C++ For Science and Engineering Lecture 12

Transcription

1 C++ For Science and Engineering Lecture 12 John Chrispell Tulane University Monday September 20, 2010

2 Comparing C-Style strings Note the following listing dosn t do what you probably think it does (assuming word is a character array): word == john ; // This s h o u l d be used with c a u t i o n It will compare the address strings addresses. The following listing compares strings using charcter arrays: #include <iostream > #include <c s t r i n g > // p r o t o t y p e f o r strcmp ( ) i n t main ( ) { using namespace std ; char word [ 5 ] =? a t e ; f o r ( char ch = a ; strcmp ( word, mate ) ; ch++){ cout << word << endl ; word [ 0 ] = ch ; cout << A f t e r loop ends, word i s << word << endl ; strcmp(a, b) returns 0 when a = b, a negative when a < b and a positive when b > a. John Chrispell, Monday September 20, 2010 slide 3/35

3 Comparing strings with the string class Note you can use the relational operators if you are using the C++ string class. #include <i o s t r e a m > #include <s t r i n g > // s t r i n g c l a s s i n t main ( ) { using namespace s t d ; s t r i n g word =? a t e ; f o r ( char ch = a ; word!= mate ; ch++){ cout << word << e n d l ; word [ 0 ] = ch ; cout << A f t e r l o o p ends, word i s << word << e n d l ; The above reworks the previous example using the C++ string class. John Chrispell, Monday September 20, 2010 slide 5/35

4 The while loop The following is syntax for a while loop: Remember the condition on which you are testing must be updated to exit the loop. John Chrispell, Monday September 20, 2010 slide 7/35

5 for vs. while The for loop and the while loop in C/C++ are essentially the same. Programmers use the following conventions: Use a for loop when you know the number of times you would like to do a specific task. Use a while loop when you are uncertain of how many times the loop will execute. Remember to block you code! I will be taking off for poor blocking of code in the future. You have been warned! John Chrispell, Monday September 20, 2010 slide 9/35

6 Using the clock! C++ has a way of adding time delays and seeing how long programs run! Note the following listing: #i n c l u d e <iostream > #i n c l u d e <ctime> // d e s c r i b e s c l o c k ( ) f u n c t i o n, c l o c k t type i n t main ( ) { using namespace std ; f l o a t secs ; cout << Enter the delay time, i n seconds : ; cin >> secs ; / =================================================== / / c o n v e r t to c l o c k t i c k s / / =================================================== / c l o c k t d e l a y = ( c l o c k t ) ( s e c s CLOCKS PER SEC ) ; cout << s t a r t i n g \a\n ; c l o c k t s t a r t = c l o c k ( ) ; while ( c l o c k ( ) s t a r t < d e l a y ){ // w a i t u n t i l time e l a p s e s / w a i t / ; // note the s e m i c o l o n cout << done \a\n ; John Chrispell, Monday September 20, 2010 slide 11/35

7 The do while loop once. Loops of this form always execute John Chrispell, Monday September 20, 2010 slide 13/35

8 Reading from the terminal One way to get info from the terminal is to use cin. This stops on white space. You typically want to read in spaces too. Even if you are looking character by character. The following listing reads in characters until we reach the # character. #i n c l u d e <iostream > i n t main ( ) { using namespace std ; char ch ; i n t count = 0 ; cout << Enter c h a r a c t e r s ; e n t e r # to q u i t : \ n ; c i n. get ( ch ) ; // use the c i n. get ( ch ) f u n c t i o n while ( ch!= # ){ cout << ch ; ++count ; c i n. get ( ch ) ; // use i t a g a i n cout << e n d l << count << c h a r a c t e r s read \n ; John Chrispell, Monday September 20, 2010 slide 15/35

9 Function Overloading Note we have used the functioncin.get() three ways now. 1 cin.get() with no arguments to read out an endline character (gets and returns a single character). 2 cin.get(char *, int) for a string of maximum size given by int. 3 cin.get(char) for a single character or char type. C would not like this. C++ allows function overloading and the compiler notes the arguments we are using and finds the correct version of the function upon compiling the code. If we want to read in the pound sign we can keep reading till the end of a file by testing for the end-of-file character. Consider the following listing: Use <c t r l ><d> or <c t r l ><z> for end of file on linux on Windows. John Chrispell, Monday September 20, 2010 slide 17/35

10 textin3.cpp #include <i o s t r e a m > i n t main ( ) { using namespace s t d ; char ch ; i n t count = 0 ; c i n. get ( ch ) ; // attempt to read a char while ( c i n. f a i l ( ) == f a l s e ){ // t e s t f o r EOF cout << ch ; // echo c h a r a c t e r ++count ; c i n. get ( ch ) ; // attempt to read a n o t h e r char cout << e n d l << count << c h a r a c t e r s read \n ; John Chrispell, Monday September 20, 2010 slide 19/35

11 textin4.cpp A slightly different approach that tests for EOF. Using characters as int values! #include <i o s t r e a m > i n t main ( void ){ using namespace s t d ; i n t ch ; // s h o u l d be i n t, not char i n t count = 0 ; while ( ( ch = c i n. get ( ) )!= EOF){ // t e s t f o r EOF cout. put ( char ( ch ) ) ; ++count ; cout << e n d l << count << c h a r a c t e r s read \n ; John Chrispell, Monday September 20, 2010 slide 21/35

12 Two dimensional arrays In C++ there is no special two dimensional array class. You simply declare an array of arrays: double maxtemps [ 4 ] [ 5 ] ; This gives an array maxtemps with 4 elements, each element is itself an array with 5 elements that are type double. We typically think of this in terms of maxtemps being an array with four rows and five columns, where each element is of type double. / Nested l o o p s to p r i n t v a l u e s / f o r ( i n t row = 0 ; row < 4 ; row++){ f o r ( i n t c o l = 0 ; c o l < 5 ; c o l ++){ cout << maxtemps [ row ] [ c o l ] << \ t ; cout << e n d l ; John Chrispell, Monday September 20, 2010 slide 23/35

13 Initializing a 2D array You initialize a 2D array in the same manner you did for a 1D array. Consider the following: i n t i n t e g e r s [ 3 ] [ 4 ] = { { 1, 2, 3, 4, // i n t e g e r s [ 0 ] { 5, 6, 7, 8, { 9, 1 0, 1 1, 1 2 ; // i n t e g e r [ 2 ] The next listing shows examples of using both a 2D array as well as an array of pointers to strings. John Chrispell, Monday September 20, 2010 slide 25/35

14 nested.cpp #i n c l u d e <iostream > const i n t C i t i e s = 5 ; const i n t Years = 4 ; i n t main ( ) { using namespace std ; const char c i t i e s [ C i t i e s ] = { // a r r a y o f p o i n t e r s // to 5 s t r i n g s G r i b b l e C i t y, G r i b b l e t o w n, New G r i b b l e, San G r i b b l e, G r i b b l e V i s t a ; i n t maxtemps [ Years ] [ Cities ] = { // 2 D array {95, 99, 86, 100, 104, // values for maxtemps [ 0 ] {95, 97, 90, 106, 102, // values for maxtemps [ 1 ] {96, 100, 940, 107, 105, // values for maxtemps [ 2 ] {97, 102, 89, 108, 104 // values for maxtemps [ 3 ] ; cout << Maximum t e m p e r a t u r e s f o r \ n\n ; f o r ( i n t c i t y = 0 ; c i t y < C i t i e s ; ++c i t y ) { cout << c i t i e s [ c i t y ] << : \ t ; f o r ( i n t y e a r = 0 ; y e a r < Years ; ++y e a r ){ cout << maxtemps [ y e a r ] [ c i t y ] << \ t ; cout << endl ; John Chrispell, Monday September 20, 2010 slide 27/35

15 The if statement The next topics we aim to under take are allowing the program to make decisions, and write to and read from files. For the first part we will use the if statement. The structure of the if statement is: i f ( t e s t e x p r e s s i o n ){ s t a t e m e n t s If the test expression is true the statements inside the code block are executed. Consider the following example that reads a sentence until a. is reached, and counts spaces. John Chrispell, Monday September 20, 2010 slide 29/35

16 if.cpp #i n c l u d e <iostream > i n t main ( ) { using namespace s t d ; char ch ; i n t s p a c e s = 0 ; i n t t o t a l = 0 ; c i n. get ( ch ) ; while ( ch!=. ){ // q u i t at end o f s e n t e n c e i f ( ch == ){ // check i f ch i s a space ++s p a c e s ; ++t o t a l ; // done e v e r y time c i n. get ( ch ) ; cout << s p a c e s << spaces, << t o t a l ; cout << c h a r a c t e r s t o t a l i n s e n t e n c e \n ; John Chrispell, Monday September 20, 2010 slide 31/35

17 ifelse.cpp Some times it is helpful to have a second option executed. #include <iostream > i n t main ( ) { using namespace std ; char ch ; cout << Type, and I s h a l l r e p e a t. \ n ; c i n. get ( ch ) ; while ( ch!=. ){ i f ( ch == \n ){ cout << ch ; // done i f n e w l i n e e l s e { cout << ++ch ; // done o t h e r w i s e c i n. get ( ch ) ; / t r y ch + 1 i n s t e a d o f ++ch f o r i n t e r e s t i n g e f f e c t / cout << \ n P l e a s e e x c u s e the s l i g h t c o n f u s i o n. \ n ; Note unlike FORTRAN or Matlab we need braces for multilple statemets to be inside the if or else blocks! John Chrispell, Monday September 20, 2010 slide 33/35

18 ifelseif.cpp #i n c l u d e <i o s t r e a m > const i n t Fave = 2 7 ; i n t main ( ) { using namespace s t d ; i n t n ; cout << Enter a number i n the range to f i n d ; cout << my f a v o r i t e number : ; do{ c i n >> n ; i f ( n < Fave ) cout << Too low g u e s s a g a i n : ; e l s e i f ( n > Fave ) cout << Too h i g h g u e s s a g a i n : ; e l s e cout << Fave << i s r i g h t! \ n ; while ( n!= Fave ) ; John Chrispell, Monday September 20, 2010 slide 35/35