1. Lab exercise C programming language

Transcription

1 Parallel and disributed systems and algorithms 2018/19 1. Lab exercise C programming language Davor Sluga "The more I C, the less I see."

2 Introduction Lab exercises are mandatory and are being graded To complete Lab exercises successfully at least 70% of assignments have to be completed Every week new assignment You can present your solution no later than 2 weeks after it was introduced Final project 2

3 Content C/C++ programming language Programming with threads pthreads OpenMP GPUs (OpenCL) MPI library national GRID 3

4 C programming language review Pointers and arrays Dynamic memory allocation Measuring time in C Visual Studio 2015 IDE 4

5 Pointers Pointer is a variable that stores memory address & operator (address operator, NOT AND operator!) Gives memory address of a variable * operator (NOT a multiplicaton operator!) Declaration of pointer variable Gives value stored at address to which the variable points 5

6 Pointer example int x,y,*p; x=3; p=&x; y=*p; *p=x+y; What are the values of variables x,y and p during the program execution? 6

7 Pointers and arrays int list[4]={1,2,3,4}; int *p=list; Equivalent expressions list[2]=5; *(p+2)=5; p[2]=5; *(list+2)=5; Differences between pointers and arrays Pointers store address of the first element, arrays store actual data Pointers -> indirect addressing Arrays -> direct addressing 7

8 Passing array as function argument Arrays in C are passed by reference. Function gets the address of the first element of array. int sum(int *T, int n){ int s=0; while(n--){ s+=t[n]; } return s; } int main(){ const int A[]={0,1,2,3}; printf("%d\n",sum(a,(sizeof A)/ sizeof(int))); } 8

9 Dynamic memory allocation The required size of the allocation is not known until run-time e.g. array size is not known because it will be read from a file Functions: void *malloc(size_t size); void *calloc(size_t nitems, size_t size); void *realloc(void *ptr, size_t size); void free(void *ptr); 9

10 Dynamic memory allocation Example 1 int main(){ A=(int *)malloc(10*sizeof(int)); B=(double *)calloc(10,sizeof(double)); C=(char *)realloc(c,10*sizeof(char)); } free(a); free(b); free(c); 10

11 Dynamic memory allocation Example 2 2D array (matrix): int B[3][4]; int **A; int main(){ A=(int **)malloc(3*sizeof(int *)); for(int i=0;i<3;i++) A[i]=(int *)malloc(4*sizeof(int)); } for(int i=0;i<3;i++) free(a[i]); free(a); 11

12 Visual Studio 2015 New project File New Project In the left column we choose project type Visual C++ Win32 In the right window we choose Win32 Console Application Enter project name and project directory OK Next Select an empty project option Finish In Solution Explorer window right click on Source Files Add New Item Select C++ File option, enter file name Add 12

13 Tips Use debugger Right click run to cursor step in, step over, step out Use Debug configuration while programming If you're happy with your program (and it works), use Release configuration, which uses compiler optimizations. Program will run much faster. 13

14 Assignment 1 Write a program with the following functions: double * Random(int n); Function dynamically allocates array of size n, where every element is a random real number between 0 and 1. Function returns pointer to this array. double ** Matrix (double *A, int n, int r); Function dynamically allocates matrix with r rows and copies array A into newly created matrix (Note: number of columns has to be calculated from variables n and r. If A is smaller than matrix, fill blank space with zeros!). Function returns pointer to the matrix. double * Max(double *A, int n); Function returns pointer to the maximum value in array A. Use all 3 functions in the main program: pass array created by Random function to the other 2 functions and print results. Measure execution time (in seconds) required by Random function to create and fill array and print it out. All required functions can be found on: 14

15 Random numbers #include <stdlib.h> srand(time(null)); //number between 0 and RAND_MAX int r = rand(); 15

16 Measuring time in C #include <time.h>... clock_t start, stop; double time;... start = clock(); somefunction(); stop = clock(); time = (double)(stop-start)/clocks_per_sec; printf("execution time: %f",time); 16

17 Example of input/output Input Enter n: 8 Enter r: 2 Output 1D: D: Maximum number: 0.99 located on address: 0012FEDC. Time required to generate random numbers: 0.01 s 17