C Programming Lecture 14 Arrays

What is an Array? b An array is a sequence of data items that are: all of the same typeall of the same type –a sequence of integers, or a sequence of characters, or a sequence of floats, etc. indexibleindexible –you can indicate or select the first element or second element or... stored contiguously in memorystored contiguously in memory –each element of the sequence is stored in memory immediately after the previous element

One-Dimensional Arrays b Declaration of a one-dimensional array. int grade[3];int grade[3]; –Provides space in memory to store three grades (as in grades on three of your quizzes) b Example of Assignments grade[0] = 95; /* 1st element index is 0 */ grade[1] = 88; /* 2nd element index is 1 */ grade[2] = 97; /* 3rd element index is 2 */

Accessing Array Elements b The following code segment will print the three grades stored in the array on the previous slide (assume i has been declared as an int variable): for (i = 0; i < 3; ++i) printf(“Grade %d is %d\n”, i, grade[i]); printf(“Grade %d is %d\n”, i, grade[i]); b Output Grade 0 is 95 Grade 1 is 88 Grade 2 is 97

Arrays and Pointers b An array name is also the name of a pointer constant to the base address (beginning) of the array in memory. b Using “made up” addresses (as we have before in discussing pointers): grade < addresses in memory < index of array element values stored in each element The address stored in grade would be 1000

Using a Symbolic Constant as the Size of the Array b It is considered good programming practice to define the size of an array as a symbolic constant: In the next example, if the size of a class changes then the symbolic constant at the top of the program is easily changed.In the next example, if the size of a class changes then the symbolic constant at the top of the program is easily changed. The program would then be recompiled so it can be used with the larger array of grades.The program would then be recompiled so it can be used with the larger array of grades.

Example of Entering Values into an Array, then Displaying the Values #include #define N 20 /* Number of students in class */ int main(void) { int grade[N]; /* declare array with N elements */ int i; /* Enter the grades for a quiz */ for (i = 0; i < N; ++i) { printf(“Enter quiz grade %d: “, i + 1); scanf(“%d”, &grade[i]); } /* display the grades that were entered */ printf(“Here are the grades that were entered\n”); for (i = 0; i < N; ++i) printf(“Grade %d is %d”, i + 1, grade[i]); } Note that the user is shown grades starting with grade 1, but what the user thinks of as grade 1 is stored in grade[0] in the array.

Initialization of Arrays As with simple variables, arrays can be initialized within a declaration. An array initializer is a sequence of initializing values written as a brace- enclosed, comma-separated list.An array initializer is a sequence of initializing values written as a brace- enclosed, comma-separated list. float x[4] = {-1.1, 0.2, 33.0, 4.4}; float x[4] = {-1.1, 0.2, 33.0, 4.4}; x[0] is initialized to -1.1 x[1] is initialized to 0.2 etc.

More on Initialization of Arrays b If a list of initializers is shorter than the number of array elements, the remaining elements are initialized to zero. b If an external or static array is not initialized, then the system automatically initializes all elements to zero. b Uninitialized automatic and constant arrays start with garbage values -- whatever happens to be in memory when the array is allocated.

Declaring and Initializing an Array Without a Size b If an array is declared without a size and is initialized to a series of values, it is implicitly given the size of the number of initializers. int a[] = {3, 4, 5, 6}; int a[] = {3, 4, 5, 6}; is equivalent to is equivalent to int a[4] = {3, 4, 5, 6}; int a[4] = {3, 4, 5, 6};

Range of Values Allowed for a Subscript b Assume a declaration: int i, a[size]; int i, a[size]; b To access an element of the array, we can write a[expr], where expr is an integral expression. b expr is called a subscript and the value of the subscript must lie in the range 0 to size - 1.

“Subscript Out of Bounds” b If a subscript from the previous example lies outside the range 0 to size - 1 then: A run-time error occurs.A run-time error occurs. The system will stop executing your program and display a message such as “subscript out of bounds”.The system will stop executing your program and display a message such as “subscript out of bounds”. Sometimes no error message is displayed, but your program will begin to produce unexpected results.Sometimes no error message is displayed, but your program will begin to produce unexpected results.

Relationship Between Arrays and Pointers b An array name represents the address of the beginning of the array in memory. When an array is declared, the compiler must allocate a base address and a sufficient amount of storage (RAM) to contain all the elements of the array.When an array is declared, the compiler must allocate a base address and a sufficient amount of storage (RAM) to contain all the elements of the array. The base address of the array is the initial location in memory where the array is stored.The base address of the array is the initial location in memory where the array is stored. –It is the address of the first element (index 0) of the array.

Passing Arrays to Functions b In a function definition, a formal parameter that is declared as an array is actually a pointer. When an array is passed, its base address is passed call-by-value.When an array is passed, its base address is passed call-by-value. The array elements themselves are not copied.The array elements themselves are not copied. As a notational convenience, the compiler allows array bracket notation to be used in declaring pointers as parameters.As a notational convenience, the compiler allows array bracket notation to be used in declaring pointers as parameters.

Example of Passing an Array to a Function int sum(int a[], int n) /* n is size of the array */ { int i, s = 0; for (i = 0; i < n; ++i) s += a[i]; return s; } alternative function definition: int sum(int *a, int n) /* n is size of the array */ { int i, s = 0; for (i = 0; i < n; ++i) s += a[i]; return s; } Notice -- no size here. That was provided in the original declaration of the array.

Equivalence of int a[]; and int *a; b As part of the header of a function definition the declaration int a[]; is equivalent to int *a; int a[]; is equivalent to int *a; b Within the body of a function, these are not equivalent. The first will create a constant pointer (and no storage for an array).The first will create a constant pointer (and no storage for an array). The second will create a pointer variable.The second will create a pointer variable.