Tutorial Week 6

<li>  
Write a program using an array to read in a set of numbers terminated by EOF 
and print them out in reverse order. Assume that no more than 100 numbers need 
to be read.

Ans: 

#include <stdio.h>
#define MAXNUM 100

int main(int argc, char *argv[])
{
    int numbers[MAXNUM];
    int number, count;

    count = 0;
    while (count < MAXNUM && scanf("%d", &number) != EOF) {
        numbers[count++] = number;
    }
    while (count > 0)
        printf("%d\n", numbers[--count];
    exit 0);
}
    

<li>  
Write a function to emulate strcmp to compare two null-terminated strings. 
It should return -1 if the first string is lexicographically less than the 
second, 1 if it is greater, and 0 if they are the same. For the second assignment, 
note the library function strncmp that compares a maximum number of elements 
of the strings.

Ans: 

int strcmp(char *s1, char *s2)
{
    while (*s1 != '\0' && *s2 != '\0') {
        if (*s1 < *s2) return -1;
        if (*s1 > *s2) return 1;
        s1++;
        s2++;
    }
    if (*s1 != '\0') return 1;    /* s1 still has chars */
    if (*s2 != '\0') return -1;   /* s2 still has chars */
    return 0;
}


<li>  
Write a program to store an array of records containing student names and IDs. 
The program should read upto 100 records. It should then enter a loop reading 
student IDs and printing the corrresponding name if found.

Ans: 

#include <stdio.h>

struct student_t {
    char *name[100];
    int ID;   /* will need to be long if int is 16 bits */
} students[MAXSTUDENT];
etc

<li>  
Write a program to read in strings (one per line), store pointers to them in 
an array and print them out. NB:You will need to use ``malloc'' to create heap 
space to store copies of the strings.

Ans: 
If you don't use malloc, but just use one string, everything in the array will 
end up pointing to the string.
#include <stdio.h>
#define MAX 100

int main(void)
{
    char *strings[MAX];
    char str[1024];          /* hope this is long enough */
    int count = 0;
    int n;

    while (count < MAX && scanf("%1023s", str) != EOF) {
        strings[count] = malloc(strlen(str) + 1); /* space for null char */
        strcpy(strings[count], str);
        count++;
    }
    for (n = 0; n < count; n++)
        puts(strings[n]);
    exit(0);
}

Laboratory Week 6

<li>  
Use the online manual to read the entry in section three for ``getwd''. Write 
a program to print your current working directory. Note the use of MAXPATHLEN 
- no complete pathname is allowed to be longer than that.

Ans: 

#include <sys/params.h>
#include <stdio.h>
int main(void)
{
    char dir[MAXPATHLEN];

    if (getwd(dir) == NULL) {
        fprintf(stderr, "cant find current dir\n");
        exit(1);
    }
    puts(dir);
    exit(0);
}

<li>  
Use the online manual to read the entry in section two for ``chdir''. Write 
a program to read directory names from standard input until end-of-file and 
change the current working directory. What patterns are recognised out of `.', 
`..', `~', etc?

Ans: 
This will recognise ., .. patterns but not ~ - only the shells do that one 
too. MAXPATHLEN is used again to give a reasonable size.
#include <stdio.h>
#include <sys/params.h>

int main(void)
{
  char path[MAXPATHLEN];

  while (gets(path) != NULL) {
    if (chdir(path) == 0) 
      printf("changed to %s\n", path);
    else printf("failed to change to %s\n");
  }
  exit(0);
}

<li>  
Write a program to read a single line of input and execute this as a command 
using the call ``system''. Can this handle ``cd'', or must you treat this as 
a special case?

Ans: 
Each command is handled by a new shell - chdir cahnges for the new shell which 
then exits, so it has no effect on the current environment. You can combine 
the last three solutions to get a simple interpreter of your own.
#include <stdio.h>
int main(void)
{  char path[2048]; /* they'll never type this much... */

  while (gets(path) != NULL)
    system(path);
  exit(0);
}