CODEWITHSUNDEEP
c++class
Tanner Ewing
Tanner Ewing

Reputation: 337

Explain the output of this program

I can't figure out how this works, the code is really complicated because it is for a programming class I'm in. I can't seem to get the program's output when I work it through manually, it is the practice test for our final next week, I wouldn't cheat on a test, the professor gave us the program and the output, I just don't understand why that is the output..:

class FinalExam
{
private:

   int This;
   int That;

public:

   FinalExam(int, int);
   void One(int);
   int  Two(int);
};

FinalExam :: FinalExam(int A = 3, int B = 5)
{
   This = A;
   That = B;
}

void FinalExam :: One(int A)
{
   This --;
   That = A;
}

int FinalExam :: Two(int A) // Two gets the int 8
{
   int Scrap; 
   Scrap = This + That - A; // 5 + 2 - 8 = -1???? 
   return Scrap;
}

main()
{

   FinalExam Block;
   FinalExam Chunk(6, 7);

   Block.One(2);

   cout << Block.Two(3) 
        << '\n'
        << Chunk.Two(8); //I get lost on this 8, It should go to "Two"
}

And the output is:

1

5

I have looked at this for about an hour and I don't understand.

Upvotes: 0

Views: 162

Answers (4)

abelenky
abelenky

Reputation: 64682

Here's a line-by-line breakdown:

1 FinalExam Block;
Uses the constructor with default values, so Block.This = 3, and Block.That = 5.

2 FinalExam Chunk(6, 7);
Uses the constructor, specifying values, so Chunk.This = 6 and Chunk.That = 7.

3 Block.One(2);
Decrements Block.This (3 ==> 2), and assigns Block.That = 2 (was previously 5).

4 Block.Two(3)
returns Block.This + Block.That-3 ==> 2+2-3 ==> 1, which is output.

5 Chunk.Two(8)
returns Chunk.This + Chunk.That - 8 ==> 6+7-8 ==> 5, which is output.

Q.E.D. the output is "1 \n 5"

Upvotes: 1

Logan Besecker
Logan Besecker

Reputation: 2761

Firstly, you'll notice that we're creating two objects of type FinalExam called Block and Chunk.

Block: since block wasn't passed any values via the parameter, it will take on the default values which come from this code block:

    FinalExam :: FinalExam(int A = 3, int B = 5)
{
   This = A;
   That = B;
}

so This = 3 and That = 5

The next line that effects Block is:

Block.One(2);

which refers to this code block:

void FinalExam :: One(int A)
{
   This --;
   That = A;
}

so This = 2 (This = This - 1 or This = 3 - 1) and That = 2 (That = A = 2(passed by value in parameters)

The last line effecting Block is:

cout << Block.Two(3)

which refers to this code block:

int FinalExam :: Two(int A) // Two gets the int 8
{
   int Scrap; 
   Scrap = This + That - A; // 5 + 2 - 8 = -1???? 
   return Scrap;
}

so we create a new integer called Scrap = 1 (This + That - A or 2 + 2 - 3(passed by value))

Chunk: the first line that refers to Chunk is:

FinalExam Chunk(6, 7);

this set A = 6 and B = 7 and because of this code block:

FinalExam :: FinalExam(int A = 3, int B = 5)
{
   This = A;
   That = B;
}

This = 6 and That = 7 (This = A = 6 and That = B = 7)

and finally, we have this line:

Chunk.Two(8);

which refers to this code block: 

int FinalExam :: Two(int A) // Two gets the int 8
{
   int Scrap; 
   Scrap = This + That - A; // 5 + 2 - 8 = -1???? 
   return Scrap;
}

A is set to 8 since we passed it by value through parameters and Scrap = 5 (b + 7 - 8 or This + That - A)

Output: We output Scrap from Block which is 1 and create a new line and output Scrap from Chunk which is 5

Hope this helps, leave a comment if you have any additional questions

Upvotes: 0

matiu
matiu

Reputation: 7725

Explanation of what's happening in the comments between the lines.

int main(int, char**) {
   FinalExam Block;
   // At this point ..
   // Block.This = 3;
   // Block.That = 5
   FinalExam Chunk(6, 7);
   // Chunk.This = 6
   // Chunk.That = 7
   Block.One(2);
   // Block.One decrement's This and assigns 2 to That so ..
   // Block.This = 2
   // Block.That = 2
   std::cout << Block.Two(3)
        // Block.Two(3) returns the result of this calculation
        // This + That - 3
        // This and That are both 2 at this point so..
        // 2 + 2 - 3 == 1
        // It returns 1 and prints out '1'
        << std::endl
        << Chunk.Two(8);
        // Chunk's This and That are 6 and 7 respectively so ..
        // cout << 6 + 7 - 8 == 5
}

Upvotes: 1

Mahesh
Mahesh

Reputation: 34625

FinalExam Block;  // Not passing any arguments to the constructor. In that case, 
                  // default argument values are taken. So, This = 3, That = 5


Block.One (2);     // This = 2; That = 2
                   // Because This is decremented and That is assigned the value
                   // passed to the method which is 2.


cout << Block.Two (3) ;  // 2 + 2 - 3 = 1 which is returned and is printed.

Similarly try the second one.

Upvotes: 3

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