C++?

Also, Is for (int i:array) equivalent to for ( int i:array ; ; ) ?

Sorry for my poor english. Thanks in advance.

Answer 1

There are now (since C++11) two distinct syntaxes for for-loops, the old C-style:

for (initialization; test; increment)

and the new

for (declaration: container)

In the new syntax, the declaration declares a variable which is successively given the value of each element of the container. Common values of "declaration" are auto val , const auto val , auto& val , and const auto& val , depending on whether you want a reference to the value in the container or a copy, and whether you want the value constant or not.

Both syntaxes are correct. It rather depends what you want to do in the loop. My preference is to use the range based for unless I am going to need the loop counter or iterator ... in which case I fall back on the old style for.

See http://en.cppreference.com/w/cpp/language/range-for for the gory details of the specification (and what is meant by "container").

Answer 2

The syntax for (int i:array) iterates through each element in the array, compared to for (int i = 0; i<sizeof(array); i++) which creates a counter that automatically increments on each iteration of the loop. The counter can then be used to access elements of the array with array[i]

As for which one you'd use, it depends on what you want to do. In your example there isn't a need to keep track of which iteration of the loop you are on, so the former will work fine. If you wanted to, say, print the iteration number each time then you would use the latter.

PS your English is perfect :)