c++ – How to loop through an array of elements without knowing its size?

Question:

char peremen_t[]  =   {10, 20, 30, 40, 50, 60, 70, 80, 90, 100};

I need to display all elements of an array. I understand that this needs a loop:

for (int i = 0; i < 10; i++) {
    qDebug() <<"peremen_t[i] " << peremen_t[i];
}

And what if we add several elements, let's say this:

char peremen_t[] =  {10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 101, 102, 103};

Then how to iterate over all these elements, constantly change the condition i < ... ? Here's the actual question: how to iterate over an array of elements without knowing how many elements there are? 🙂

Answer:

There are several approaches. First, the size of an array can be calculated using the formula sizeof( массива ) / sizeof( элемента массива ) . for instance

for ( size_t i = 0; i < sizeof( peremen_t ) / sizeof( *peremen_t ); i++ ) 
{
    qDebug() <<"peremen_t[i] " << peremen_t[i];
}

Second, you can use a for loop based on a range. If you need the index of the element, you can determine it before the loop. For instance,

size_t i = 0;
for ( auto x : peremen_t ) 
{
    qDebug() <<"peremen_t[" << i << "] " << x;
    ++i;
}

You can also use an iterator loop. For instance,

#include <iterator>

// ...

size_t i = 0;
for ( auto it = std::begin( peremen_t ); it != std::end( peremen_t ); ++it, ++i ) 
{
    qDebug() <<"peremen_t[" << i << "] " << *it;
}

And finally, you can write a template function that does the required task. For instance,

template <typename T, size_t N>
inline void f( const T ( &peremen_t )[N] )
{
    for ( size_t i = 0; i < N; i++ ) 
    {
        qDebug() <<"peremen_t[i] " << peremen_t[i];
    }
}

In addition, you can use some boundary value in the array. For example, for character arrays, this could be '\0' , provided that the actual elements of the array cannot contain that character, or some other unique value. For instance,

char peremen_t[]  =   {10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 0 };
//                                                             ^^^ 
for ( size_t i = 0; permen_t[i] != '\0'; i++ ) 
{
    qDebug() <<"peremen_t[i] " << peremen_t[i];
}