3) please explain the below c program for sorting

[rangerskm]

compiler used=gcc
os -fedora 19

#include <stdio.h>
   #include <string.h>

   #define MAXLINES 5000     /* max #lines to be sorted */

   char *lineptr[MAXLINES];  /* pointers to text lines */

   int readlines(char *lineptr[], int nlines);
   void writelines(char *lineptr[], int nlines);

   void qsort(char *lineptr[], int left, int right);

   /* sort input lines */
   main()
   {
       int nlines;     /* number of input lines read */

       if ((nlines = readlines(lineptr, MAXLINES)) >= 0) {
           qsort(lineptr, 0, nlines-1);
           writelines(lineptr, nlines);
           return 0;
       } else {
           printf("error: input too big to sort\n");
           return 1;
       }
   }

   #define MAXLEN 1000  /* max length of any input line */
   int getline(char *, int);
   char *alloc(int);

   /* readlines:  read input lines */
   int readlines(char *lineptr[], int maxlines)
   {
       int len, nlines;
       char *p, line[MAXLEN];

       nlines = 0;
       while ((len = getline(line, MAXLEN)) > 0)
           if (nlines >= maxlines || p = alloc(len) == NULL)
               return -1;
           else {
               line[len-1] = '\0';  /* delete newline */
               strcpy(p, line);
               lineptr[nlines++] = p;
           }
       return nlines;
   }

   /* writelines:  write output lines */
   void writelines(char *lineptr[], int nlines)
   {
       int i;

       for (i = 0; i < nlines; i++)
           printf("%s\n", lineptr[i]);
   }

i got the error message

[sathish@localhost ~]$ vi pointarr.c
[sathish@localhost ~]$ gcc pointarr.c
pointarr.c:29:8: error: conflicting types for ‘getline’
int getline(char *, int);
^
In file included from pointarr.c:1:0:
/usr/include/stdio.h:678:20: note: previous declaration of ‘getline’ was here
extern _IO_ssize_t getline (char **__restrict __lineptr,
^
pointarr.c: In function ‘readlines’:
pointarr.c:40:40: error: lvalue required as left operand of assignment
if (nlines >= maxlines || p = alloc(len) == NULL)

 

[errakeshpillai]

Why did you create an entirely new thread for the same discussion again. You could have continued it there itself...??

Also the reason for first error has been posted there.

You second error is due to parenthesis mistake , i guess.
It should be like this.

           if (nlines >= maxlines || ((p = alloc(len)) == NULL))

 

[Jinzpaul4u]

Hi rangerskm ,

[sathish@localhost ~]$ vi pointarr.c
[sathish@localhost ~]$ gcc pointarr.c
pointarr.c:29:8: error: conflicting types for ‘getline’
int getline(char *, int);

I hope its a local and global scope problem
try these..

   #define MAXLINES 5000     /* max #lines to be sorted */
    #define MAXLEN 1000  /* max length of any input line */
   
   char *lineptr[MAXLINES];  /* pointers to text lines */
      int getline(char *, int);
   char *alloc(int);
   int readlines(char *lineptr[], int nlines);
   void writelines(char *lineptr[], int nlines);

   void qsort(char *lineptr[], int left, int right);

   /* sort input lines */
   main()
   {

}

In file included from pointarr.c:1:0:
/usr/include/stdio.h:678:20: note: previous declaration of ‘getline’ was here
extern _IO_ssize_t getline (char **__restrict __lineptr,

/usr/include/stdio.h in this inbuild libray fuction is already using getline declaration. you have to rename your declaration.

pointarr.c: In function ‘readlines’:
pointarr.c:40:40: error: lvalue required as left operand of assignment
if (nlines >= maxlines || p = alloc(len) == NULL)

p = alloc(len)????

You need to compare, not assign:

About the error:

lvalue required as left operand of assignment

lvalue means an assignable value (variable), and in assignment the left value to the = has to be lvalue (pretty clear).

Both function results and constants are not assignable (rvalues), so they are rvalues. so the order doesn't matter and if you forget to use == you will get this error. (editI consider it a good practice in comparison to put the constant in the left size, so if you write = instead of ==, you will get a compilation error. for example:

int a = 5;
if (a = 0) // Always evaluated as false, no error.
{
    //...
}

int a = 5;
if (0 = a) // Generates compilation error, you cannot assign a to 0 (rvalue)
{
    //...
}

update me.

Best regards,

 

[jayanth.devarayanadurga]

If this is correct then

---

Code C - [expand]
1
int getline(char *, int);

---

this is wrong

---

Code C - [expand]
1
(len = getline(line, MAXLEN)) > 0

---

The first argument is a pointer to a char and it expects address of a char variable in the calling function.

It should be

---

Code C - [expand]
1
(len = getline(&line, MAXLEN)) > 0

---

Change

---

Code C - [expand]
1
if (nlines >= maxlines || p = alloc(len) == NULL)

---

to

---

Code C - [expand]
1
if ((nlines >= maxlines) || (p = alloc(len) == NULL))

---

 

[rangerskm]

i tried the code by making changes .but its shows still error

[sathish@localhost ~]$ gcc sort1.c
/tmp/ccZ1YzHU.o: In function `readlines':
sort1.c:(.text+0x96): undefined reference to `alloc'
sort1.c:(.text+0xf9): undefined reference to `gettline'
collect2: error: ld returned 1 exit status

 

[errakeshpillai]

The function to allocate memory is malloc/calloc , not simply alloc.

And the second error will be due to some typo mistake. You may have forgot to change the name of the function (adding an extra 't' ) somewhere in the program.
Just check it and update.