Question

Write two versions of each string-comparison function in Fig. 8.30. The first version should use array subscripting, and the second should use pointers and pointer arithmetic.

Short answer

Implement using both array subscripts and pointers, both compare character by character.

Step-by-step solution

Understanding Array Subscript Method

In the array subscript method, we use indexes to access each character of the strings. For example, if we have a string `str`, `str[i]` gives the character at the ith position of the string. We will use this approach first for each string comparison function.

Implementing strcmp Using Array Subscript

To compare two strings `s1` and `s2` using array subscripting, use a loop to iterate over the characters while they are equal. If a difference is found or the end of both strings is reached, return the difference between the current characters or zero if they are identical. ``` int strcmp_subscript(const char s1[], const char s2[]) { int i = 0; while (s1[i] && s1[i] == s2[i]) { i++; } return s1[i] - s2[i]; } ```

Understanding Pointer Arithmetic Method

In the pointer arithmetic method, you use pointers to iterate over the strings. You can directly compare the values that the pointers point to and move the pointer by incrementing it, i.e., `*pointer` gives the character value and `pointer++` moves to the next character.

Implementing strcmp Using Pointer Arithmetic

Implement the `strcmp` function using pointers. Initialize pointers to point at the start of each string, then increment them while characters are equal. Stop if a difference is found or both strings end, and return the character difference or zero. ``` int strcmp_pointer(const char *s1, const char *s2) { while (*s1 && *s1 == *s2) { s1++; s2++; } return *s1 - *s2; } ```

Compare and Conclude

Both methods achieve the same result, but the pointer method may be more efficient in certain compilers due to direct memory access. However, the array subscript method is often simpler and more readable for new programmers.

Key concepts

String Comparison

String comparison in C++ involves evaluating two strings to determine their relative ordering or equality. This operation is key in many applications, such as sorting or checking for unique values.
To compare strings, we often look at them as sequences of characters and compare each character pairwise. If we find any differences, we can conclude which string is greater or if they are equal. This is typically done using functions like `strcmp` in C++, which return zero if two strings are identical, a positive value if the first non-matching character in `s1` is greater, and a negative value if it's smaller.
In general, string comparison requires careful attention to character encoding and null-terminations to avoid errors. By understanding string comparison, we can manipulate and control data in programs more effectively.

Pointer Arithmetic

Pointer arithmetic is a technique in C++ that involves performing operations directly with memory addresses. It allows programmers to navigate through array-like data structures more efficiently. In pointer arithmetic, you work with the address rather than the index.
When using pointer arithmetic, incrementing a pointer (e.g., `pointer++`) moves it to the next block of memory of the data type it points to. This is valuable since it abstracts the index calculations and can lead to performance improvements in some cases.
By using pointers in string comparison, you directly access each character and can take advantage of lower-level operations. This technique can be a bit abstract for beginners but offers a powerful tool in optimizing code.

Array Subscripting

Array subscripting is a common method to access elements in a sequence using an index. In C++, you use the square brackets (e.g., `arr[i]`) to access the ith element of an array. This is a very intuitive and user-friendly approach, especially for beginners.
When comparing strings using array subscripting, you iterate over the characters using indices. With `str[i]`, you get direct access and can easily implement logic for comparison.
While subscripting is simple and readable, it abstracts away some of the details of memory access, potentially leading to less efficient code. Still, it is often the go-to approach because of its simplicity and ease of understanding.

Function Implementation

Function implementation in C++ involves writing the actual code for functions that define specific behavior in a program. When implementing a function like `strcmp`, you are responsible for deciding the logic flow and how it achieves its purpose.
For string comparison functions, your implementation might include loops, conditionals, and return statements to convey the logic of comparing two sequences of characters. You decide whether to use array subscripting or pointer arithmetic based on your optimization goals and readability.
Strong function implementation skills are crucial for writing efficient, clear, and maintainable code. Understanding the underlying concepts like pointer arithmetic or array subscripting enhances your ability to implement functions optimally in various contexts.