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 builtwith operations (for example, arithmetical

expressoins with “+“, “-“, “\“,“*“ )

Getting Started: Input and Output of Data#include <stdio.h>

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 itwith

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 = 232334; 971

Memory:

......

reserved for a

Types of Expressions and Casting

The types of single variables schould be declared by programmer:

float f_number = 3.141592; double d_number = 3.14159265358; int i_number=3;

Many operations cause automatic conversions of operands, in order to bring them to common type:

i_number+f_number; -> 6.141592 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:

01

0)1()(

n

nnnfnf

1*...*)1(*)( nnnf

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 <math.h>;

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}};

5

3

1

x

765

123

321

A

Arrays : Strings Strings can be represented in C as arrays of

characters (type char):

For example:

Memory:

o \0leh l

char str[]=“hello“;

int i=0;while(str[i]!=‘\0‘){ printf(“%c\n“, str[i]); i++;}

Arrays: Pointers

o \0leh l

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 \0leh l...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

765

123

321

Afloat A[3][3]={{1,2,3}, {3,2,1}, {5,6,7}};

2 ...321 3A[2] ...A[1]

Multidimensional arrays

2 ...321 3A[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 <stdio.h>

// the computed f(n)// will be stored hereint 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<stdlib.h>

int *results;

main(){ results=malloc(100*sizeof(integer));

factorial(3);factorial(44); factorial(100);

free(results);}

Preprocessing File inclusion

#include <filename> #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=gccCFLAGS = -Wall –pedanticOBJ = helper.o init.o boundary.o uvp.o main.o

all: $(OBJ) $(CC) $(CFLAGS) –o sim $(OBJ) –lmclean: rm $(OBJ)

helper.o : helper.hinit.o : helper.h init.h

....