Examples
Command Line arguments
#include <stdio.h>
#include <stdlib.h>
/* This shows how args can be read from the Unix command line */
int main(int argc, char *argv[]){
int i;
printf("The %d arguments are\n", argc);
for (i=0; i<argc; i++)
printf("%d %s\n",i, argv[i]);
exit (0);
}
Using qsort, random numbers and the clock
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
* compile on HPs using c89 -D_HPUX_SOURCE -o filename filename.c */
#define NUM 10
int comp(const void *a, const void *b )
{
return *(int *)a - * (int *)b;
}
int main(int argc, char *argv[])
{
int numbers[NUM];
int i;
srand48((long)time(NULL));
printf("\nUnsorted numbers are:-\n");
for (i=0; i< NUM; i++){
numbers[i]= 1000 * drand48();
printf("%d: %3d\n", i, numbers[i]);
}
/* See the qsort man page for an explanation of the following */
qsort((void*) numbers, (size_t) NUM, sizeof(int), comp);
printf("\nSorted numbers are:-\n");
for (i=0; i< NUM; i++)
printf("%d:%3d\n", i, numbers[i]);
exit(0);
}
Calling other programs
The commands used from the command line can be called from C.
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char *argv[]){
FILE *popen();
FILE *fp;
char string[32];
/* First use the system() call. Output will go to stdout. */
system("date");
/* Now `capture' the output of date using popen() */
fp = popen("date","r");
if (fp == NULL)
fprintf(stderr,"Cannot run date\n");
else{
fgets(string, 32, fp);
printf("The date command returns [%s]\n", string);
pclose(fp);
}
}
Linked Lists
The following program creates a singly linked list. Pointers are maintained to the head and tail of the list.
#include <stdio.h>
typedef struct _list_item {
int val;
struct _list_item *next;
} list_item;
/* prototypes */
list_item *add_list_item(list_item *entry, int value);
void print_list_items(void);
list_item *head=NULL;
list_item *tail=NULL;
main(int argc, char *argv[])
{
tail=add_list_item(tail,5);
tail=add_list_item(tail,7);
tail=add_list_item(tail,2);
print_list_items();
}
list_item *add_list_item(list_item *entry, int value)
{
list_item *new_list_item;
new_list_item=(list_item*)malloc(sizeof(list_item));
if (entry==NULL){
head=new_list_item;
printf("First list_item in list\n");
}
else {
entry->next = new_list_item;
printf("Adding %d to list. Last value was %d \n",value,entry->val);
}
new_list_item->val = value;
new_list_item->next = NULL;
return new_list_item;
}
void print_list_items(void)
{
list_item *ptr_to_list_item;
for (ptr_to_list_item= head;ptr_to_list_item!= NULL;
ptr_to_list_item=ptr_to_list_item->next) {
printf("Value is %d \n", ptr_to_list_item->val);
}
}
Using pointers instead of arrays
#include "stdio.h"
char *words[]={"apple","belt","corpus","daffodil","epicycle","floppy",
"glands","handles","interfere","jumble","kick","lustiness",
"glands","handles","interfere","jumble","kick","lustiness",
"glands","handles","interfere","jumble","kick","lustiness",
"glands","handles","interfere","jumble","kick","lustiness",
"glands","handles","interfere","jumble","kick","lustiness",
"glands","handles","interfere","jumble","kick","lustiness",
"glands","handles","interfere","jumble","kick","lustiness",
"glands","handles","interfere","jumble","kick","lustiness",
"glands","handles","interfere","jumble","kick","lustiness",
"mangleworsel","nefarious","oleangeous","parsimonious",NULL};
void slow(void)
{
int i,j,count=0;
for (i=0; words[i] != NULL ; i=i+1)
for (j=0; j <= strlen(words[i]) ; j=j+1)
if(words[i][j] == words[i][j+1])
count= count+1;
printf("count %d\n",count);
}
void fast(void)
{
register char **cpp; /* cpp is an array of pointers to chars */
register char *cp;
register int count=0;
for (cpp= words; *cpp ; cpp++) /* loop through words. The final
NULL pointer terminates the loop */
for (cp = *cpp ; *cp ; cp++) /* loop through letters of a word.
The final '\0' terminates the loop */
if(*cp == *(cp+1))
count++;
printf("count %d\n",count);
}
/*count the number of double letters, first using arrays, then pointers */
int main(int argc, char *argv[]){
slow();
fast();
}
A data filter
The program reads from stdin an ASCII file containing values of a variable y for integral values of x running from 0 to n-1 where n is the number of values in the file. There may be several values on each line. The program outputs the x, y pairs, one pair per line, the y values scaled and translated by factors built into the main routine..
#include <stdio.h>
#include <stdlib.h>
int answer;
float offset;
float scale;
char buf[BUFSIZ];
int xcoord = 0;
char *cptr;
int transform(int a)
{
return a * scale + offset + 0.5;
}
char* eat_space(char *cptr){
/* This while loop skips to the nonspace after spaces.
If this is the end of the line, return NULL
`While a space, keep going'
*/
while (*cptr ==' '){
if (*cptr == '\0')
return NULL;
else
cptr++;
}
return cptr;
}
char * next_next_num(char *cptr){
/* This while loop skips to the 1st space after a number.
If this is the end of the line, return NULL
`While NOT a space, keep going'
*/
while (*cptr !=' '){
if (*cptr == '\0')
return NULL;
else
cptr++;
}
/* Now move to the start of the next number */
return eat_space(cptr);
}
int main(int argc, char *argv[])
{
offset = 2.3;
scale = 7.5;
while(1){
/* if we haven't reached the end of the file ...*/
if(fgets(buf, BUFSIZ,stdin)!= NULL){
/* initialise cptr to point to the first number ...*/
cptr = eat_space(buf);
do{
/* convert the representation of the num into an int */
sscanf(cptr,"%d", &num);
/* print x and y to stdout */
printf("%d %d\n",xcoord, tranform(num));
/* skip to the start of the next number on the line */
cptr=next_next_num(cptr);
xcoord++;
}while ( cptr!=NULL);
}
else{
exit(0);
}
}
}
Reading Directories
#include <stdio.h>
#include <sys/types.h>
#include <dirent.h>
#include <sys/stat.h>
#define REQUEST_DIR "/"
int main(int argc, char *argv[]){
FILE *fp;
DIR *dirp;
struct dirent *dp;
struct stat buf;
dirp = opendir(REQUEST_DIR);
chdir(REQUEST_DIR);
/* Look at each entry in turn */
while ((dp = readdir(dirp)) != NULL) {
/* Now stat the file to get more information */
if (stat(dp->d_name, &buf) == -1)
perror("stat\n");
if (S_ISDIR(buf.st_mode))
printf("%s is a directory\n", dp->d_name);
else if (S_ISREG(buf.st_mode))
printf("%s is a regular file\n", dp->d_name);
}
(void) closedir(dirp);
}
Calling fortan NAG library routines
#include <stdio.h>
#include <stdlib.h>
/* c89 -L/usr/local/lib g01aaf.c -lnag -lcl */
main(){
int i;
int n, iwt, ifail;
double x[10], wt[10];
double xmean, s2, s3, s4, xmin, xmax, wtsum;
iwt = 0;
n = 10;
ifail = 0;
for (i=0;i<10;i++){
wt[i]=1;
x[i]= 3.5 * i;
}
g01aaf(&n, x, &iwt, wt, &xmean, &s2, &s3, &s4,
&xmin, &xmax, &wtsum, &ifail);
if (ifail == 0){
printf("Mean is %lf, Std deviation is %lf\n", xmean, s2);
printf("Max is %lf, min is %lf\n", xmax, xmin);
}
else
printf("G01AAF failed\n");
}
Queens: recursion and bit arithmetic
This program counts the number of ways that 8 queens can be placed on a chess board without any 2 of them being on the same row, column or diagonal. It was written by M. Richards at cl.cam.ac.uk
#include <stdio.h>
int count;
void try(int row, int ld, int rd){
if (row == 0xFF)
count++;
else{
int poss = 0xFF & ~(row | ld | rd);
while (poss){
int p = poss& -poss;
poss = poss -p;
try(row+p, (ld+p)<<1, (rd+p)>>1);
}
}
}
int main(int argc, char *argv[]){
printf("Eight Queens\n");
count = 0;
try(0,0,0);
printf("Number of solutions is %d\n", count);
exit(0);
}