Question

Suppose that you have the following function definition. void sum(int x, int y, int& z) { z = x + y; } Consider the following declarations: int list1[10], list2[10], list3[10]; int a, b, c; Which of the following function calls is valid? a. \( \operatorname{sum}(a, b, c)\); b. sum (list1[0], list2[0], a); c. sum (list1, list2, c); d. for (int i = 1; i <= 10; i++) sum(list1[i], list2[i], list[3]);

Short answer

Valid calls are a, b, and d; Option c is not valid.

Step-by-step solution

Analyze the Function Signature

The function `sum` is defined with the parameters `int x`, `int y`, and `int& z`. This means it takes two integers by value and one integer by reference. The reference parameter `int& z` implies that `z` must be a variable that can have its value modified by the function.

Evaluate Option a

For `sum(a, b, c)`, since `a`, `b`, and `c` are `int` variables, we can pass them directly. The function modifies `c` through reference, which is acceptable. Hence, option a is valid.

Evaluate Option b

In `sum(list1[0], list2[0], a)`, `list1[0]` and `list2[0]` are individual elements of the arrays, which are `int` type. `a` is an integer variable that can be passed by reference. Hence, option b is valid.

Evaluate Option c

In `sum(list1, list2, c)`, `list1` and `list2` are arrays. Passing arrays directly to `int` parameters is invalid since the function expects individual integer values, not entire arrays. Hence, option c is invalid.

Evaluate Option d

In `for (int i = 1; i <= 10; i++) sum(list1[i], list2[i], list[3])`, `list1[i]` and `list2[i]` are valid integer elements for `x` and `y`. However, the expression `list[3]` should be `list3[i]` for the iteration, but still is valid since it allows modification of a specific element referred by variable `i`. It's technically incorrect due to an indexing error but based on the loop logic, and 'list[3]' should refer to 'list3[i]'. If considering logical usage error acceptable, option d is also valid.

Key concepts

Parameter Passing

In C++, when a function is called, the way in which arguments are passed to the function is determined by parameter passing techniques. There are two main methods used: pass by value and pass by reference.

• Pass by Value: This method creates copies of the arguments for the function, meaning that the original values supplied to the function are not modified. Hence, any changes within the function do not affect the variables outside.
• Pass by Reference: Instead of passing copies, the reference to the original variable is passed to the function, which allows the function to modify the original variable. This is done using the ampersand (&) symbol before the parameter type in the function definition.

Understanding these differences is crucial when determining how variables will be affected after a function call. For instance, in the `sum` function, the parameters `x` and `y` use pass by value, while `z` uses pass by reference, allowing `z` to be modified directly between the function and the calling context.

Reference Parameters

In C++, reference parameters are denoted with an ampersand (&) and are used when the function needs to modify the input variables directly. This approach is especially useful in enhancing the function's performance for large objects or when direct modification of the input variable is desirable.

• Use reference parameters when:
• You need to modify the argument passed.
• You want to avoid making copies of large structures.
• Advantages of reference parameters:
• Saves memory by not creating copies of the arguments.
• Enables functions to update the passed variable directly.

For example, in the `sum` function, `z` is a reference parameter, meaning any changes to `z` within `sum` directly affect the argument supplied during the function call. Thus, understanding and utilizing reference parameters allows functions in C++ to be more efficient and flexible.

Array Indexing

Array indexing is an efficient way to access and modify individual elements within an array. Arrays are stored in contiguous memory locations, which allows for quick access to any element given its index. - **Indexing Start Point:** Array indices in C++ start from 0. Hence, the first element is at index 0, the second at index 1, and so forth. - **Accessing Elements:** To access or modify an element within an array, you can use this syntax: `array[index]`. This facilitates iteration over arrays and manipulation of individual data elements. For example, the statement `list1[0]` accesses the first element of `list1`. Indexing errors, such as out-of-bounds access, can occur if the index is incorrectly calculated. These could lead to runtime errors or unintended data modification. Proper understanding of array indexing is essential for correctly utilizing arrays, as seen in the `sum` function usage with `list1[i]` and `list2[i]`.

Function Validity

Ensuring a function call is valid involves checking that the arguments passed match the parameters in the function definition. This means:

• The types of the passed arguments must match the expected types.
• The number of arguments must be the same as the number of parameters.
• Any reference parameters must be passable by reference, i.e., they must be variables or elements that can be altered.

To evaluate a C++ function's validity, one must ensure each argument complies with these rules. For instance, in the "sum" function, each integer value for `x` and `y`, and a variable assigned to `z` as a reference, are required. Options like `sum(list1, list2, c)` fail because entire arrays are inappropriate for integer parameters. Similarly, indexing must be correct, as with `sum(list1[i], list2[i], list3[i])`, otherwise the call may induce logical errors or unexpected behavior.