Fundamentals of Programming Session 15

Transcription

1 Fundamentals of Programming Session 15 Instructor: Reza Entezari-Maleki 1 Fall 2013 These slides have been created using Deitel s slides Sharif University of Technology

2 Outlines Array Examples Passing Arrays to Functions 2

3 Array Examples 3 Arrays that are static are initialized once at compile time. If you do not explicitly initialize a static array, that array s elements are initialized to zero by the compiler. Figure 6.11 demonstrates function staticarrayinit with a local static array and function automaticarrayinit with a local automatic array. Function staticarrayinit is called twice (lines 12 and 16). The local static array in the function is initialized to zero by the compiler (line 25). The function prints the array, adds 5 to each element and prints the array again.

4 4 Array Examples

5 5 Array Examples

6 6 Array Examples

7 7 Array Examples

8 Passing Arrays to Functions 8 To pass an array argument to a function, specify the name of the array without any brackets. For example, if array hourlytemperatures has been defined as int hourlytemperatures[ 24 ]; the function call modifyarray( hourlytemperatures, 24 ) passes array hourlytemperatures function modifyarray. and its size to Unlike char arrays that contain strings, other array types do not have a special terminator. For this reason, the size of an array is passed to the function, so that the function can process the proper number of elements.

9 Passing Arrays to Functions 9 C automatically passes arrays to functions by reference the called functions can modify the element values in the callers original arrays. The name of the array evaluates to the address of the first element of the array. Because the starting address of the array is passed, the called function knows precisely where the array is stored. Therefore, when the called function modifies array elements in its function body, it s modifying the actual elements of the array in their original memory locations. Figure 6.12 demonstrates that an array name is really the address of the first element of an array by printing array, &array[0] and &array using the %p conversion specifier a special conversion specifier for printing addresses.

10 Passing Arrays to Functions The %p conversion specifier normally outputs addresses as hexadecimal numbers. Although entire arrays are passed by reference, individual array elements are passed by value exactly as simple variables are. Such simple single pieces of data (such as individual ints, floats and chars) are called scalars. To pass an element of an array to a function, use the subscripted name of the array element as an argument in the function call. 10

11 11 Passing Arrays to Functions

12 Passing Arrays to Functions 12 For a function to receive an array through a function call, the function s parameter list must specify that an array will be received. For example, the function header for function modifyarray (that we called earlier in this section) might be written as void modifyarray( int b[], int size ) indicating that modifyarray expects to receive an array of integers in parameter b and the number of array elements in parameter size. The size of the array is not required between the array brackets. If it s included, the compiler checks that it s greater than zero, then ignores it. Specifying a negative size is a compilation error. Figure 6.13 demonstrates the difference between passing an entire array and passing an array element.

13 13 Passing Arrays to Functions

14 14 Passing Arrays to Functions

15 15 Passing Arrays to Functions

16 16 Passing Arrays to Functions

17 Passing Arrays to Functions There may be situations in your programs in which a function should not be allowed to modify array elements. Because arrays are always passed by reference, modification of values in an array is difficult to control. C provides the type qualifier const to prevent modification of array values in a function. Figure 6.14 demonstrates the const qualifier. 17

18 18 Passing Arrays to Functions

19 19 Passing Arrays to Functions

20 Question 1 Write a program to take a sentence and reverse it using recursion. You cannot use arrays! 20

21 Answer 1 21 void Reverse(); int main(){ printf("enter a sentence:"); Reverse(); return 0; } void Reverse(){ char c; scanf("%c", &c); if( c!= '\n'){ Reverse(); printf("%c",c); } }

22 Question 2 What is the output of the following code? void foo( int [] ); int main(){ int ary[4] = {1, 2, 3, 4}; foo(ary); printf("%d ", ary[0]); } 22 void foo(int p[4]){ int i = 10; p = &i; printf("%d", p[0]); } Answer: 101

23 Question 3 Given array ary with no repetitive elements and size of N, and number Sum. Write a function to recursively determine if ary has a subset that sum of its elements is equal to Sum. Print one of those subsets. 23

24 Answer 3 24 int hassubset(int array[], int N, int Sum) { if (N == 0) { return (Sum == 0); } if (hassubset(array, N-1, Sum - array[n-1])) { printf("%d ", array[n - 1]); return 1; } else if (hassubset(array, N-1, Sum)) { return 1; } else { return 0; } }