Introduction to C For the participants of Practical Course „Scientific Computing and Visualization“

Contents Getting Started: the first program data input and result output compilation and execution Variables, expressions, types, functions Control program flow loops, conditional statements Arrays and pointers Standard C libraries Preprocessing Makefiles

Getting started: C Program structure Any C programm consists of functions variables Functions consist of statements ( a=b+c, function call, etc. ) Statements includes expressions built with operations ( for example, arithmetical expressoins with “+“, “-“, “\“,“*“ )

Getting Started: Input and Output of Data #include main() { int a, b, result; printf(“Input a,b:\t“); scanf(“%d,%d“,&a,&b); result=a+b; printf(“%d plus %d is equal to %d \n“, a,b,result); }

Getting Started: Compilation In order to execute the programm, compile it with  gcc filename The executable file a.out will be produced, start it with:  a.out

Operators and Expressions in C binary (-,+,*,/,%,&&,||) x+y; year % 4; b && true Unary (-,+,*,&) int a=+2; int *pointer_to_a = &a; int b=-a; int sum = * pointer_to_a + b; postfix and prefix operators (++,--) a=2; b=++a; c=a++;

Variables and their Types There are following main types in C : bool (1 byte) char (1 byte) int (4 bytes) long, short, signed, unsigned int (4, 8 bytes) float (4 bytes) double (8 bytes) void For example: bool b=true; long int a; char c =`a`; a = ; 971 Memory:... reserved for a

Types of Expressions and Casting The types of single variables schould be declared by programmer: float f_number = ; double d_number = ; int i_number=3; Many operations cause automatic conversions of operands, in order to bring them to common type: i_number+f_number; -> int -> float sizeof(f_number + d_number)==8 float -> double sizeof(f_number + (float)d_number)==4 double -> float

Functions The function definition in C: ret_type name (arg_type1 arg_name1,...,arg_typeN argnameN) {... } double parabola(double x) { //declare the new variable, that will be returned return x*x; } double f0 = parabola(0); double f1 = parabola(1);

Are functions computing arithmetic expressins powerfull enough ? Now let us try to define the well-known factorial function in C. This function can be specified as follows:

Flow control: Conditional Statement To test logical conditions and make decisions use conditional statement IF-ELSE: if (condition) {... Statemen_block1... }else {... Statemen_block1... } For example: int f(int n){ if ( n>0 ) return n*f(n-1); else return 1; }

Loops: While and For for (init; cond; post){... } for (i=1; i<=n; i++){ fac=fac*i; } while (cond){... } while (i<=n){ fac=fac*i; i=i+1; } int i = 1; int fac = 1; do{... }while (cond) do{ fac=fac*i; i=i+1; }while (i<=n)

Arrays: Vectors and Matrices float x[3]={1,3,5}; #include ; float vector_norm=sqrt(x[1]*x[1]+x[2]*x[2]+x[3]*x[3]); float matrix_trace = A[1][1]+A[2][2]+A[3][3]; float A[3][3]={{1,2,3}, {3,2,1}, {5,6,7}};

Arrays : Strings Strings can be represented in C as arrays of characters (type char): For example: Memory: o\0lehl char str[]=“hello“; int i=0; while(str[i]!=‘\0‘){ printf(“%c\n“, str[i]); i++; }

Arrays: Pointers o\0lehl char str[]=“hello“; int i=0; while(str[i]!=‘\0‘){ printf(“%c\n“, str[i]); i++; }...str

Arrays: Pointers while(*str!=‘\0‘){ printf(“%c\n“,*(str++)); } o\0lehl...str

Indirection and Address Operators (*, &): Examples int x=1, y=2, z[10]; int *ip=&x; y=*ip; *ip=0; ip= &z[3];

Indirection and Address Operators (*, &): Examples void swap (int a, int b) { int tmp=a; a=b; b=tmp; } swap(a,b); // wrong Swap(&a,&b); // correct

Multidimensional Arrays: Pointers to Pointers float A[3][3]={{1,2,3}, {3,2,1}, {5,6,7}}; A[2]...A[1]

Multidimensional arrays A[2]...A[1] for(i=0;i<3;i++){ for(j=0;j<3;j++){ printf(“%f “, y[i][j]); } printf(“\n“); }

Arrays: Dynamic Programming As another example, let us implement the factorial calculation in a more efficient way. The main idea is to reuse once calculated results. We store them in an array.

Arrays: Dynamic Programming #include // the computed f(n) // will be stored here int results[10]; main(){ int i; /* Array initialization: write 0 in each field */ for(i=0; i<10;i++){ results[i]=0; } factorial(3);factorial(4); factorial(10); } int factorial(int n){ int res=1; for(i=n; i>0; i--) { // has the result been // already computed ? if(results[i]==0){ res=res*i; results[i]=res;} else return results[i]; } return fac; }

Memory allocation C provides a number of functions to allocate memory to store data in “ stdlib.h“ void *malloc(unsigned int nbytes); void free(void *ap)

Memory allocation #include int *results; main(){ results=malloc(100*sizeof(integer)); factorial(3);factorial(44); factorial(100); free(results); }

Preprocessing File inclusion #include #include “filename“ Macro substitutions #define max(A,B) (A)>(B) ? (A):(B) #define CONSTANT = 25 Conditional inclusion #ifndef __HELPER_H__ #define __HELPER_H__... #endif

Standard libraries stdio.h standard input and output printf, scanf, fprintf, fscanf, fopen, flcose,... string.h string processing Strlen, strcpy, strcat, strcmp,...

Standard libraries stlib.h memory allocation, random numbers, type conversion atof, atoi, rand, alloc, malloc, free math.h mathematical functions sin, cos, sqrt, strcmp, remainder,...

Compilation with makefiles Makefiles consist of macro definitions rules of compilation dependences declaration

Compilation with makefiles CC=gcc CFLAGS = -Wall –pedantic OBJ = helper.o init.o boundary.o uvp.o main.o all: $(OBJ) $(CC) $(CFLAGS) –o sim $(OBJ) –lm clean: rm $(OBJ) helper.o : helper.h init.o : helper.h init.h....