C++ For Science and Engineering Lecture 16

Transcription

1 C++ For Science and Engineering Lecture 16 John Chrispell Tulane University Friday October 1, 2010

2 arrfun1.cpp Notes on the previous listing: The name of an array is the same as if it were a pointer. c o o k i e s == &c o o k i e s [ 0 ] // a r r a y name i s a d d r e s s o f f i r s t element. There are two exceptions to this rule. 1 Array declarations use the array name to lable the storage. 2 Applying sizeof to an array name yields the size of the whole array in bytes. The notations i n t a r r and i n t a r r [ ] // a r e o n l y synonymous i n the f u n c t i o n header and p r o t o t y p e. we can not use int arr[] to declare a pointer in the body of a function. RECALL a r r [ i ] == ( a r r + i ) // v a l u e s i n two n o t a t i o n s &a r r [ i ] == a r r + i // a d d r e s s e s i n two n o t a t i o n s Consider the following example: John Chrispell, Friday October 1, 2010 slide 3/25

3 arrfun2.cpp #include <iostream > const i n t A r S i z e = 8 ; i n t sum arr ( i n t a r r [ ], i n t n ) ; // use s t d : : i n s t e a d o f u s i n g d i r e c t i v e i n t main ( ) { i n t c o o k i e s [ A r S i z e ] = { 1, 2, 4, 8, 1 6, 3 2, 6 4, ; std : : cout << cookies << = array address, ; s t d : : cout << s i z e o f c o o k i e s << = s i z e o f c o o k i e s \n ; i n t sum = sum arr ( cookies, ArSize ) ; s t d : : cout << Total c o o k i e s e a t e n : << sum << s t d : : e n d l ; sum = sum arr ( c o o k i e s, 3 ) ; // a l i e s t d : : cout << F i r s t t h r e e e a t e r s a t e << sum << c o o k i e s. \ n ; sum = sum arr ( c o o k i e s + 4, 4 ) ; // a n o t h e r l i e std : : cout << Last f o u r e a t e r s ate << sum << c o o k i e s. \ n ; return 0 ; / ==================================== / / r e t u r n the sum o f an i n t e g e r a r r a y / / ==================================== / i n t sum arr ( i n t a r r [ ], i n t n ){ i n t t o t a l = 0 ; s t d : : cout << a r r << = a r r, ; s t d : : cout << s i z e o f a r r << = s i z e o f a r r \n ; f o r ( i n t i = 0 ; i < n ; i ++){ t o t a l = t o t a l + a r r [ i ] ; return t o t a l ; John Chrispell, Friday October 1, 2010 slide 5/25

4 Think about your program Consider writing a program that takes an array of values. For our purposes they can be collectible pez dispenser values. Consider your collection to have a max size of 1000 dispensers. You wish to store and update the value of each integer indexed dispenser. You program should have a nice way to display the current value. We can create a program for this example using three user defined functions. Each function performs a simple task. These functions make writing the main function of the program quick and easy. Consider the following code: John Chrispell, Friday October 1, 2010 slide 7/25

5 arrfun3.cpp #include <iostream > const i n t Max = 100 0; // f u n c t i o n p r o t o t y p e s i n t f i l l a r r a y ( double ar [ ], i n t l i m i t ) ; void s h o w a r r a y ( const double ar [ ], i n t n ) ; // don t change data void r e v a l u e ( double ar [ ], i n t n ) ; i n t main ( ) { using namespace s t d ; double p r o p e r t i e s [ Max ] ; i n t s i z e = f i l l a r r a y ( p r o p e r t i e s, Max ) ; s h o w a r r a y ( p r o p e r t i e s, s i z e ) ; r e v a l u e ( p r o p e r t i e s, s i z e ) ; s h o w a r r a y ( p r o p e r t i e s, s i z e ) ; return 0 ; John Chrispell, Friday October 1, 2010 slide 9/25

6 arrfun3.cpp count / ============================================== / / Simple f u n c t i o n used to f i l l the a r r a y and / / to determine i t s length. / / ============================================== / i n t f i l l a r r a y ( double ar [ ], i n t l i m i t ){ using namespace std ; double temp ; i n t i ; f o r ( i = 0 ; i < l i m i t ; i ++) { cout << Enter v a l u e # << ( i + 1) << : ; cin >> temp ; i f (! c i n ){ // bad i n p u t c i n. c l e a r ( ) ; while ( c i n. get ( )!= \n ){ continue ; cout << Bad i n p u t ; i n p u t p r o c e s s terminated. \ n ; break ; e l s e i f ( temp < 0){ // s i g n a l to t e r m i n a t e break ; ar [ i ] = temp ; return i ; John Chrispell, Friday October 1, 2010 slide 11/25

7 arrfun3.cpp count / =============================================== / / The f o l l o w i n g f u n c t i o n can use, but not a l t e r, / / the a r r a y whose a d d r e s s i s ar / / =============================================== / void s h o w a r r a y ( const double ar [ ], i n t n ){ using namespace std ; f o r ( i n t i = 0 ; i < n ; i ++) { cout << Pez # << ( i + 1) << : $ ; cout << ar [ i ] << e n d l ; / ======================================= / / m u l t i p l i e s each element o f ar [ ] by a / / r e q u e s t e d f a c t o r / / ======================================= / void r e v a l u e ( double ar [ ], i n t n ){ s t d : : cout << Enter r e v a l u a t i o n f a c t o r : ; double f a c t o r ; s t d : : c i n >> f a c t o r ; f o r ( i n t i = 0 ; i < n ; i ++){ ar [ i ] = f a c t o r ; John Chrispell, Friday October 1, 2010 slide 13/25

8 Array Approach Two approaches to array data. In C/C++ the traditional approach to dealing with array data is to pass a pointer to the array to a function. A second argument to the function will be the size of the array. A second approach is to pass a pointer to the start and to the end of an array. The following listing is a modified version of the arrfun2.cpp lsiting given earlier. It passes a pointer to the start and the end of the array. John Chrispell, Friday October 1, 2010 slide 15/25

9 arrfun4.cpp #i n c l u d e <i o s t r e a m > const i n t A r S i z e = 8 ; i n t sum arr ( const i n t begin, const i n t end ) ; i n t main ( ) { using namespace std ; i n t c o o k i e s [ A r S i z e ] = { 1, 2, 4, 8, 1 6, 3 2, 6 4, ; i n t sum = sum arr ( cookies, cookies + ArSize ) ; cout << Total c o o k i e s eaten : << sum << endl ; sum = sum arr ( cookies, cookies + 3 ) ; // f i r s t 3 elements cout << F i r s t t h r e e e a t e r s a t e << sum << c o o k i e s. \ n ; sum = sum arr ( cookies + 4, cookies + 8 ) ; // l a s t 4 elements cout << Last f o u r e a t e r s ate << sum << c o o k i e s. \ n ; return 0 ; i n t sum arr ( const i n t begin, const i n t end ){ const i n t pt ; i n t t o t a l = 0 ; f o r ( pt = begin ; pt!= end ; pt++){ t o t a l = t o t a l + pt ; return t o t a l ; John Chrispell, Friday October 1, 2010 slide 17/25

10 Pointers and const We have now come across two distinct uses of const with pointers. Declaring a constant pointer. This prevents you from ever changing the address it points to. i n t myage = 3 0 ; const i n t pt = & myage ; The integer pointer pt will always points to the address of integer variable myage. We can therefor not use assignments with the pointer pt to change the value of myage. pt += 1 ; // BAD pt p o i n t s to a c o n s t a n t i n t e g e r. c i n >> pt ; // BAD Again pt p o i n t s to a c o n s t a n t i n t e g e r. myage = 20 // GOOD As myage i s not c o n s t a n t. Note these assignments woudl work if pt was not of constant type. Using a constant prototype to protect array values (see previous example). const i n t months [ 1 2 ] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31;... i n t sum ( i n t a r r [ ], i n t n ) ; // p r o t o t y p e with u n p r o t e c t e d v a l u e s. i n t sum ( months, 1 2 ) ; // W i l l not work as months i s a c o n s t a n t. John Chrispell, Friday October 1, 2010 slide 19/25

11 Pointers and const Consider the following i n t age = 3 9 ; const i n t pt = &age ; The const in the second declaration is only declaring the integer to which the pointer points is constant. Not the pointers address. Thus, we could change the address by: i n t v a l u e = 1 2 ; pt = &v a l u e ; // This i s p e r f e c t l y ok. / The v a l u e pt p o i n t s to i s now 12 / A second posibility is: i n t s l o t h = 3 ; const i n t ps = &s l o t h ; // p o i n t e r to c o n s t i n t. i n t const s n i p e = &s l o t h ; // a c o n s t p o i n t e r to i n t. Note we can even declare both value and the pointer to be constants. This is useful if you are passing around data using pointers. John Chrispell, Friday October 1, 2010 slide 21/25

12 Functions and Two-D arrays If we qre given the two dimensional array and sum fucntion: i n t data [ 3 ] [ 4 ] = {{1,2,3,4, { 9, 8, 7, 6, { 2, 4, 6, 8 ; i n t t o t a l = sum ( data, 3 ) ; What does the function prototype look like? John Chrispell, Friday October 1, 2010 slide 23/25

13 Functions and Two-D arrays If we qre given the two dimensional array and sum fucntion: i n t data [ 3 ] [ 4 ] = {{1,2,3,4, { 9, 8, 7, 6, { 2, 4, 6, 8 ; i n t t o t a l = sum ( data, 3 ) ; What does the function prototype look like? Idea: the data element is an array with each element an array of size 4. i n t sum ( i n t ( ar2 ) [ 4 ], i n t s i z e ) ; // or e q u i v e l e n t l y i n t sum ( i n t ar2 [ ] [ 4 ], i n t s i z e ) ; Note we need the parentheses, as int *ar2[4] declares and array of 4 pointers to type int instead of the desired pointer single pointer to an array of four ints. The first cannot work as C/C++ dosn t allow passing of arrays. John Chrispell, Friday October 1, 2010 slide 23/25

14 the sum function The sum function in the previous example would look something like: i n t sum ( i n t ar2 [ ] [ 4 ], i n t s i z e ){ i n t t o t a l = 0 ; f o r ( i n t r =0; r < s i z e ; r++){ f o r ( i n t c = 0 ; c < 4 ; c++){ t o t a l += ar2 [ r ] [ c ] ; return t o t a l ; Note we are passing in the number of rows to be looped over. Note the equivalence of the following: ar2 [ r ] [ c ] == ( ( ar2 + r ) + c ) // These a r e the same. John Chrispell, Friday October 1, 2010 slide 25/25