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Introduction to C/C++

Introduction to C/C++. Doug Sondak SCV sondak@bu.edu. Outline. Goals C/C++ History Basic syntax makefiles Additional syntax . Goals. To be able to write simple C/C++ programs To be able to understand and modify existing C/C++ code To be able to write and use makefiles.

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Introduction to C/C++

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  1. Introduction to C/C++ Doug Sondak SCV sondak@bu.edu

  2. Information Services & Technology Outline • Goals • C/C++ History • Basic syntax • makefiles • Additional syntax

  3. Information Services & Technology Goals • To be able to write simple C/C++ programs • To be able to understand and modify existing C/C++ code • To be able to write and use makefiles

  4. Information Services & Technology Compiled vs. Interpreted Languages • Interpreted languages • when you type something, e.g., “x=y+z”, it is immediately converted to machine language and executed • examples: Matlab, Python • advantage • lots off convenient features • disadvantage • can be slow and memory-intensive

  5. Information Services & Technology Compiled (cont’d) • Compiled languages • examples: C, C++, Fortran • source code is written using a text editor • source code does nothing by itself – it’s just text • we will use source file suffix .cpp • source code must be processed through a compiler • translates source code into machine language • creates executable • this is the code that you actually run • like .exe file in Windows

  6. Information Services & Technology C History • Developed by Dennis Ritchie at Bell Labs in 1972 • Originally designed for system software • Impetus was porting of Unix to a DEC PDP-11 • PDP-11 had 24kB main memory! • 1978 book “The C Programming Language” by Kernighan & Ritchie served as standard • Official ANSI standard published in 1989 • Updated in 1999

  7. Information Services & Technology C++ History • C++ was developed by BjarneStroustrup at Bell Labs in 1979 • implemented object-oriented features of another language, “Simula,” in C • originally called “C with Classes” • name changed to C++ in 1983 • first commercial compiler in 1985 • official standard published in 1988

  8. Information Services & Technology C vs. C++ • C is essentially a subset of C++ • There are some “convenience features” in C++ that we will utilize here • Since we will be writing rudimentary codes, we will not get into object-oriented C++ constructs • We will use the GNU C++ compiler g++

  9. Information Services & Technology Types of Variables • each variable and value has a type • some common types • int • short for “integer” • number with no decimal places • 1, 857436 • float • short for “floating-point” • number with decimal places • 1.234, 4.0

  10. Information Services & Technology Types of Variables (cont’d) • char • short for “character” • enclosed in single quotes • ‘x’, ‘$’ • character string is string of chars enclosed in double quotes • “This is a character string.”

  11. IS&T Organization C/C++ Syntax • Case-sensitive • Spaces don’t matter except within character strings • I use them liberally to make code easy to read • Source lines end with semicolons (as in Matlab) • Comments • notes for humans that are ignored by the compiler • C: enclosed by /* */ • C++: // at beginning of comment • many C compilers also accept this syntax • Use them liberally!

  12. IS&T Organization C/C++ Syntax (cont’d) • source code contains functions • each one performs some task • you write some of them • some are intrinsic to the language • every code contains at least one function, called main • functions often, though not always, return a value • function is characterized by the type of value it returns • int, float, char, etc. • if function does not return anything, we will declare (characterize) it as an “int” function

  13. Information Services & Technology C/C++ Syntax (3) • functions may, but do not have to, take arguments • “arguments” are input values to the function • code blocks, including entire functions, are enclosed within “curly braces” { } • main function is defined in source code as follows: int main( ) { function statements } function arguments (we have no arguments here but still need parentheses) function name type declaration

  14. Information Services & Technology C/C++ Syntax (4) • Style note: some people like to arrange the brackets like int main( ) { function statements } • Either way is fine • Be consistent!

  15. Information Services & Technology C/C++ Syntax (5) • coutis a stream that is used to direct output to the screen, e.g. cout << “my string”; • sends the string to cout, i.e., to the screen • The above syntax does not include a carriage return at the end of the line. We can add the carriage return with: cout << “my string” << endl;

  16. Information Services & Technology C/C++ Syntax (6) • some program elements (built-in functions, variables, etc.) are contained in header files • to use these program elements you need to “include” the appropriate header files in your source code • in C, header files have .h suffixes • may or may not have .h suffix in C++ • syntax for inclusion of header files: #include <header_file_name> • Included before function definition • < and > are part of the syntax • Note that the #include statement does not end with a ;

  17. Information Services & Technology C/C++ Syntax (7) • C++ (but not C) has a feature, namespace, that defines packages of functions, etc. • This is fairly new, and you might not see it in older C++ programs • The cout stream is defined in the iostreamheader file, which is part of the “std” namespace • The syntax to use the std namespace is using namespace std;

  18. Information Services & Technology Exercise 1 • Write a “hello world” program in an editor • Program should print a character string • General structure of code, in order: • include the file “iostream” • not iostream.h, just “iostream” • use the “std” namespace • define main function • use cout and endl to print string to screen • Save it to the file name hello.cpp • solution

  19. Information Services & Technology Compilation • A compiler is a program that reads source code and converts it to a form usable by the computer • Code compiled for a given type of processor will not generally run on other types • AMD and Intel are compatible • We’ll use g++, since it’s free and readily available

  20. Information Services & Technology Compilation (cont’d) • Compilers have numerous options • See gcc compiler documentation at http://gcc.gnu.org/onlinedocs/ • gcc refers to the “GNU compiler collection,” which includes the C compiler (gcc) and the C++ compiler (g++) • For now, we will simply use the –o option, which allows you to specify the name of the resulting executable

  21. Information Services & Technology Compilation (3) • In a Unix window: g++ –o hello hello.cpp • “hello” is name of executable file (compiler output) • “hello.cpp” is source file name (compiler input) • Compile your code • If it simply returns a Unix prompt it worked • If you get error messages, read them carefully and see if you can fix the source code and re-compile

  22. Information Services & Technology Compilation (4) • Once it compiles correctly, type hello at the Unix prompt, and it will run the program • should print the string to the screen

  23. Information Services & Technology Declarations • different variable types (int, float, etc.) are represented differently internally • different bit patterns • must tell compiler the type of every variable by declaring them • example declarations: int i, jmax, k_value; float xval, elapsed_time; char aletter, bletter;

  24. Information Services & Technology Arithmetic • +, -, *, / • No power operator (see next bullet) • Math functions in math.h • pow(x,y) raises x to the y power • sin, acos, tanh, exp, sqrt, etc. • for some compilers, need to add –lm flag to compile command to access math library • Exponential notation indicated by letter “e” 4.2e3 • Goodpractice to use decimal points with floats, e.g., x = 1.0 rather than x = 1

  25. Information Services & Technology Arithmetic (cont’d) • ++ and -- operators • these are equivalent: i = i+1; i++; • always increments/decrements by 1 • += • these are equivalent: x = x + 46.3*y; x += 46.3*y;

  26. Information Services & Technology Arithmetic (3) • Can convert types with cast operator float xval; int i, j; xval = (float) i / (float) j; • Pure integer arithmetic truncates result! 5/2 = 2 2/5 = 0

  27. Information Services & Technology Exercise 2 • Write program to convert a Celcius temperature to Fahrenheit and print the result. • Hard-wire the Celcius value to 100.0 • We’ll make it an input value in a subsequent exercise • Don’t forget to declare all variables F = (9/5)C + 32 • solution

  28. Information Services & Technology cin • cinreads input from the screen cin >> var; • Note: >> rather than << as with cout • Writes input value to variable var • Often use cout and cin to prompt for a value: cout << “Enter value: ”; cin >> x;

  29. Information Services & Technology Exercise 3 • Modify Celcius program to read value from keyboard • Prompt for Celcius value using cout • Read value using cin • Rest can remain the same as last exercise • solution

  30. Information Services & Technology Arrays • Can declare arrays using [ ] float x[100]; char a[25]; • Array indices start at zero • Declaration of x above creates locations for x[0] through x[99] • Multiple-dimension arrays are declared as follows: int a[10][20];

  31. Information Services & Technology Arrays (cont’d) • Character strings (char arrays) always end with the character \0 • You usually don’t have to worry about it as long as you dimension the string 1 larger than the length of the required string char name[5]; name = “Fred”; char name[4]; name = “Fred”; works doesn’t work

  32. Information Services & Technology For Loop • for loop repeats calculation over range of indices for(i=0; i<n; i++){ a[i] = sqrt( pow(b[i],2) + pow(c[i],2) ); } • for statement has 3 parts: • initialization • completion condition • what to do after each iteration

  33. Information Services & Technology Exercise 4 • Write program to: • declare two vectors of length 3 • prompt for vector values • calculate dot product • print the result • solution

  34. Information Services & Technology Pointers • Memory is organized in units of words • Word size is architecture-dependent • Pentium: 4 bytes • Xeon, Itanium: 8 bytes • Each word has a numerical address 32 16 8 0

  35. Information Services & Technology Pointers (cont’d) • When you declare a variable, a location of appropriate size is reserved in memory • When you set its value, the value is placed in that memory location float x; x = 3.2; 32 3.2 16 8 0 address

  36. Information Services & Technology Pointers (3) • A pointer is a variable containing a memory address • Declared using * prefix float *p; • Address operator & • Address of specified variable float x, *p; p = &x;

  37. Information Services & Technology Pointers (4) float x, *p; p = &x; 1056 32 p 1052 16 16 1048 8 1040 0 address address

  38. Information Services & Technology Pointers (5) • Depending on context, * can also be the dereferencing operator • Value stored in memory location pointed to by specified pointer *p = 3.2; • Common newbie error float *p; *p = 3.2; float x, *p; p = &x; *p = 3.2; Wrong! – p doesn’t have value yet correct

  39. Information Services & Technology Pointers (6) • The name of an array is actually a pointer to the memory location of the first element • a[100] • “a” is a pointer to the first element of the array (a[0]) • These are equivalent: x[0] = 4.53; *x = 4.53;

  40. Information Services & Technology Pointers (7) • If p is a pointer and n is an integer, the syntax p+nmeans to advance the pointer by n memory locations • These are therefore equivalent: x[4] = 4.53; *(x+4) = 4.53;

  41. Information Services & Technology Pointers (8) • In multi-dimensional arrays, values are stored in memory with last index varying most rapidly (a[0][0], a[0][1], … ) • Opposite of Matlab and Fortran • The two statements in each box are equivalent for an array declared as int a[20][20]: a[0][17] = 1; a[1][0] = 5; *(a+17) = 1; *(a+20) = 5;

  42. Information Services & Technology sizeof • Some functions require size of something in bytes • A useful function – sizeof(arg) • The argument arg can be a variable, an array name, a type • Returns no. bytes in arg float x, y[5]; sizeof(x) ( 4) sizeof(y) (20) sizeof(float) ( 4)

  43. Information Services & Technology Dynamic Allocation • Suppose you need an array, but you don’t know how big it needs to be until run time. • Use malloc function malloc(n) • n is no. bytes to be allocated • returns pointer to allocated space • lives in stdlib.h

  44. Information Services & Technology Dynamic Allocation (cont’d) • Declare pointer of required type float *myarray; • Suppose we need 101 elements in array • malloc requires no. bytes, cast as appropriate pointer myarray = (float *) malloc(101*sizeof(float)); • free releases space when it’s no longer needed: free(myarray);

  45. Information Services & Technology Exercise 5 • Modify dot-product program to handle vectors of any length • Prompt for length of vectors • Read length of vectors from screen • Dynamically allocate vectors • Don’t forget to include stdlib.h so you have access to the malloc function • solution

  46. Information Services & Technology if/else if/else • Conditional execution of block of source code • Based on relational operators < less than > greater than == equal <= less than or equal >= greater than or equal != not equal && and || or

  47. Information Services & Technology if/else if/else (cont’d) if( x > 0.0 && y > 0.0 ){ z = 1.0/(x+y); }else if( x < 0.0 && y < 0.0){ z = -1.0/(x+y); }else{ printf(“Error condition\n”); }

  48. Information Services & Technology if/else if/else (3) • Can use “if” without “else” if( x > 0.0 && y > 0.0 ){ printf(“x and y are both positive\n”); }

  49. Information Services & Technology Exercise 6 • In dot product code, check if the magnitude of the dot product is less than using the absolute value function fabsf. If it is, print a warning message. • With some compilers you would need to include math.h for the fabsf function • With some compilers you would need to link to the math library by adding the flag –lm to the end of your compile/link command • solution

  50. Information Services & Technology Functions • Function returns a single item (number, array, etc.) • Return type must be declared • Argument types must be declared • Sample function definition: float sumsqr(float x, float y){ float z; z = x*x + y*y; return z; }

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