//GLobal Variables
uint8_t my_array[ MAX ];

...

int manipulateArray( uint8_t value )
{
    for( uint8_t i=0; i<MAX; i++ )
    {
         if( -1 == my_array[ i ] )
              my_array[ i ] = value;
    }
}

//GLobal Variables
uint8_t my_array[ MAX ];
uint8_t idx = 0;

...

int manipulateArray( uint8_t value )
{
    my_array[ idx++ ] = value;
}