C C Overview InputOutput New Data Types Expressions

C++ • • • C++ Overview Input/Output New Data Types Expressions Declarations, Operators Control Structures 1

C++ 159. 234 Bjarne Stroustrup of AT&T Bell Laboratories Designed and implemented a new language in the early 1980’s. Initially called “C with Classes” Classes it became known as C++, an incremental step up from C. It was designed to deliver: the flexibility and efficiency of C with the facilities for program organisation of the Simula language (usually referred to as object-oriented programming). 2

C++ 159. 234 A word from the inventor "C makes it easy to shoot yourself in the foot; C++ makes it harder, but when you do, it blows away your whole leg. " - Bjarne Stroustrup http: //www. research. att. com/~bs/C++. html 3

Object Oriented Programming (OOP) language? A programming language in which programmers define an abstract data type consisting of both: • data and • the operations (functions) that can be applied to data. 4

C++ 159. 234 AIMS Make programming more enjoyable. Be a general purpose language that: is a better C supports data abstraction supports object-oriented programming supports generic programming 5

C++ 159. 234 DESIGN PRINCIPLES Don’t get involved in a quest for perfection. Be useful immediately. Don’t try to force people. It is more important to allow a useful feature than to prevent every misuse. Leave no room for a lower-level language below C++ (except assembler). 6

C++ 159. 234 Language rules: Provide as good support for user-defined types as for built-in types. Locality is good. Preprocessor usage should be eliminated. No gratuitous incompatibilities with C. If you don’t use a feature, you don’t pay for it. 7

Comments 159. 234 COMMENTS C++ allows the use of // as a comment all text to the end of the line is ignored. Use /*. . . */ for large multi-line blocks of comments // for small comments. // allows nesting of comments. Commenting out a large block of code does not work in C, if the block already contains a small comment. /* for (i=0; i<10; i++) { k = i*i; /*k /* is the square of i*/ } */ /* for (i=0; i<10; i++) { k = i*i; // this is ok } */ 8

Comments 159. 234 COMMENTS Sometimes its useful to use a pre-processor directive to comment a large chunk of code: #if 0 commented out code with /* comments */ #endif 9

Input and Output 159. 234 Input and Output: C++ uses a new way to perform console I/O. Its major advantage over printf and getchar, is that it is both consistent and safer. printf("%s", i); /* but i is an integer */ is not detected as an error by most compilers C++ instead uses the notion of streams manipulators overloading it overloads the two operators >> and << 10

C++ streams A stream is a sequence of bytes moving to/from a device or file. 11

cin 159. 234 Input and Output #include <iostream>. . . cout << "Hello World“ << endl; writes "Hello World" on the console and then goes to the next line. cout << "i = " << i; //outputs the value of i For input: cin >> i; //The >> point the way the data is going. There is no need to use pointers! To get a complete line of text (the newline is read and discarded) cin. getline(s, max_no_of_chars); 12

Text Formatting 159. 234 To alter the appearance of output, we insert manipulators into the stream, or call functions. #include <iostream> endl outputs a new line dec uses decimal values hex uses hexadecimal values width(int w) field width of next output fill(char c) fill character setf(flag) set special flags ios: : left align ios: : right align cout. fill('0'); cout. setf(ios: : right); cout << hours << ": "; cout. width(2); cout << minutes << ". "; cout. width(2); cout << seconds << endl; sprintf is still used to format a string before displaying it. 13

C++ 159. 234 NEW SIMPLE TYPES C++ defines two new simple types: bool Boolean variables can represent either true or false. bool test; test = true; test = (x<5); bool, true and false are new keywords in C++ and cannot be used as identifiers. On output the values are shown as 0 (false) and 1 (true). wchar_t is a 'wide' character type is designed to allow alphabets with many characters (e. g. Japanese characters). 14

C++ Enumerated types: #define is used to link names to constants. Use of the pre-processor is frowned on in C++. Enumerated types are one way of replacing it. enum day {sun, mon, tue, wed, thu, fri, sat}; defines a new type day. 159. 234 Use this to create variables that can store days: day d 1, d 2; d 1 = tue; d 2 = d 1; if (d 2 == wed) {. . . } Enumerated types allow the compiler to check the values associated with them. d 1 = 1; // will produce a warning enum is implemented using integers. 15

C++ 159. 234 Enumerated types: In the above example sun is represented by 0, mon by 1 etc. There is no easy way to print out enumerated types. switch (d 1) { case sun: cout << "Sunday"; break; case mon: . . . } Another way of avoiding the pre-processor is by using const: const int MAX = 4000; MAX is a non-changeable integer variable. You can initialise constants but not assign to them. 16

C++ 159. 234 Values of expressions: We are familiar with the normal arithmetic operators: + - * / a + b is an expression and its value is the sum of the contents of the two variables. However, in C++, the following statements below: b = 2; c = 3; a = b + c; can also been written as: a = (b=2) + (c=3); DON’T do this. 17

C++ 159. 234 Values of expressions: However consider: a = b = c = 1; This sets several variables to one - and is used. The brackets have been omitted - if we insert them, we get: a = (b = (c = 1)); 18

C++ 159. 234 Comma operator: for (i=0; i<10; i++) { } In most uses of a for loop, the initialisation phase and the increment phase only have one expression. To initialise two variables we can use the comma operator for (i=0, j=0; i<10; i++, j++) { } The comma operator can be used anywhere: i = 0, j = 0; //this is OK. Its value is the value of the left-hand expression. k = (i=4), (j=5); k gets the value 4. But DON’T do this. 19

C++ 159. 234 The conditional operator: A 'ternary' 'ternary operator - it takes three arguments: expr 1 ? expr 2 : expr 3 If expr 1 is true then expr 2 is evaluated otherwise expr 3 is evaluated. It allows a shorthand form of an if statement: if (y < z) { x = y; } else { x = z; } can also be written: x = (y < z) ? y : z; 20

C++ 159. 234 break is used in a switch statement to stop execution falling through from one case to the next: switch (c) { case 'A': … do something break; case 'B': … etc break; } break can also be used in loops as well. while (true) { if (a == b) { break; }. . . } break ends the inner-most loop that encloses it. 21

C++ 159. 234 continue goes to the beginning of the loop. Inside a for loop the 'increment' expression is evaluated after a continue : for (i=0; i<total; i++) { cin >> c; if (c > 'A') { continue; // i is incremented } } This is different from a while loop i = 0; while (i<total) { cin >> c; if (c > 'A') { continue; //i is not incremented } i++; } 22

C++ 159. 234 goto The use of goto is frowned upon! Try not to use it. It can be helpful in getting out of deeply nested loops, when a break isn't much use. Explicit jumps around pieces of code are used a lot in assembler programming. A label has the same syntax as an identifier, with a colon added at the end for (i=0; i<10; i++) { for (j=0; j<20; j++) {. . . if (a[i][j] == 10) goto found; } }. . . found: cout << "I found it! " << endl; Better: create a function and use return. 23

C++ 159. 234 Declarations: C allows declarations at the start of a { } block. Variables only exists while the block is being executed. We normally declare variables at the start of a function, but this is also ok: int main () { int i; for (i=0; i<10; i++) { int temp; temp = i; } } temp only exists within the for loop. 24

C++ 159. 234 DECLARATIONS C++ allows declarations anywhere. The variable exists from that point to the end of the enclosing block. for (int j=0; j<10; j++) { cout << j; int k = j; cout << k; } for (int j=0; j<10; j++) { cout << j << k; // is an error! int k = j; cout << k; } j exists while the loop is executing and maybe longer depending on compiler settings. k exists from its declaration to the end of the loop. 25

C++ streams A stream is a sequence of bytes moving to/from a device or file. 26

Input from Keyboard Store in i the value entered from the keyboard: cin >> i; By default cin skips all white spaces (blank, tab, newline). Get a complete line of text (the newline character is read & discarded): cin. getline(s, max. No. Of. Chars); s must be a string and max. No. Of. Chars an integer (getline is a method of the cin object, set up for us by the system ) 27

Reading From File C-way #include <stdio. h> FILE *f; f = fopen(“data. txt”, “r”); fscanf(f, "%d %d", &n 1, &n 2); fclose(f); C++ way #include <fstream. h> ifstream in. File; in. File. open(“data. txt”); in. File >> n 1 >> n 2; in. File. close((); 28

Sending Data To File C-way #include <stdio. h> FILE *f; f = fopen(“result. txt”, “w”); • • • fprintf(f, "%d %d", n 1, n 2); fclose(f); C++ way #include <fstream. h> • • • ofstream out. File; out. File. open(“result. txt”); out. File << n 1 << n 2; out. File. close(); 29

File Name Selected By User char* file. Name; . . . cout << "Enter file name: "; cin >> file. Name; ifstream in. File; in. File. open(file. Name); if( !in. File){ cerr <<"No file-error here!"; exit(EXIT_FAILURE); //include <stdlib. h> } in. File. close(); cout <<"Done"; . . . 30

Reading data ( assume char ch; char str[60]; ) cin my. File Doing: Need #include<iostream> Need #include<fstream> cin >>ch my. File >> ch cin. get(ch) my. File. get(ch) Skip white spaces Reads every char ch cin. get(str, 40, ‘#’) cin. get(str, 40) Read C-strings (leaves termin. ch. in the stream), appends ‘’. cin. getline(str, 60, ’ my. File. getline(str, #’) 60, ’#’) cin. getline(str, 60) my. File. getline(str, 6 0) Read C-strings (removes termin. ch. from stream), appends ‘’. 31

Output data cout #include <iostream> cout <<value my. File #include<fstream> Doing: my. File<<value Write out value (char, int, C/C++ strings …) cout. put(ch) my. File. put(ch) Write out char ch 32

Attention! 1. Check for the existence of the file. if(!my. File){ //no file to work with !my. File cerr <<“Failed to find/create file<<endl; return 1; //notify the system something was wrong } This (common) trick works because my. File will have a NULL pointer if the file open operation failed. NULL is generally the same as zero so !NULL evaluates true. When the file is in a different directory, e. g “A: mystuffinput. txt” We must use: ifstream my. File(“A: \mystuff \input. txt”); \ \ 33

Attention! 2. Mixing numbers and strings when working with files needs careful consideration. while(in. File >> num 1 >> num 2 >> ch >> word) Is OK for this in. File: But it does not work for 23. 4 12 4 56 M Demlow A Smith 23. 4 56 M Ann Demlow 12 4 A Smith 34

#include <iostream> #include <fstream> using namespace std; int main(){ cout << “Hi there!" << endl; int i; while( cin >> i ){ ) cout << i << endl; } return 0; • Output is: Hi there! 1 1 2 2 Terminated with ^D or ^Z } 35

#include <iostream> #include <fstream> using namespace std; int main(){ ifstream my. Input. Stream( "testinput. txt" ); if( !my. Input. Stream ){ cerr << "error opening input file" << endl; exit(1); } ofstream my. Output. Stream( "testoutput. txt" ); if( !my. Output. Stream ){ cerr << "error opening output file" << endl; exit(1); } while( my. Input. Stream >> i ){ my. Output. Stream << i * 10 << endl; } my. Input. Stream. close(); my. Output. Stream. close(); return 0; } • Input of: 123456789 0 • Gives output file: 10 20 30 40 50 60 70 80 90 0 36

Summary Disk file ---- memory ifstream var ---- Keyboard Monitor cin cout cerr ---- >>… <<… ofstream var ---- Disk file 37

Exercise Write a complete program that will read the following data from a text file, then generate an output file. Example: Number of items 3 1 2. 2 3 4. 4 5. 5 6. 6 7 8. 8 1. 1 2. 1 3. 1 4. 1 Store these items into an array of struct x 1, y 1, x 2 , y 2 Use dynamic memory allocation for the array 38

Exercise Use the following format for the output file w. Bound. x 1 = v 1, w. Bound. y 1 = v 2, w. Bound. x 2 = v 3, w. Bound. y 2 = v 4 Where vi is a value read from the file Sample output file: out. txt w. Bound. x 1 = 1, w. Bound. y 1 = 2. 2, w. Bound. x 2 = 3, w. Bound. y 2 = 4. 4 w. Bound. x 1 = 5. 5, w. Bound. y 1 = 6. 6, w. Bound. x 2 = 7, w. Bound. y 2 = 8. 8 w. Bound. x 1 = 1. 1, w. Bound. y 1 = 2. 1, w. Bound. x 2 = 3. 1, w. Bound. y 2 = 4. 1 39

Summary File streams work like streams coming from/going to the keyboard (cin)/ screen (cout), except that the other end is a file instead of an input/output device. To use cin/cout, we must include <iostream> To use ifstream/ofstream, we must include <fstream> Next: Formatting output, program structure, types in C++, enum, const. Textbook p. 30 -34, 37(const), 383, browse p. 414 -417 40