1 Dynamic memory allocation and arrays

Transcription

1 C-programming exercises I: Solutions Martin Gren, Christopher Linderälv, Joakim Löfgren and Mattias Ångqvist Chalmers University of Technology, Department of Physics 1 Dynamic memory allocation and arrays 1. Write a program with a function that computes the scalar product between to arrays. In the main function, read a length from the command line and create two vectors of that length for which you calculate the scalar product. int len ; // read the length of the vectors from the terminal printf (" Enter an array length : "); scanf ("% d", & len ); // note that we need to pass a pointer to scanf // dynamically allocate two 1D arrays double *v1 = malloc ( len * sizeof ( double )); double *v2 = malloc ( len * sizeof ( double )); double res ; // intialize to some arbitrary values v1[ i] = i; v2[ i] = 2.0 * i; // compute the scalar product res = scalar_product ( len, v1, v2); printf (" The scalar product is %.4 f\n", res ); // dont forget to free the memory free (v1); free (v2); 2. The C99-standard introduces the concept of variable length arrays (VLAs). Without allocating memory dynamically, repeat the previous exercise using VLAs. Do you need to modify the signature of the function that calculates the scalar product? No, because an array passed to a function will automatically decay into a pointer the first element. int len ; printf (" Enter an array length : "); scanf ("%d", & len ); /* create two variable length arrays

2 * the memory is managed for us so * no need to call free () later */ double v1[ len ]; double v2[ len ]; double res ; v1[ i] = i; v2[ i] = 2.0 * i; res = scalar_product ( len, v1, v2); printf (" The scalar product is %.4 f\n", res ); 3. Dynamically allocate a 2D array and use it to create an n 3 array filled with doubles of your choice. Interpreting each row in this array as the coordinate vector of a point in space, write a function that calculates the distance between two given points. double get_pair_distance ( double pos [][3], int i1, int i2); int nrow ; int i1, i2; // read number of points from terminal printf (" Enter number of points : "); scanf ("%d", & nrow ); /* dynamically allocated matrix where each row * will represent the coordinates of a point */ double (* pos ) [3] = malloc ( sizeof ( double [ nrow ][3]) ); // intialize the positions to some arbitrary values for ( int i = 0; i < nrow ; ++i) { pos [i][j] = i /5.0 - j /10.0; // try it! printf (" Get distance between points : "); scanf ("%d %d", &i1, &i2); dist = get_pair_distance ( pos, i1, i2); printf (" Euclidean distance : %.4 f\n", dist ); free ( pos ); // euclidean distance between rows i1 and i2 in the n x 3 array pos double get_pair_distance ( double pos [][3], int i1, int i2) { dist += ( pos [i1 ][j] - pos [i2 ][j]) * ( pos [i1 ][j] - pos [i2 ][j]); dist = sqrt ( dist );

3 return dist ; 2 Header files and separate sources Rewrite your code from the previous exercise so that the two functions (scalar product and pairdistance) are located in a separate C-file. Create a corresponding header file where you put the function prototypes. Recompile your program and make sure that everything works. Compilation process: gcc -c my_funcs.c gcc -c main.c gcc -o main main.o my_funcs.o -lm - Header file (with include guard) my_funcs.h # ifndef MY_FUNCS_H # define MY_FUNCS_H // includes // prototypes double get_pair_distance ( double pos [][3], int i1, int i2); # endif File with functions my_funcs.c # include " my_funcs.h" double get_pair_distance ( double pos [][3], int i1, int i2) { dist += ( pos [i1 ][j] - pos [i2 ][j]) * ( pos [i1 ][j] - pos [i2 ][j]); dist = sqrt ( dist ); return dist ; Main program main.c # include " my_funcs.h" // some code for testing here 3 Using external libraries Use gsl to generate a large array filled with random numbers drawn from the uniform distribution on [0, 1].

4 # include <time.h> # include <gsl / gsl_randist.h> gsl_rng * initialize_rng (); void write_array ( char * file_name, double arr [], int len ); FILE * outfile ; int nrand ; double r; // ask how many random numbers to generate printf (" Number of random numbers to be generated : "); scanf ("%d", & nrand ); // allocate array to hold random numbers double * rand_arr = malloc ( nrand * sizeof ( double )); // initialize rng by calling helper function gsl_rng * my_rng = initialize_rng (); // generate random numbers for ( int i = 0; i < nrand ; ++i) { r = gsl_rng_uniform ( my_rng ); rand_arr [ i] = r; write_array (" random_numbers. txt ", rand_arr, nrand ); free ( rand_arr ); // helper function to initialize the rng gsl_rng * initialize_rng () { const gsl_rng_type * rng_type ; gsl_rng * my_rng ; gsl_rng_env_setup (); rng_type = gsl_rng_default ; my_rng = gsl_rng_alloc ( rng_type ); gsl_rng_set ( my_rng, time ( NULL )); return my_rng ; // function for writing a 1D array to file void write_array ( char * file_name, double arr [], int len ) { FILE *fp; fp = fopen ( file_name, "w"); fprintf (fp, " %.4 f ", arr [i]); fclose (fp); 4 Getting started on H1: Molecular Dynamics Create an array of coordinates representing atomic positions in an fcc lattice. Create fcc structures of different volume by varying the lattice constant and calculate the energy of each generated structure. Write the lattice constant and the corresponding energies to file and determine the equilibrium lattice constant by doing a quadratic fit in, e.g., MATLAB or Python. # include <time.h> # include " initfcc.h" # include " alpotential. h"

5 int main () { int ncell ; // need at least 3 for the potential to work // read number of unit cells from terminal printf (" How many unit cells in each direction? "); scanf ("%d", & ncell ); int natom = 4 * ncell * ncell * ncell ; double a, a0 = 4.05; double cell_length ; double energy = 0.0; int nsweep = 20; FILE *fp; // allocate a matrix to hold atomic coordinates double (*x) [3] = malloc ( sizeof ( double [ natom ][3]) ); // open a file to write to fp = fopen (" energy_lattconst. txt ", "w"); /* initialize fcc cells with different lattice constants * and calculate their energies */ for ( int i = 0; i < nsweep ; ++i) { a = a0 * ( *( i - nsweep /2) ); init_fcc (x, ncell, a); cell_length = ncell * a; energy = get_energy_al (x, cell_length, natom ); fprintf (fp, "%f %f\n", a, energy ); fclose (fp); free (x);