C++ Primer 1.4.4. Can't quite understand the need of first IF statement

Question

I'm new here, so please, bear with me. I'm currently trying to learn c++ via C++ Primer 5th edition, and as I was looking at SO about a doubt I had on section 1.4.4 (which I managed to find the answer), I realized I don't understand the need or purpose of the first IF statement on this code:

#include <iostream>
int main()
{    
int currVal = 0, val = 0;

if (std::cin >> currVal) 
{
    int cnt = 1;  
    while (std::cin >> val) 
    { 
        if (val == currVal)   
            ++cnt;            
        else 
        { 
            std::cout << currVal << " occurs " << cnt << " times" << std::endl;
            currVal = val;    
            cnt = 1;          
        }
    }          
    std::cout << currVal <<  " occurs " << cnt << " times" << std::endl;
} 
return 0;
}

So I tried to change it in a way that looks more "logical" to me in order to try and understand the need for that IF, and ends up that the program seems to work exactly the same way...here's the modification:

#include <iostream>
int main()
{   
int currVal = 0, val = 0, cnt=1;    
std::cin >> currVal;         
    while (std::cin >> val) { 
        if (val == currVal) 
            ++cnt; 
        else { 
            std::cout << currVal << " occurs "
                << cnt << " times" << std::endl;
            currVal = val; 
            cnt = 1; 
        }
    }         
    std::cout << currVal << " occurs "
        << cnt << " times" << std::endl;     
return 0;
}

Can anybody be so kind and explain it for me? I would Appreciate it.

Answer 1

In case of an empty input or non numerical input, your second version will print

std::cout << currVal << " occurs "
    << cnt << " times" << std::endl;

With currVal having a value of 0 and a count of 1, which is wrong.

See: operator>>

Answer 2

std::cin >> currVal is an expression that returns an istream reference.

The istream has an implicit conversion to bool provided by an overloaded operator. This evaluates to false if there are no more data to be read.

The if is exploiting that. If you omit this then your program will not run correctly since currVal will be set to zero.