C++ Primer ex 7.12

[arnuld]

it compiles and runs and at one place it produces strange output becaus
eof a semantic-bug. i can't find it:
/* C++ Primer - 4/e
*
* exercise 7.12
* STATEMENT:
* write a programme to calculate the sum of the elements in an
array. write th e function 3 times.each time using a different approach
to manage the array bounds.
*
*/
#include <iostream>

/* i wanted both parameters, the pointer itself the int to which the
pointer points, to be const but for me this "const" thing in C++ is too
complex to be understood :( , any help on that */

/* Method-1: using Pointers (like iterators) */
int print_sum_iter( const int *pbegin, const int *pend ) {
int sum = 0;
while( pbegin != pend )
{
sum += *pbegin++;
}

return sum;
}

/* Method-2: using indexing */
int print_sum_index( const int ia[], size_t arr_size ) {
int sum = 0;
for( size_t i=0; i != arr_size; ++i)
{
sum += ia[i];
}

return sum;
}
/* Method-3: using NULL terminator */

int print_sum_NULL( const int *parr ) {
int sum = 0;
while( *parr != '\0' )
{
sum += *parr++;
}

return sum;
}
int main()
{
const int arr_size = 3;
int ia[arr_size] = { 0, 2, 4 };

std::cout << "print_sum_iter: "
<< print_sum_iter( ia, ia + arr_size ) << "\n";

std::cout << "print_sum_index: "
<< print_sum_index( ia, arr_size )
<< "\n";

std::cout << "print_sum_NULL: "
<< print_sum_NULL( ia )
<< std::endl;

return 0;
}

/* OUTPUT
~/programming/cpp $ g++ -ansi -pedantic -Wall -Wextra ex_07-12.cpp
~/programming/cpp $ ./a.out
print_sum_iter: 6
print_sum_index: 6
print_sum_NULL: 0
~/programming/cpp $

*/

--
http://arnuld.blogspot.com

[Ian Collins]

arnuld wrote:

it compiles and runs and at one place it produces strange output becaus
eof a semantic-bug. i can't find it:

int print_sum_NULL( const int *parr ) {
int sum = 0;
while( *parr != '\0' )
{
sum += *parr++;
}

return sum;
}
int main()
{
const int arr_size = 3;
int ia[arr_size] = { 0, 2, 4 };
std::cout << "print_sum_NULL: "
<< print_sum_NULL( ia )
<< std::endl;

The first element in ia is 0, so the loop end condition in
print_sum_NULL will be true.

--
Ian Collins.

[Frank Birbacher]

Hi!

arnuld schrieb:

it compiles and runs and at one place it produces strange output becaus
eof a semantic-bug. i can't find it:
/* C++ Primer - 4/e
*
* exercise 7.12
* STATEMENT:
* write a programme to calculate the sum of the elements in an
array. write th e function 3 times.each time using a different approach
to manage the array bounds.
*
*/
#include <iostream>

/* i wanted both parameters, the pointer itself the int to which the
pointer points, to be const but for me this "const" thing in C++ is too
complex to be understood :( , any help on that */

You have to read types from right to left:
1. "int const *p" - p is a pointer to a const int
2. "const int *p" - p is a pointer to int which is const (same as 1)
3. "int *const p" - p is a const pointer to int
4. "int const *const p" - p is a const pointer to const int

/* Method-3: using NULL terminator */

int print_sum_NULL( const int *parr ) {
int sum = 0;
while( *parr != '\0' )
{
sum += *parr++;
}

return sum;
}

'\0' is a character constant, value 0
0 is an integer constant, value 0
NULL is a macro for an integer constant, value 0

This way your function find the first "0"-element in the array and
terminates. Also note:

const int arr_size = 3;
int ia[arr_size] = { 0, 2, 4 };

Your array has exactly three elements. There will be no extra "NULL" at
the end. This is different to string litterals. They get an extra 0 char
at the end:

char text[] = "Hallo";

The "text" array has 6 (!) elements because of the extra 0.

So finding the end of an array by a sentinel (this is what your approach
is called) is usually avoided because it can break so easily. Suppose
you hadn't had a zero in your array at all. Your loop would happily
search through your whole memory to find a zero.

Frank