Self-test 4:
Function basics

Which of the following is NOT true?
1. No return statement is allowed in a function with return type "void".
2. Return statement(s) is/are necessary for a function with non-void return type.
3. There can be many return statements in a function but at most one return statement is executed each time the function is executed.
4. A return statement can at most return one object to the caller of the function.
Solution
A.

A is NOT true because an empty return statement "return;" can still be used. Consider the following code:
```
void test()
{
    return; // can be used, but can also be omitted
}
```
B is true because for function with non-void return type, a return statement is necessary, otherwise, the result will be undefined and that is not allowed.

C is true. Consider the following code:
```
int compare(int x, int y)
{
    if (x > y)
        return 1;
    else if (x < y)
        return -1;

    return 0;
}
```
In this function, we have 3 return statements. Only one of them will be executed.

D is true because we cannot return two objects (e.g. two integers) in a single return type (unless you wrap the 2 objects into a user-defined composite data type such as struct or class that you will later learn). In some sense, pass-by-reference will achieve the same effect. For example:
```
void sum_and_diff(int a, int b, int& sum, int& diff)
{
    sum = a+b;
    diff = a-b;
}
```
Consider the following function:
```
int update(int a, int b)
{
    a += b;
    return a;
}
```
(i) Which of the following statement(s) is/are valid function calls, where a and b are two int variables; x is a double variable.
1. a += update(a, b);
2. x += update( update(a, b), a);
3. x = update(x, 10);
4. a = update(x+b, a);
Solution

All are correct. x and the floating-point value of other expressions will be coerced to int before passing to the update function.
(ii) Let a = 2, b = 3, and x = 4.5. What are the values of a, b, and x after each of the valid function calls above?

Solution
Similar to the last question, consider the following function:
```
int update(int &a, int b)
{
    int c = a; 
    a += b;
    return c;
}
```
(i) Which of the following statement(s) is/are valid function calls, where a and b are two int variables; x is a double variable.
1. a += update(a, b);
2. x += update( update(a, b), a);
3. x = update(x, 10);
4. a = update(x+b, a);
Solution

A only. B and D are not valid because the first formal parameter of update requires pass-by-reference but the corresponding actual parameter is not a variable. C is not valid because one cannot initialize a reference variable of type int using a variable of type double.
(ii) Let a = 2, b = 3, and x = 4.5. What are the values of a, b, and x after each of the valid function calls above?

Solution

a = 7, b = 3, x = 4.5 for the first call only since it is the only valid function call.
Note that update(a,b) increases value of a by b but returns the original value of a before it is incremented. So after the call to update(a,b), value of a is changed to 5, and update(a,b) returns 2. Therefore, a += 2 is evaluated to a equal to 7.

What is/are the output(s) of the following program?

#include <iostream>
using namespace std;

void test(int two, int three)
{
    int one = two * three;
    cout << one << three << two << endl;
}

int main(void)
{
    int a = 7, b = 5, c = 3;
    test(a, b);
    cout << a << b << c << endl;
}

Solution

3557
753

Step 1: a = 7, b = 5, c = 3   (Note: they are variables in main function)
Step 2: call the test function as: test(7, 5)
Step 3: two = 7, three = 5  (Note: they are variables in test function)
Step 4: one = 7*5 = 35  
Step 5: 3557 is printed (because no space is printed between these three numbers)
Step 6: 753 is printed

What is/are the output(s) of the following program?

#include <iostream>
using namespace std;

int test(int n1, int n2)
{
    cout << n1 << n2 << endl;
    return n1/n2;
}

int main(void)
{
    int n1 = 5, n2 = 3;
    n2 = test(n2, n1);
    cout << n1 << n2;
}

Solution

35
50

Step 1: n1 = 5, n2 = 3; (Note: they are main function variables)
Step 2: n2  =  test(3, 5); // the function test is called
Step 3: n1 = 3, n2 = 5; (Note: they are test function variables)
Step 4: 35 is printed (because no space is printed between two numbers)
Step 5: return 3/5 => return 0 (integer division)
Step 6: n2 = test(3, 5) = 0;  (Note: this n1 is in the main function)
Step 7: 50 is printed

What is/are the output(s) of the following program?

#include <iostream>
using namespace std;

int test(int a, int b, int c)
{
    b = c;
    cout << c << b << a << endl;
    return (a-b+c);
}

void main(void)
{
    int a = 5, b = 4, c = 3;
    a = test(c, b, a);
    cout << a << b << c << endl;
}

Solution

553
343

Step 1: a = 5, b = 4, c = 3 (Note: they are variables in main function)
Step 2: a = test(3, 4, 5)
Step 3: a = 3, b = 4, c = 5 (Note: they are variables in test function)
Step 4: b = 5           (Note: b is a variable in test function)
Step 5: 553 is printed
Step 6: return (3-5+5) => return 3
Step 7: a = test(3, 4, 5) = 3 (Note: a is a variable in main function)
Step 8: 343 is printed

What is/are the output(s) of the following program?

#include <iostream>
using namespace std;

int test(int n1, int n2)
{
    cout << n2 << n1 << endl;
    return n2*n1;
}

void main(void)
{
    int n1 = 4, n2 = 3, n3 = 8;
    n2 = test(n1, n3);
    cout << n1 << n2 << n3 << endl;
}

Solution

84
4328

Step 1: n1 = 4, n2 = 3, n3 = 8 (Note: they are variables in the main function)
Step 2: n2 = test(4, 8)
Step 3: n1 = 4, n2 = 8       (Note: they are variables in the test function)
Step 4: 84 is printed
Step 5: return 8*4 => return 32
Step 6: n2 = test(4, 8) = 32 (Note: n2 is a variable in the main function)
Step 7: 4328 is printed

Give the outputs of the following code.

#include <iostream>
using namespace std;

void fun1(int a)
{
    cout << "a is " << a << endl;
}

void fun2(void)
{
    int a;
    a = 2;
    cout << "a is " << a << endl;
}

int main(void)
{
    int a = 0;
    fun1(a);
    fun2();
    cout << "a is " << a << endl;
    return 0;
}

Solution

a is 0
a is 2
a is 0

because (a) fun1 uses pass-by-value and will not change its input, and (b) variable a in fun2 is not the same as the variable a in other functions of the program.

What are the outputs of the following program?

#include <iostream>
using namespace std;

void fun2(int& a, int& b, int c, int& d)
{
    int temp;
    temp = a;
    a = b;
    b = c;
    c = d;
    d = temp;
}

int main(void)
{
    int a, b, c, d;
    a = b = c = d = 0;

    for(int i = 0; i < 3; i = i+1)
    {
        fun2(a, b, c, d);
        a = a + 4;
        b = b - 3;
        c = c * 2;
        d = d - 1;
    }

    cout << a << " " << b << " " 
         << c << " " << d << endl;
    return 0;
}

Solution

1 -3 0 0

    Explanation:
                        | a | b | c | d |
    Initially           | 0 | 0 | 0 | 0 |
    After 1st fun2 call | 0 | 0 | 0 | 0 |
    After 1 iteration   | 4 |-3 | 0 |-1 |
    After 2nd fun2 call |-3 | 0 | 0 | 4 |
    After 2 iteration   | 1 |-3 | 0 | 3 |
    After 3rd fun2 call |-3 | 0 | 0 | 1 |
    After 3 iteration   | 1 |-3 | 0 | 0 |

What are the outputs of the following program?

#include <iostream>
using namespace std;

int foo(int& a, int b, int& c)
{
    int d = 5;
    a = b++;
    b = d;
    cout << a << " " << b << " " << c << " " << d << endl;
    return -d;
}

int bar(int a, int& b, int& c)
{
    int d = --a;
    b = a;
    cout << a << " " << b << " " << c << " " << d << endl;
    return d;
}

int main(void)
{
    int a = 4, b = 6, c = 2, d = 9;
    cout << a << " " << b << " " << c << " " << d << endl;
    d = foo(a, b, c);
    cout << a << " " << b << " " << c << " " << d << endl;
    a = 0;
    d = bar(a, b, c);
    cout << a << " " << b << " " << c << " " << d << endl;
}

Solution

4 6 2 9
6 5 2 5
6 6 2 -5
-1 -1 2 -1
0 -1 2 -1

Self-test 4: Function basics

Self-test 4:
Function basics