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Overview of programming in C C is a fast, efficient, flexible programming language Paradigm: C is procedural (like Fortran, Pascal), not object oriented.

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Presentation on theme: "Overview of programming in C C is a fast, efficient, flexible programming language Paradigm: C is procedural (like Fortran, Pascal), not object oriented."— Presentation transcript:

1 Overview of programming in C C is a fast, efficient, flexible programming language Paradigm: C is procedural (like Fortran, Pascal), not object oriented (like C++, Java) C code is compiled, not interpreted (like python, perl, java)  higher speed High degree of memory control (pointers, dynamic allocation), compiled code much faster than mathematica et al. Very efficient compilers available, very fast and portable Similar syntax to Java Together with Fortran most commonly used platform for HPC All of Unix is written in C!

2 Basic programming cycle Write your program in a text editor (for example emacs) /* hello world */ #include int main() { printf("Hello world.\n"); return 0; } Compile: invoke gcc -o hello hello.c on the command line Run: type hello Analyse output (gnuplot, etc) Note: name of compiler may vary: gcc (GNU), icc (intel), cc (Sun), pgcc (portland group)

3 Structure Comments appear in /* */ or start with // for one line comment Each line ends with ; A C program consists entirely of functions It must have at least one special function: int main () { variable declarations … function1(); function2(); … } which can call additional functions to perform operations Functions other than main() must be first declared, then defined: Ex: int function1();

4 Functions Example: int multbytwo(int x) { int retval; retval = x * 2; return retval; } You can declare a prototype int multbytwo(int); before main() and define the function later Functions of type int or double require a return statement; void returns nothing

5 A typical program /* include statements */ #include // basic io-capability #include // basic math: exp,sin,cos,… /* function declarations */ void initialize(); // read in parameters for simulation void compute(int); // perform actual computation void output(); // produce output, write data to file int main() { initialize(); compute(int); output(); } /* function definitions */ void initialize(){…} In longer programs, function declarations are moved to "header" (.h) files

6 Data types int: integer (4 bytes on 32-bit machines  encodes 2 32 numbers) Ex: int n=100; float: real number (4 bytes) Ex: float x=0.4502; double: high (double) precision real number (8 bytes) char: alphanumerical character (1/2 bytes) The sizeof() command returns the # of bytes reserved for variable Qualifiers: const: will not change long: increase storage (eg long int  64 bit) Note: in C, all variables must be declared in the beginning before any non-declaratory statement; cannot use keywords!

7 Assignments and arithmetics Variables must be declared at start of function, same type can be grouped together, can be initialized at declaration: int l,m; m=5; l=10; l=m+l; double x=5.8,y,z; y=4.6; z=x*x+y*y; z=sin(x); Casting: change type of variable l=(int)z; z=(double)m/(double)l; //ensures floating point division

8 Arithmetic operators Assignment: = Binary arithmetic: +, -, *, /, % Can be applied to int, float or double except the modulus operator % (int only) Shortcuts: +=, -=, *=, /= x+=2; // equivalent to x = x+2; x*=2; // equivalent to x = x*2; Increment/Decrement: ++,-- x++; // acts like x = x+1; x--; // acts like x = x-1; Both x++ and ++x are expressions, but ++x acts like x+1; Example: b=10; c=b++; // c is 10 and b is 11; b=10; c=++b; // c and b are now 11;

9 Output to screen: printf() Output to screen: printf("format string", var1, var2, var3); Format string can be text or % for variables, followed by type %d: integer %ld: long integer %f: float %c: character %s: string more general: %length.precision type, eg. %10.8f prints 10 total digits, 8 after the. point \n: newline (cr) \t: tab Example: int x = 3; int y = 15; char op = '+'; printf("\t%d %c %d = %d\n", x, op, y, x + y); Result: 3 + 5 = 18; Note: input from screen: scanf() (later)

10 Scope of variables If declared within a function, the variables are only known to that function (local variable), i.e. within the {} If declared before main(), the variable is known to all functions and can be manipulated everywhere (global variable) int i,j;// global variables int main{ double x; // local variable } Global variables can be used to share information between functions, but this is not recommended/good style Better: variable pass by value or by pointer in function argument (later)

11 Flow control Branching with if … else #include int main() { int cows = 6; if (cows > 1) printf("We have cows\n"); if (cows > 10) printf("loads of them!\n"); } Format: if (expression) {statement;} else if (expression) {statement;} else {statement;} Useful operators in logical expression: relational:, =, equality: ==, inequality: != logical: and &&, or ||

12 Loops while loop (repeating if, condition checked at start): Format: while (expression) {statement;} for loop (for lists): Format: for (initial-stmt; cond; interation-stmt) {statement;} Ex: for (i=0; i<10; i++) {statement;} do...while loop (condition checked at end): Format: do {statement;} while (expression); switch (nested if...else): Format: switch (expression){ case const1 {statement;break;} case const2 {statement;break;} }

13 Examples Summation int main() { int i=1, sum=0; for (i=1;i<=10;i++) { sum=sum+i; } printf("The sum is: %d\n",sum); } Switch switch (month) { case 1 : case 3 : case 5 : case 7 : case 8 : case 10 : case 12: printf("31 days\n"); break; case 2: printf("28 or 29 days\n"); break; case 4 : case 6 : case 9 : case 11 : printf("30 days\n"); break; }

14 Pointers A variable is stored in a certain memory location or address Pointers allow to manipulate these addresses explicitly Declaration: add a star to the type you want to point to Ex: int *a; declares variable of type int that holds the address (a pointer to) an int Two operators: - & operator: "address of", can be applied to any variable, result has "one more star" - * operator: "dereference", accesses the object that the pointer points to, can only be applied to pointers, result has "one less star"

15 Pointer examples int main() { int i=5,j=2,z=3; //regular variables int *ip; //pointer declaration ip = &i; //pointer now points to location of i j = *ip; //accesses value of object; j now has value 5 *ip = 7; // } Do not confuse notation: int *a declares the pointer; * operator ("content-of") dereferences ("makes less of a pointer") Can assign pointer values to other pointers: int *ip2; ip2=ip; // ip2 points to the same location as ip

16 Why pointers? Consider swapping two values: void swap(int x, int y){ int temp; temp=x; x=y; y=temp; } int main() { int a=3,b=5; swap(a,b); printf("a is %d, b is %d\n",a,b); } Result: a is 3, b is 5! What went wrong? C is "pass by value", the called function cannot manipulate the original variables

17 A correct swap Cure: pass pointers and not variables: void swap(int *px, int *py){ int temp; temp=*px; *px=*py; *py=temp; } int main() { int a=3,b=5; swap(&a,&b); printf("a is %d, b is %d\n",a,b); } Pointers are most useful for passing data between functions

18 Arrays Often we need to manipulate large vectors or matrices of data Arrays allow to refer to a number of instances of the same name Ex: int data[5]; declares an integer array of length 5 Note: in C arrays are zero-indexed - numbering begins at zero!! data[expr] accesses elements of the array where expr evaluates to an integer Careful: there is no range checking. Only access elements 0 to N-1 for an array of size N. Multidimensional arrays: int data[N][M][L];

19 Arrays and Pointers An integer array a is just a pointer, it is of type (int *). a points to first memory location of the array. int *ip; ip=&a[0]; // sets ip to point to element zero of a (ip+i) points to the i-th element  a[i] and *(ip+i) are equivalent!  ip[i] and *(ip+i) are equivalent! Since an array is a pointer it can be manipulated by a function: Ex: void func (int *a){} int main (){ int b[3]={85,6,7}; func(b); } Note: declare function to accept pointer!

20 What is the output? int main(){ int a[4] = { 0, 1, 2, 3}; int *pa; pa = a+1; printf ("%ld\n", *pa); printf ("%ld\n", pa[2]); pa++; printf( "%d\n",pa[0]); scanf("%d", pa+1); printf("You typed: %d\n",a[3]); } 1 3 2 6 You typed: 6

21 Structured Data Types The structure mechanism allows to aggregate variables of different types: struct complex { double real; double im; } void func(){ struct complex a,b; a.real=3;; b=a; } "." is the member operator; connects structure and member

22 File I/O Use fprintf or fscanf for input/output from/to files Accessing files requires first the allocation of a file pointer: FILE *results; then opening the file for read/write/append with results=fopen("data.dat","w"); or "r" or "a" Write to file (formatted output): fprintf(results, "format string\n", var1, var2, var3); Read from file (formatted input): fscanf(results, "format string\n", &var1, &var2, &var3); Note: fscanf expects a location for the data, hence the &-operator. Finally: fclose(results); Note: standard input/output are handled as files; printf/scanf are just special cases of fprintf/fscanf applied to stout/stdin

23 Dynamic memory allocation Often we do not know the amount of required memory at compilation time (eg. variable size problem) int data[size] is a static array cannot change its size Need to allocate memory dynamically, "on the fly" malloc(size): allocates size bytes and returns a pointer to the allocated memory (or new in C++/Java) free(ptr): frees the memory space pointed to by ptr Example: #include int *data; int length=1000; data = malloc(sizeof(int)*length); //allocates int array free(data);

24 Dynamic memory allocation If malloc is unable to allocate memory, it returns NULL pointer; good to check whether malloc was successful: if(ip==NULL) { printf("out of memory\n"); exit(1); } It is possible to extend allocated memory: datanew = realloc(data, sizeof(int)*(length*2)); creates contiguous block in memory; appends to existing data

25 Multidimensional arrays To allocate multidimensional arrays, we use pointer to pointers: #include int **array; array = malloc(nrows * sizeof(int)); if(array == NULL) { fprintf(stderr, "out of memory\n"); exit(1);} for(i = 0; i < nrows; i++) { array[i] = malloc(ncolumns * sizeof(int)); if(array[i] == NULL) { fprintf(stderr, "out of memory\n"); exit(1);} } Can access like static array: array[i][j];

26 Tips for compiling Compiling has three stages: 1) preprocessing 2) compile to object code (.o) 3) link all pieces and create executable Compiler options help control the process (see man gcc). All options start with -, some important options are: -o: name of executable -O, -O2: turn on optimizer -I: additional search path for include-files (.h) -c: create only object code, do not link (for larger projects) -l: link library, e.g -lm for link to math library -L: additional search path for library-files -g: produce debugging info (for gdb or ddd) -Wall: turn on all warnings

27 Preprocessor statements #include inserts the entire content of a file, verbatim. #include "filename": searches for file in current directory #include : searches standard paths, typically for standard library header files #define allows to set up macros #define SIZE 10 : the preprocessor replaces all occurrences of SIZE with 10. - Makes global changes easier - Makes programs easier to read Conditional compilation : #ifdef name … #endif control with -Dname at compilation

28 Working with libraries Libraries provide convenient access to additional functions Ex: sin(x), cos(x), exp(x) are all in the math library To access these functions, first include the corresponding header file (eg. math.h), then link the library with -lm when compiling We will use libraries like the GNU scientific library libgsl.a In this case: 1) include (in code) 2) gcc prog.c -o prog -lm -lgsl -I/usr/local/lib (on Note: each library is searched only once, so the order matters!

29 Example: random number generator #include #include // include header files int main () { const gsl_rng_type * T; // define pointer to rng type gsl_rng * r; // define pointer to rng double u; gsl_rng_env_setup(); // reads env variables (seed) T = gsl_rng_default; // sets default rng r = gsl_rng_alloc (T); // returns pointer to rng T u = gsl_rng_uniform (r); // samples from the rng printf ("%.5f\n", u); } gsl_rng_free (r); // free up memory return 0; }

30 Other libraries -lmpich: message passing interface (MPI) for parallel code -lfftw: fast fourier transforms -llinpack: linear algebra -lblas: basic linear algebra … and many more

31 The make utility make is a powerful UNIX tool to manage the development of larger projects Executables are viewed as targets that have dependencies A Makefile consists of definitions of one or more targets and how to "make" them Each target definition contains: 1. name of the target, followed by : 2. list of dependencies 3. set of actions that define how to make the target Example: prog1: prog1.c gcc -g -O prog1.c -o prog1 Note: all lines stating actions MUST begin with a TAB to compile, simply type "make"

32 Makefiles continued For portability, make can access environment variables $(HOME) (note syntax) We can define our own macros: MYLIB = $(HOME)/lib CCFLAGS= -g -O -I$(HOME)/include (useful for automating the compilation process) Note: to continue beyond one line use the \ (continuation construct) FILES= a.c\ b.c\ c.c One can have generic targets:.c.o: gcc $(CCFLAGS) $.c tells make how to make a.o file from a.c file

33 A longer Makefile example OBJ= prog1.o CXXFLAGS=-g -O2 -lm -Wall LINKFLAGS=-lm CXX=gcc.c.o: $(CXX) $(CXXFLAGS) -c $*.c prog1: $(OBJ) $(CXX) $(LINKFLAGS) -o $@ $(OBJ) clean: rm -f *.o rm prog1

34 Data visualization gnuplot is a simple but powerful plotting tool Plotting of functions: f(x)=some function p f(x) Plotting of data: p "data.dat" using ($1):($2) plots first column of data against second column data can be processed by functions: eg. log($1):log($2) Print to file: set term post eps enhanced //switch output to encaps. ps set out "plot.eps" //name of output file p "…" // plot command set term x11 // switch output to screen Can automate using gnuplot scripts

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