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Magazine Chapter 10: Problem 29
Given the variable initializations
int \(a[5]=\\{0,10,20,30,40\\}\)
int \(k=3\) int \(* p=a+1\)
determine the output from each of the following statements:
A) cout \(<
Short Answer
Expert verified
To sum up, the outputs of each given statement are as follows:
A: 30
B: 30
C: 0
D: 0
E: 20
F: 10
G: 10
H: 20
Step by step solution
01
1. Evaluate statement A: cout
To evaluate `a[k]`, we need to find the value stored at index k in array a: a[3]. The array a has the values \(\{0,10,20,30,40\}\). At index 3, element 30 is stored. So the output for this statement would be 30.
02
2. Evaluate statement B: cout
In this statement, we first find the value of `a+k`. Remember that in C++, the name of an array gives the address of its 0th element, thus a = &a[0]. Therefore, `a+k` means the memory address of the element at index k (a[3]). Then *(a+k) dereferences this memory address, giving us the value stored in a[3]. So the output would be 30.
03
3. Evaluate statement C: cout
`*a` is equivalent to `*(&a[0])`, which is the value stored in a[0]. In this case, `*a` will result in the value 0. So the output for this statement would be 0.
04
4. Evaluate statement D: cout
First, find the value of `*a`, which is 0. Then `a[*a]` is equivalent to `a[0]`. In this case, `a[0]` will result in the value 0. So the output for this statement would be 0.
05
5. Evaluate statement E: cout
First, find the value of `*a`, which is 0. Then `a[*a+2]` evaluates to `a[0+2]`, which is `a[2]`. In this case, `a[2]` will result in the value 20. So the output for this statement would be 20.
06
6. Evaluate statement F: cout
To evaluate `*p`, we need to remember that p is a pointer initialized to a+1, which points to the second element in the array a with index 1. Thus, *p = a[1], which is the value stored in a[1]. In this case, `a[1]` equals 10, so the output for this statement would be 10.
07
7. Evaluate statement G: cout
This statement is equivalent to `*(p+0)` since indexing an array in C++ involves pointer arithmetic. Since p is equivalent to a+1, `p[0]` is equivalent to `*(a+1+0)` which is `a[1]`. So the output would be 10.
08
8. Evaluate statement H: cout
This statement is equivalent to `*(p+1)`. As explained earlier, p is equivalent to a+1, so `p[1]` is equivalent to `*(a+1+1)` which is `a[2]`. In this case, `a[2]` equals 20, so the output for this statement would be 20.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Pointer Arithmetic
Pointer arithmetic in C++ enables us to perform operations on pointers that allow us to navigate through an array or a sequence of memory locations. In the context of arrays, adding an integer value to a pointer shifts the position of the pointer by that number of array elements.
For instance, if we have an integer array defined as
For instance, if we have an integer array defined as
int a[5] = {0,10,20,30,40} and a pointer int *p = a + 1, p points to the second element of a because it adds the size of one integer worth of memory to the address of a. When we look at the exercises covered, such as cout << *(a+k) or cout << p[1], the pointer moves to the kth and second index in the array, respectively, and the values 30 and 20 are printed. Array Indexing
Array indexing is a way to access individual elements within an array using an index number. In C++, an array's index starts from 0, meaning the first element is at index 0, the second at index 1, and so on. In the exercise, when we see statements like
Therefore, accessing an array element
cout << a[k] and cout << a[*a+2], we're retrieving the value stored at the kth position, and at the position of *a+2, which is the third element, respectively.Therefore, accessing an array element
a[i] directly gives us the value at the ith position in the array. This method of accessing elements is fundamental in understanding how arrays work in C++. Dereferencing Pointers
Dereferencing a pointer means obtaining the value stored in the memory location to which the pointer points. The dereference operator (*) is used in C++ to do this. Whenever you see a statement like
In the solutions for statements like
cout << *p, the program will print the value stored at the address p is pointing to.In the solutions for statements like
cout << *a or cout << *(a+k), we're dereferencing the pointer to get and display the value at the beginning of the array, and the value at the kth position, respectively. Dereferencing is a core concept when it comes to working with pointers and dynamic memory in C++. C++ Syntax
The syntax in C++ can be rather strict and may seem complex at first, but it's built upon consistent rules. If we break down the exercise statements, they follow specific syntactical patterns of C++. The array declaration
The pointer declaration
Through these examples, we can see how operators like
int a[5] = {0,10,20,30,40} assigns a block of memory large enough for five integers.The pointer declaration
int *p = a + 1 initializes a pointer p which points to the memory location immediately following a's first element, according to pointer arithmetic rules.Through these examples, we can see how operators like
&, *, and the array index operator [] are used in the language, which illustrates the importance of understanding C++ syntax for effective programming. 
