How to initialize all elements in an array to the same number in C++

Question

I\'m trying to initialize an int array with everything set at -1.

I tried the following, but it doesn\'t work. It only sets the first value at -1.

in         


        

Answer 1

u simply use for loop as done below:-

for (int i=0; i<100; i++)
{ 
a[i]= -1;
}

as a result as u want u can get A[100]={-1,-1,-1..........(100 times)}

Answer 2

I would suggest using std::array. For three reasons:
1. array provides runtime safety against index-out-of-bound in subscripting (i.e. operator[]) operations,
2. array automatically carries the size without requiring to pass it separately
3. And most importantly, array provides the fill() method that is required for this problem

#include <array>
#include <assert.h>

typedef std::array< int, 100 > DirectoryArray;

void test_fill( DirectoryArray const & x, int expected_value ) {
    for( size_t i = 0; i < x.size(); ++i ) {
        assert( x[ i ] == expected_value );
    }
}

int main() {
    DirectoryArray directory;
    directory.fill( -1 );
    test_fill( directory, -1 );
    return 0;
}

Using array requires use of "-std=c++0x" for compiling (applies to the above code).

If that is not available or if that is not an option, then the other options like std::fill() (as suggested by GMan) or hand coding the a fill() method may be opted.

Answer 3

It does work right. Your expectation of the initialiser is incorrect. If you really wish to take this approach, you'll need 100 comma-separated -1s in the initialiser. But then what happens when you increase the size of the array?

Answer 4

Can't do what you're trying to do with a raw array (unless you explicitly list out all 100 -1s in the initializer list), you can do it with a vector:

vector<int> directory(100, -1);

Additionally, you can create the array and set the values to -1 using one of the other methods mentioned.

Answer 5

I'm surprised at all the answers suggesting vector. They aren't even the same thing!

Use std::fill, from <algorithm>:

int directory[100];
std::fill(directory, directory + 100, -1);

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Not concerned with the question directly, but you might want a nice helper function when it comes to arrays:

template <typename T, size_t N>
T* end(T (&pX)[N])
{
    return pX + N;
}

Giving:

int directory[100];
std::fill(directory, end(directory), -1);

So you don't need to list the size twice.

Answer 6

If you really need arrays, you can use boosts array class. It's assign member does the job:

boost::array<int,N> array; // boost arrays are of fixed size!
array.assign(-1);