C++ For Science and Engineering Lecture 10

Transcription

1 C++ For Science and Engineering Lecture 10 John Chrispell Tulane University Wednesday 15, 2010

2 Introducing for loops Often we want programs to do the same action more than once. #i n c l u d e <iostream > i n t main ( ) { using namespace s t d ; i n t i ; // c r e a t e a c o u n t e r / i n i t i a l i z e ; t e s t ; update / f o r ( i = 0 ; i < 5 ; i ++){ cout << Counting Sheep << i <<. \ n ; cout << ZZZZZZZZZZzzzzzzz..... \ n ; return 0 ; You may use the increment operator anywhere, not just in loops. John Chrispell, Wednesday 15, 2010 slide 3/27

3 Introducing for loops For loops handel the following steps: 1 Setting a value initially. 2 Performing a test to see whether the loop shoudl continue. Typically a relational statement or one that compares two values. At its heart this is a true or false statement. Every expression has value as will be seen in an upcoming listing. 3 Executing the loop actions. 4 Updating value(s) used for the test. John Chrispell, Wednesday 15, 2010 slide 5/27

4 The design of for loops John Chrispell, Wednesday 15, 2010 slide 7/27

5 Value of expressions express.cpp Consider the value of some expressions: #i n c l u d e <iostream > i n t main ( ) { using namespace std ; i n t x ; cout << The e x p r e s s i o n x = 100 has the v a l u e ; cout << ( x = 100) << endl ; cout << Now x = << x << endl ; cout << The e x p r e s s i o n x < 3 has the v a l u e ; cout << ( x < 3) << endl ; cout << The e x p r e s s i o n x > 3 has the v a l u e ; cout << ( x > 3) << endl ; / ======================= / / a newer C++ f e a t u r e / / ======================= / cout. s e t f ( i o s b a s e : : b o o l a l p h a ) ; cout << The e x p r e s s i o n x < 3 has the v a l u e ; cout << ( x < 3) << endl ; cout << The e x p r e s s i o n x > 3 has the v a l u e ; cout << ( x > 3) << endl ; return 0 ; John Chrispell, Wednesday 15, 2010 slide 9/27

6 numtest.cpp The following listing has code that counts down! #i n c l u d e <iostream > i n t main ( ) { using namespace s t d ; cout << Enter the s t a r t i n g countdown v a l u e : ; i n t l i m i t ; c i n >> l i m i t ; i n t i ; f o r ( i = l i m i t ; i ; i ){ // q u i t s when i i s 0 cout << i = << i << \n ; cout << Done now t h a t i = << i << \n ; return 0 ; Note we don t enter the loop if limit is 0. John Chrispell, Wednesday 15, 2010 slide 11/27

7 for loops in C++ and C There is a difference in for loops in C and C++. In C++ you can declare and initialize a variable in a for loop. f o r ( i n t i = 2 0 ; i > 0 ; i ) cout << i = << i << \n ; / i o n l y e x i s t s i n the l o o p / cout << i << e n d l ; // Not v a l i d as i dosn t e x i s t. Note we can omit the braces around the for loop if we feel lazy. Now lets us a loop to calculate factorials! John Chrispell, Wednesday 15, 2010 slide 13/27

8 factorial.cpp #i n c l u d e <iostream > using namespace s t d ; const i n t A r S i z e = 1 6 ; / example o f e x t e r n a l d e c l a r a t i o n / i n t main ( ) { double f a c t o r i a l s [ A r S i z e ] ; f a c t o r i a l s [ 1 ] = f a c t o r i a l s [ 0 ] = 1. 0 ; f o r ( i n t i = 2 ; i < A r S i z e ; i ++){ f a c t o r i a l s [ i ] = i f a c t o r i a l s [ i 1]; cout << Values : << e n d l ; f o r ( i n t j = 0 ; j < A r S i z e ; j ++) cout << j <<! = << f a c t o r i a l s [ j ] << e n d l ; return 0 ; John Chrispell, Wednesday 15, 2010 slide 15/27

9 forstrings.cpp // f o r s t r 1. cpp u s i n g f o r with a s t r i n g #include <i o s t r e a m > #include <s t r i n g > i n t main ( ) { using namespace s t d ; cout << Enter a s t r i n g : ; s t r i n g word ; g e t l i n e ( cin, word ) ; / ================================ / / d i s p l a y l e t t e r s i n r e v e r s e o r d e r / / ================================ / f o r ( i n t i = word. s i z e ( ) 1 ; i >= 0 ; i ){ cout << word [ i ] ; cout << \ndone!. \ n ; return 0 ; John Chrispell, Wednesday 15, 2010 slide 17/27

10 ++ vs. The increment (++) and decrement ( ) operators can be use two ways. 1 Prefix : --i 2 Postfix : i++ One advances the index after the execution of the statement. The other indexes the variable then executes the statement. C++ compilers make no garuntees when you mix these operators within a statement. v a l u e = i ( i ) ; // This s h o u l d be a v o i d e d. In a loop there is no difference between: f o r ( n = l i m ; n > 0 ; n ) s tatement ; f o r ( n = l i m ; n >0; n ) s tatement ; There may be a small difference in speed of execution. John Chrispell, Wednesday 15, 2010 slide 19/27

11 ++, and pointers The increment (++) and decrement ( ) operators can be use with pointers. Consider the following double a r r [ 5 ] = { , , , , ; / Set the p o i n t e r pt to a r r [ 0 ], i. e. to 21.1 / double pt = a r r ; / Advance the p o i n t e r to a r r [ 1 ], i. e / pt++; You can also change the value pointed to by using the dereference operator. / I n c r e m e nt the p o i n t e d to v a l u e / ( pt )++; // or ++ pt / D e r e f e r e n c e o r i g. l o c a t i o n, then i n c r e m e n t p o i n t e r / pt++; John Chrispell, Wednesday 15, 2010 slide 21/27

12 more for your bag of tricks C++ and even C allow for combinations of operators: / Combined Assignment O p e r a t o r s / += A s s i g n s L + R to L = A s s i g n s L R to L = A s s i g n s L R to L /= A s s i g n s L/R to L %= A s s i g n s L%R to L You may use these at will. i += 5 ; j = v a l u e ; But now that we have loops we need to make sure we format the code so that we can read it. Please USE BLOCKS!!! Note variables declared in blocks live only in those blocks. The also mask variables of the same name! John Chrispell, Wednesday 15, 2010 slide 23/27

13 block.cpp #i n c l u d e <iostream > i n t main ( ) { using namespace std ; cout << The Amazing Accounto w i l l sum and average ; cout << f i v e numbers f o r you. \ n ; cout << P l e a s e e n t e r f i v e v a l u e s : \ n ; double number ; double sum = 0. 0 ; f o r ( i n t i = 1 ; i <= 5 ; i ++){ // b l o c k s t a r t s h e r e i n s i d e l o o p cout << Value << i << : ; c i n >> number ; sum += number ; cout << F i v e e x q u i s i t e c h o i c e s i n d e e d! ; cout << They sum to << sum << endl ; cout << and a v e r a g e to << sum / 5 <<. \ n ; cout << The Amazing Accounto b i d s you adieu! \ n ; return 0 ; John Chrispell, Wednesday 15, 2010 slide 25/27

14 forstr2.cpp An example of how a, can be used to put two expressions into a single statement. #i n c l u d e <iostream > #i n c l u d e <s t r i n g > i n t main ( ) { using namespace std ; cout << Enter a s t r i n g : ; s t r i n g word ; g e t l i n e ( cin, word ) ; char temp ; i n t i, j ; / Use, to s e p a r a t e two e x p r e s s i o n s / f o r ( j = 0, i = word. s i z e ( ) 1 ; j < i ; i, ++j ){ temp = word [ i ] ; word [ i ] = word [ j ] ; word [ j ] = temp ; cout << word << \ndone\n ; return 0 ; John Chrispell, Wednesday 15, 2010 slide 27/27