CMU Statistics Summer Computing 2013 Day 2: Introduction to C/C++

Transcription

1 1) QH1 Problems 1 to 3 CMU Statistics Summer Computing 2013 Day 2: Introduction to C/C++ 2) QH1 Problem 4: Setting up C and quick intro to C 3) Preview of HW1 and discussion of documentation (suggest viewing draft R coding conventions) 4) C is a compiler that converts your source code into native machine language for whichever operating system and computer architecture you are using. Therefore it is fast! It requires more work and skill than R (an interpreter rather than a compiler) and is far more dangerous, e.g., it is easy to change variables accidentally. 5) C++ is an object oriented compiler that adds many high level features to C (especially features similar to objects and methods in R). It is a bit harder to learn the C, but tends to be much safer. Its speed is close to C (use of safer features does tend to slow it down some). It allows more complex programming that C with fewer lines of code. It is often a better choice than C when you are writing complex code, particularly with complex, user defined data types. We will use C++ because if you happen to need any of it s features, it will be easy to add them to your existing code. 6) I will assume that you are using GNU g++. Besides being free, well maintained, well documented ( widely used, and widely accepted, it has the GNU Scientific Library (GSL) and support for the best matrix algebra system (BLAS). It is on the department machines and may already be on your Mac. If you have a Windows PC, the options include a. install the mingw version of GNU C on your PC b. install Visual C++ (expensive but very good) or Visual C++ Express (free with some limitations) c. SSH into a Department or University Linux computer (also a Mac choice). 7) Quick Unix/Linux primer a. ssh into unix.andrew.cmu.edu or another machine, allowing security as needed. b. Commands and file names are case sensitive! c. Directory levels are separated by slash not back-slash. Tilde (~) at the beginning of a file name starts at your home directory. Slash (/) starts at the computer s home directory. E.g., ls ~/projects/ shows the files in the projects subdirectory of your home directory and ls /bin shows the files in the system bin directory. 1

2 d. Environmental variables hold key information. The printenv command shows the current values. The value of the PATH variable indicates where programs will be looked for. Use setenv myvar myvalue to set a variable. e. Ending a command with & makes it run independently, so your terminal is not tied up and you get a new command prompt to run other things simultaneously. f. Basic commands are i. List files in the current ( working ) directory: ls ii. Print working directory: pwd iii. Change directories (absolute or relative): cd 1. Starting with ~ or / is absolute, e.g., ls ~ 2.../ goes up one level form the current location, e.g., ls../over 3. subdirname goes down a level, e.g., ls mysub iv. Make a new directory (absolute or relative): mkdir v. Rename or move a file: mv, e.g., mv old new or mv old.. vi. Delete a file: rm vii. Edit a file: xemacs myfile & viii. Run a program in your path: progname & ix. Run a program in your current directory:./progname & x. List currently running jobs: jobs xi. Kill currently running job #1: kill %1 xii. Filename completion with tab. Ctrl-a / ctrl-e to start / end of line. 8) Here are the steps to writing a program in C++ a. Plan your program and testing procedures. Break down the problem into a main program and useful functions. Consider details of input and output including command line arguments. b. Using a text editor create the main() function and all of the additional functions, usually in separate.cpp files (.c for plain c). Create.h header files with macros, prototypes and includes as needed. Much of C++ functionality is not built in per se, but comes from libraries, and you must include the corresponding header files to have access to named constants and appropriate function prototypes. Include appropriate commenting. 2

3 c. Compile each.cpp (or.c) file using -Wall warning level. Correct syntax errors and warnings. A sample command is g++ -c -Wall myfile1.cpp, and this produces the object file myfile1.o. (Correct syntax errors and re-run.) d. Link the object files together with the standard library and any necessary additional libraries to create the executable program. (The last two steps are greatly facilitated by a make file.) A sample command is g++ -o myprog myfile1.o myfile2.o and this produces the executable program myprog. (If you forget the -o outfile, the program will be called a.out.) e. Test and debug your program. (It s nice to add. to your PATH.) f. Simple first example: In a file named hello.cpp put: /* This is hello.cpp, a simple C++ program */ /* compile: g++ -c -Wall hello.cpp */ /* link: g++ -o hello hello.o */ /* run: hello */ #include <iostream> /* using namespace std; */ int main() { std::cout << "C++ rules!\n"; return(0); } Then at the Unix prompt type g++ -c -Wall hello.cpp to convert the source code to object code (assuming no syntax errors). Then at the Unix prompt type g++ -o hello hello.o to create the executable hello program from the object file hello.o and the standard C++ library functions. The hello file is run by typing hello at the Unix prompt (if your path does not include., you ll need./hello to run it), and what we see is: C++ rules! 9) Variables in C++ a. Your variable names (identifiers) must start with a letter, and then may be followed by up to 30 more letters and numbers plus underscore ( _ ). Identifiers are case sensitive. Make the names meaningful to aid others who may read your code (including your future self)! Here is a list of reserved keywords that cannot be used as variable names: i. C keywords: auto, const, double, float, int, short, struct, unsigned, break, continue, else, for, long, signed, switch, void, case, default, enum, goto, register, sizeof, typedef, volatile, char, do, extern, if, return, static, union, while ii. Additional C++ keywords: asm, dynamic_cast, namespace, reinterpret_cast, try, bool, explicit, new, static_cast, typeid, catch, false, 3

4 operator, template, typename, class, friend, private, this, using, const_cast, inline, public, throw, virtual, delete, mutable, protected, true, wchar_t, and, bitand, compl, not_eq, or_eq, xor_eq, and_eq, bitor, not, or, xor iii. Also, these, while not keywords, are pre-defined identifiers: cin, endl, INT_MIN, iomanip, main, npos, std, cout, include, INT_MAX, iostream, MAX_RAND, NULL, string b. Basic Variable Types i. char: one byte (8 bits) representing -128 to This can be interpreted as holding one standard ASCII character. These are represented as something like 'A', '4', and '\n', where the latter is the ASCII newline character (code 10). Note that the quotes must be single, and the contents must be only one character or a predefined '\*' symbol. ii. int: the natural size of an integer for the machine, typically 32 or 64 bits (4 or 8 bytes) on modern machines but not guaranteed to be bigger than 2 bytes. 32 bit integers go from -2,147,483,648 to 2,147,483,647. (The rarely used short int is allowed to be implemented in as few as 16 bits). iii. float: a single precision floating point number typically not used in statistical computing. These are 32 bit IEEE 754 representations with about 7 significant (decimal) figures. Float constants can be typed as decimals followed by the letter f, e.g., 3.4f. iv. double: a double precision floating point number using for representing non-integer numeric data in statistical computing. These are 64 bit (8 byte) IEEE 754 representations of real numbers or their best approximations. The format has approximately 16 significant (decimal) figures and a range of approximately 10 +/-308. Double constants can be typed as decimals (using a trailing.0 for whole numbers) or in scientific notation as, e.g., -3.45e-12. v. By default, the above are signed, but unsigned versions of char and int are available. The unsigned versions go from 0 to 255 and (for 32 bit systems) 0 to 4,294,967,295 (2 32-1). vi. Also, a long versions of int is available that is guaranteed to be at least 4 bytes. A long double is at least 8 bytes, but may use 10 or 16 bytes. vii. The header files <climits> and <cfloat> contains the actual limits for whichever system you are using. (Use, e.g., <float.h> in plain C.) viii. C++ also has wchar_t which is holds Unicode multi-byte text. ix. A const modifier makes a variable unchangeable, e.g. (from math.h), const double pi =

5 c. Pointer variables contain the location of another variable. You can add or subtract something from a pointer to get another pointer which may or may not point to appropriate information. Pointers are powerful and dangerous. You can create a pointer to: i. Any other variable, including a class object or another pointer ii. A function Pointer variables are declared to point to a specific type of variable, and incrementing or decrementing by 1 changes their value by the number of bytes of whatever it is pointing to (see sizeof() operator). This allows implementation of efficient one-dimensional arrays if an appropriate block of memory is available. The size of a pointer variable itself is the computer s word size: 32 or 64 bits. d. Structure variables are user defined complex data types, similar to lists in R. Union variables allow a stretch of memory to be interpreted in multiple ways. e. Classes such as string are a powerful C++ improvement over structures. String class objects are easier to manipulate and less subject to misuse (though a bit slower) than c style strings which are really a character array with a terminating NULL (ASCII 0) character. They may contain various data elements and functions that operate on those data elements. f. Using variables i. A variable declaration introduces an identifier (variable) and describes its type: native data type, object, or function. A declaration is what the compiler needs to accept references to an identifier that is defined elsewhere. Unless the variable is defined somewhere in your program, the code will compile, but fail at linkage time. Multiple (identical) declarations for the same variable are allowed. A declaration must occur before the first use of a variable in any program unit. ii. A variable definition actually instantiates/implements an identifier. It is what the linker needs in order to link references to those entities. It tells the compiler to allocate space for variables and optionally how to initialize the variables. Each full program must define each variable exactly once. All definitions are also declarations. 5

6 iii. The form type identifier; is used to declare a variable of a given type with a specific name. When used with native data types, this form is also a definition (without initialization). Also type identifier = initialvalue; is a definition with initialization. The alternate form extern type identifier; ensures that the statement is just a declaration. For functions and classes, the extern is optional and the lack of a body for the function or class is enough to ensure that the statement is only a declaration. A class or function definition is of the form class identifier {body}; or returntype function(argument list) identifier {body};. Here are some definitions: int numkids; double height = 3.4; char first_letter = A ; double function sqr(double x) { return(x*x); } And here are some declaration examples: extern double height; extern char first_letter; double function(double x); The last declaration is also called a function prototype. iv. A pointer is declared or defined using an asterisk: int *pkidnum; /* int* pkidnum is also allowed */ double *pheight; double (*pfun)(int); /* pointer to function with int argument and double return */ v. An array is defined using square brackets: int kidspersubject[numsubjects]; double riverlengths[3]; char name[17]; /* 16 char. name + terminator */ char names[nsubjects][17]; The array variable is a pointer to a block of memory. You cannot be sure what the initial values are with the above forms. Arrays can be initialized at definition like this: double myarray[3] = { 0.0, 0.0, 0.0 }; Declarations to arrays use empty brackets: extern double myarray[ ]; or equivalently: extern double *myarray; 6

7 Future references with [ ] access elements, numbered 0 to length-1. Future references without the [ ] are pointers to the first element. Multidimensional arrays are simulated. E.g., in the above example of an array of length nsubjects, each element is an array of length 17. The variable name itself is a pointer to an array of nsubjects pointers each of which points to a character array of length 17. C-style strings (as opposed to C++ strings from the string class) are an array of characters terminated by a '\0' (ASCII code 0) null character. They may be initialized using double quotes: int name[ ] = "Hello"; This creates a memory block with 6 bytes including the terminal null. It can be change to hold any string up to 5 characters long. You can also use, e.g.: int name[256] = "Hello"; to hold any string up to 255 characters long. vi. Dynamic memory allocation uses the keyword new : int * array_of_ints;... array_of_ints = new int[5000];... delete [] array_of_ints;... array_of_ints = new int[10000]; The first statement allocates a single uninitialized pointer to a potential array of integers. The new statement allocates space for 5000 integers and sets the pointer to point to them. The delete statement returns the memory to the operating system. The second new allocates a new, larger block of space. If the dynamic memory allocation does not occur within a try/catch block, the program ends with the message terminate called after throwing an instance of 'std::bad_alloc'. An alternate approach if you want to internally recover from an attempt to allocate too much memory is: double *ages = new (nothrow) double[nsubjects]; The added (nothrow) will cause the pointer to be equal to NULL if not enough memory is available, and you can test and die with a nice message if needed instead of using the more complex try/catch approach. 7

8 vii. Function declarations, also called function prototypes, tell the compiler how functions are defined in terms of the object type returned and the object type(s) of the arguments (if any). Functions may return type void, which is essentially returning nothing. Each file is compiled separately and should (must in most circumstances) be told the prototype of every function it calls (and may be told the prototype of functions it does not use without any penalty). Therefore, it is common, good practice to put all of the prototypes of your project into an include (.h) file and include that file at the top of every.cpp file in your project. Prototypes look like function definitions but have no function body. Examples are: double powerfun(double value, int power); char *wordreverser(char *word); void printinfo(char *word, int times, double v); Special syntax exists for a variable number of arguments. viii. Variables have a storage class which determines their lifetime, i.e., when they occupy memory during the program run. The storage classes are: 1. Automatic: allocated and initialized automatically when a function is called (includes parameters) and/or a braced { } block is entered, and automatically destroyed when these complete. The stack mechanism manages these automatic variables. 2. Static: allocated and initialized when the program starts, and destroyed when it ends. 3. Dynamically allocated: these data are stored on the heap. If you don t manage their pointers appropriately, memory fragmentation and/or leak will occur. (A garbage collector may help with this problem.) E.g., if you have an automatic local pointer in a function named *pdata, and you use new to assign to it the location of a dynamically allocated block of heap memory, and then the function returns to its caller without running delete [ ], that memory block will be orphaned until the program ends. ix. Variables have a scope, which is the portion of the program within which they are available. 1. Local variables are visible only within the function or smaller braced {} block where they are defined and are completely invisible if a nested {} block creates a new variable of the same name. They are also invisible when another function is entered. 2. Global variables are defined outside of any functions and are visible throughout the program. They are not directly available if another variable using the same name is in scope, but are indirectly available from everywhere using a form like ::myglobalvar. 3. Static variables have a scope limited to the function where they are defined using the static keyword prefix, but have a full program lifetime. 8

9 10) Functions in C/C++ use pass-by-value. This means that a copy of each argument from the calling program is passed to the function. Therefore the original value cannot be changed by the function. Normally some variable in the original program is changed by using the form var = function(var, othervar);. To change more than one variable in the calling function, you can pass a pointer to the variable, which gives the calling program full access to the original variable (but has some danger). In addition to the opportunity to use the pointer to any single variable, all arrays are passed by reference, i.e., the name of any array is really a pointer to the first value, so the called function, which receives a copy of the pointer, can modify it. Use const if you want to prevent the called function from potentially changing the array contents. 11) Source code in C++ consists of preprocessor directives, variable (data) declarations and definitions function prototypes (declarations), function definitions, and class declarations and definitions. Functions are not variables in the same way as they are in R. Instead, they are special primary objects, and most C++ code is just function and class definitions. 12) Almost all C++ programs contain some preprocessor directives. These start with # and do not end in a semicolon. The main directives are: a. #include <somestandardincludefile> or #include "myincludefile.h" which simply adds in the code of that file at the point of the directive b. #define IDENTIFIER VALUE to define either a macro or a constant c. #if/#ifdef/#ifndef/#elif/#else/#endif to include or exclude specific bits of code Although any valid C++ code be in the include file, typically it consists of constant definitions, macro definitions, declarations (including prototypes), and directives to include other include files. 13) Comments can occur anywhere and are bracketed by /* and */ in C or C++ or just preceded by // in C++. 14) C++ has a fairly small vocabulary. The keywords include those for function, class and variable declaration, those for program control (e.g., if/else, continue/break, for, switch/case/default, do/while), several used for casting, plus sizeof and return. 15) C/C++ code syntax: simple code lines end with a semicolon ( ; ) while complex ones consist of a series of semicolon-terminated simple commands enclosed in a set of curly braces ( { and } ). All practical functions consist of a single complex command. 9

10 16) Math is fairly standard, but be careful of integer arithmetic. doublevar = intvar/5; /* assigns 4.0 if intvar=24 */ doublevar = intvar/5.0; /* assigns 4.8 if intvar=24 */ Times and divide have higher precedence than add and subtract. A%B computes A modulo B. There is no power operator (but you can use the pow(x,y) function to compute x y ). Shortcut operators exist for changing variables directly: i++; /* is the same as i=i+1, while i--; subtracts 1 */ i+=3; /* adds 3 to i. *=, /= and -= also work */ Special bitwise operators are occasionally useful. The sizeof() operator finds the size of a variable or variable type in bytes. The dereferencing operator is *. If you declare pch to be a pointer to char, and if you set pch to point to some allocated memory, *pch refers to the character itself. The & operator extracts the address of a variable, i.e., it creates a pointer to the variable. The + and - operators (as well as ++, --, +=, and -=) work on pointers, so you can use: char string[] = " ABCDEF; char *palpha = string+10; // points to "ABCEDEF" char Dee, Eff; palpha +=3; // now points to "DEF Dee = *palpha; // Dee holds 'D' Eff = palpha[2]; // Eff holds 'F' 17) The keyword const can be used in definitions and declarations to help C++ help you avoid common errors. It assures that, after the initial definition some memory location(s) cannot be changed in the future. E.g., const int pi= ; assures that variable pi cannot be accidentally modified. As another example, const char * name; is a pointer to a c-style string that can be changed to point to a different string, but cannot be (accidentally) used to change any name because name[j]= K ; gives a compiler error. As an example, consider this code fragment: char name1[7] = "Howard"; char name2[8] = "Seltman"; const char *pc = name1; cout << "name1 is " << name1 << "\n"; pc = name2; // pc[3]='k'; // error: "assignment of read-only location" cout << "name2 is " << name2 << "\n"; 18) Casting is an important and somewhat confusing topic. It is similar to the as.xxx functions in R. a. Explicit casting: either I = int(d); or I = (int) d; takes the integer part of double d and stores it in I, truncated, not rounded. b. static_cast: I = static_cast<int>(d); is the newer way to do the above. 10

11 c. const_cast can be used (carefully) to reinterpret the const-ness of a variable. d. dynamic_cast performs run time checking for appropriateness, and is useful for complex use of classes. 19) Output to the terminal can be of the form: cout << "string text" << variable << "more" << another << "\n"; or if you really want to flush the terminal cout << "string text" << variable << "more" << another << endl; 20) Input from the keyboard can be of the form cin >> var; but this cause problems because it leaves everything after the first space for the next cin >> var statement. Usually you want to use something like: string intext; cout << "Enter an number> "; getline(cin, intext); d = atof(intext.c_str()); cout << "Your number is " << d << endl; The getline() function safely stores a line of input of any size in the C++ string class string intext (automatically allocating an appropriate amount of space). The c_str() string class member function returns a pointer to an old c-style string, which is what the atof() function needs to convert the text to a double (despite the fact that it should be named atod() ). There is no error checking, and numbers followed by non-numerics are read as the number, while initial non-numerics cause a zero result. 21) A good place to look for pre-defined functions to do whatever task you need is 22) Logical expressions can use ==,!=, <, >, <=, and >= as well as (),!(), &&, and some other operators. 11

12 23) The syntaxes for program control are: if (someexpression) somecommand; if (someexpression) { somecommands; } else if (otherexpression) { someothercommands; } else { evenmorecommands; } for (i=0; i<5; i++) { /* do something with i in {0,1,2,3,4) */ if (someexpression) continue; /* next i, without commands below */ if (otherexpression) break; /* leave the loop now! */ } switch (somevariable) { case somevalue: somecommands; break; case othervalue: othercommands; break;... default: defaultcommands; } while (someexpression) { commandspossiblydone; } do { commandsdoneatleastonce; } while (someexpression); 24) Include files and warnings #include <somename> /* loads from installed source code somewhere */ #include myinclude.h /* loads from the current directory */ If you use a function which has a prototype in an include file that you did not include, you ll get a warning like: arrtest.c:10: warning: implicit declaration of function exit arrtest.c:10: warning: incompatible implicit declaration of built-in function exit In Unix, just type man S3 exit to see that it is stdlib.h that you need to include. 12

13 25) Namespaces Namespaces are a feature designed to allow you to use functions, classes, and constants from several different libraries or different parts of one of your large projects, even if they happen to use the same name. Essentially, C++ actually uses a long name of the form namespace::function to refer to any of these. All of the built in functions are in the std namespace, so should be typed in as, e.g., as std::cout. The cheap, dirty, and slow way to handle this is to have a using namespace std; statement at the top of your file, after the #include s and before the main code. It s probably better to have several separate statements like using std::cout;, or even safer, explicitly type std::cout each time instead of importing the entire namespace. The namespaces control only what goes into the object files at compile time. If you refer to anything outside the standard library, you still need to tell the linker where to find those items at link time. 26) Command line arguments and return codes: The pre-defined arguments for the main() function are main(int argc, char **argv), and argc will be the argument count including the program name as argument 0. All arguments are stored as strings (char[ ]) and argv is a vector of these, so argv[1] is the first supplied argument and is in the form of a \0 terminated string. You can use atoi(argv[j]) or atof(argv[j]) to convert to int or double respectively. The value inside the final return(); is passed to the operating system when the program ends as the return code. By convention, you should return 0 if the program ends successfully and some non-zero code(s) if it fails. This allows shell scripts to do different things based on success or failure of your program. 27) Redirection of input and output Unix and command-line Windows allow a file or the output of another program to substitute for keyboard input and/or to capture terminal output. The syntax is: myprog <altinput.txt >altoutput.txt Also: myprog1 myprog2 > altoutput.txt uses the output of myprog1 as the input for myprog2. 13