Jumping into C++ Chapter 13 Practice Prob No4 - Pointers - c++

Question

I've having trouble understanding the wording of this question and what it means by returning the second value through a pointer parameter?
The problem is:
Write a function that takes input arguments and provides two seperate results to the caller, one that is the result of multiplying the two argumentsm the other the result of adding them. Since you can directly return only one value from a funciton you'll need the seecond value to be returned through a pointer or references paramter.
This is what I've done so far.
int do_math(int *x, int *y)
{
int i =*x + *y;
int u = *x * *y;
int *p_u = &u;
return i;
}
void caller()
{
int x = 10;
int y = 5;
std::cout << do_math(&x, &y);
//std::cout << *u;
}

Answer 1

I think all they're wanting you to do is to demonstrate your understanding of the difference between passing arguments by value and passing them by reference.
Here is a sample code that shows that although my function is only returning one value "i = X+Y", It is also changing the value of Y to (Y * X).
Of course if you do need Y's value to stay unchanged, you could use a third variable that is equal to Y's value and pass its reference as an extra argument to your function.
You could run the code bellow to see what's happening to X and Y before and after calling the function.
Hope this helps.
#include <iostream>
using namespace std;
int do_math(int value1, int *pointer_to_value2)
{
int i = value1 * *pointer_to_value2;
*pointer_to_value2 = *pointer_to_value2 + value1; // y changes here
return i;
}
int main( int argc, char ** argv ) {
int x = 10;
int y = 5;
cout << "X before function call " << x << endl;
cout << "Y before function call " << y << endl;
int product = do_math(x, &y);
cout << "X after function call " << x << endl;
cout << "Y after function call " << y << endl;
cout << "What the function returns " << product << endl;
return 0;
}

Answer 2

In the assignment there is written
Write a function that takes input arguments ...
So there is no any need to declare these input parameters as pointers.
The function could look like
int do_math( int x, int y, int &sum )
{
sum = x + y;
return x * y;
}
or
int do_math( int x, int y, int *sum )
{
*sum = x + y;
return x * y;
}
In these function definitions the sum and the product can be exchanged as the parameter and return value
As for me the I would write the function either as
void do_math( int x, int y, long long &sum, long long &product )
{
sum = x + y;
product = x * y;
}
or
#include <utility>
//...
std::pair<long long, long long> do_math( int x, int y )
{
return std::pair<long long, long long>( x + y, x * y );
}
void caller()
{
int x = 10;
int y = 5;
std::pair<long long, long long> result = do_math( x, y );
std::cout << "Sum is equal to " << result.first
<< " and product is equal to " << result.second
<< std::endl;
}
Edit: I would like to explain why this statement
std::cout << "sum is " << do_math(x, y, result) << " and result is " << result;
is wrong.
The order of evaluation of subexpressions and function argument is unspecified. So in the statement above some compilers can output value of result before evaluation function call do_math(x, y, result)
So the behaviour of the program will be unpredictable because you can get different results depending on using the compiler.
Edit: As for your code from a comment then it should look like
#include <iostream>
int do_math( int x, int y, int *p_u )
{
int i = x + y;
*p_u = x * y;
return i;
}
int main()
{
int x = 10;
int y = 5;
int u;
int i = do_math( x, y, &u );
std::cout << i << std::endl;
std::cout << u << std::endl;
}
Also take into account that in general case it is better to define variables i and u as having type long long because for example the product of two big integers can not fit in an object of type int.

Answer 3

The wording is kind of contrived but I believe the task asks you to
return the multiplication as the return value of the function, and
since you can't return two types at once (except if you wrap them up somehow), you should use a third parameter as a storage area for the sum:
#include <iostream>
/* Multiplication in here */ int do_math(int x, int y, int& result/* Addition in here */)
{
result = x + y;
return x*y;
}
int main() {
int x = 10;
int y = 5;
int addition = 0;
int multiplication = do_math(x, y, addition);
std::cout << "multiplication is " << multiplication << " and sum is " << addition;
}
Example
It's not specifically asking you to use two parameters for the function.

Answer 4

A typical solution to the intent of the exercise text…
” Write a function that takes input arguments and provides two seperate results to the caller, one that is the result of multiplying the two argumentsm the other the result of adding them. Since you can directly return only one value from a funciton you'll need the seecond value to be returned through a pointer or references paramter
… is
auto product_and_sum( double& sum, double const a, double const b )
-> double
{
sum = a + b;
return a*b;
}
#include <iostream>
using namespace std;
auto main() -> int
{
double product;
double sum;
product = product_and_sum( sum, 2, 3 );
cout << product << ", " << sum << endl;
}
This code is unnatural in that one result is returned while the other is an out-argument.
It's done that way because the exercise text indicates that one should do it that way.
A more natural way to do the same is to return both, as e.g. a std::pair:
#include <utility> // std::pair, std::make_pair
using namespace std;
auto product_and_sum( double const a, double const b )
-> pair<double, double>
{
return make_pair( a*b, a+b );
}
#include <iostream>
#include <tuple> // std::tie
auto main() -> int
{
double product;
double sum;
tie( product, sum ) = product_and_sum( 2, 3 );
cout << product << ", " << sum << endl;
}
As the second program illustrates, the last sentence of the exercise text,
” Since you can directly return only one value from a funciton you'll need the seecond value to be returned through a pointer or references paramter
… is just not true. I suspect the author had meant the word “directly” to clarify that this excluded the case of a non-basic type. But even so the conclusion is incorrect.

Answer 5

What you need to do is provide another parameter to the function - the pointer or the reference to the variable where you want to store your other result:
int do_math(int *x, int *y, int &res) //or int *res
{
...
res = *x * *y;
...
}
Then make a result variable in main and pass it to the function