Remove duplicate elements from pointer array C

Question

I am trying to take user input and print out each word on a separate line(without duplicates). What i have done so far is able to take user input and print each line separately in an alphabetical order. What i need to do right now is be able to remove the duplicates within the array that's char* argue[]

My Input:

./a.out banana apple apple apple zoo cat fork

My output:

apple
apple
apple
banana
cat
fork
zoo

what needs to be done is print one apple instead of three.

Here is what i have done so far and I have commented the part of code where the problem is

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

int main(int argc, char* argv[]) {
  int i, j, k, size;
  size = argc -1;
  char *key;
  char* a[argc-1];

  for (i = 2; i < argc; i++) {
    key = argv[i];

    j = i-1;
    while (j >= 1 && strcmp(argv[j], key) > 0) {
      argv[j+1] = argv[j];
      j--;
    }

    argv[j+1] = key;
  }

  //Problem                                                                                                                                                   
  //for(i = 2; i < size; i++){                                                                                                                                
  //    if(argv[i-1] != argv[i])                                                                                                                              
  //      a[i] = argv[i-1];                                                                                                                                   
  //}                                                                                                                                                         

  //for(i=0; i< size; i++)                                                                                                                                    
  //  puts(a[i]);                                                                                                                                             

  for(i=1; i< argc; i++)
    puts(argv[i]);

  return 0;
}

Pointer comparison is not the same as string comparison.

jxh
– jxh

2016-11-21 20:12:33 +00:00
Commented Nov 21, 2016 at 20:12 — jxh
– jxh, Commented Nov 21, 2016 at 20:12

Vlad from Moscow · Accepted Answer · 2016-11-21 20:47:26Z

2

First of all you could use standard C function qsort declared in header <stdlib.h>.

If you want to output the parameters excluding duplicates then there is no need to remove parameters. You can just output unique parameters.

The program can look the following way

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

int cmp(const void *left, const void *right)
{
    return strcmp(*(const char **)left, *(const char **)right);
}

int main( int argc, char * argv[] )
{
    if (argc > 1)
    {
        qsort(argv + 1, argc - 1, sizeof(*argv), cmp);

        for (int i = 1; i < argc; )
        {
            puts(argv[i]);
            while (argv[++i] != NULL && 
                   strcmp(argv[i - 1], argv[i] ) == 0);
        }
    }

    return 0;
}

If to supply these command line parameters

banana apple apple apple zoo cat fork

then the program output will be the following

apple
banana
cat
fork
zoo

If you are indeed going to "remove" the duplicated parameters then argc shall have a correct value relative to the modified list of the parameters and argv[argc] shall be equal to NULL.

The program can look the following way

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

int cmp(const void *left, const void *right)
{
    return strcmp(*(const char **)left, *(const char **)right);
}

int main( int argc, char * argv[] )
{
    if (argc > 1)
    {
        qsort(argv + 1, argc - 1, sizeof(*argv), cmp);

        int n = 1;

        for (int i = 1; i < argc; i++)
        {
            int j = 1;
            while (j < n && strcmp(argv[j], argv[i]) != 0) j++;

            if (j == n)
            {
                if (n != i) argv[n] = argv[i];
                ++n;
            }
        }

        argc = n;
        argv[argc] = NULL;
    }

    for ( int i = 1; i < argc; i++ ) puts(argv[i]);

    return 0;
}

Its output will be the same as it is shown above.

edited Nov 21, 2016 at 20:47

answered Nov 21, 2016 at 20:21

Vlad from Moscow

313k27 gold badges204 silver badges358 bronze badges

Sign up to request clarification or add additional context in comments.

7 Comments

Bodo Thiesen Over a year ago

it is addressed, but it wont help the OP, he's obviously learning C and is supposed to understand the concept of strings and pointers. This answer won't help him there.

iMataMi Over a year ago

remove from char* or just use another array doesn't really matter. as long as duplicates are gone @Arkadiy

tdao Over a year ago

this is neat solution

iMataMi Over a year ago

I don't really understand how the while loop works thats within the for loop. I am confused as why it ends in ; and has no condition. An explanation would be great. Thanks! :)

Vlad from Moscow Over a year ago

@sleep-elite First of all as I already wrote in my answer argv[argc] shall be equal to null. The while loop has a condition but it has an empty statement as its body. The while loop is stop its iteration when either two string are non equal each other or the last NULL pointer is encountered.

|

Bodo Thiesen · Accepted Answer · 2016-11-21 20:42:48Z

0

In C, strings are not a data type itself but a convention introduced by the language. It is important to keep that in mind.

Now, if you have two char pointers, then even if the strings they point to are identical, the addresses at which they are stored may not be the same. So, the comparison

if (argv[i-1] != argv[i])

will not check for the strings to be identical, but instead will only compare the addresses. So:

char * a = "Hello world!\n";
char * b = a;

if (a == b) {
    puts("Always true\n");
}

Because here, you don't actually copy the string, you just copy the address where the string is stored. This also yields to this effect:

char a[20] = "Hello world!\n";
char * b = a;
a[0] = 'X';
puts(b); // Will print "Xello world!\n";

So, what you need is some way to compare the string pointed to by the two variables. That's what strcmp is for. It will return 0, if the strings are identical, or -1 or +1 if the first string comes alphabetically before or after the second string.

So use the test

if (!strcmp(argv[i-1], argv[i]))

~~and all should work as expected~~. Keep in mind, that your array a doesn't contain copies of the strings, but instead only point to the strings in argv.

/edit: Actually, there are some other small bugs, but once this problem is solved, I guess you will be able to fix the rest too.

=== recommendation hinted to in the comment ===

In a code like this:

for (i = 2; i < size; i++) {
    if (argv[i-1] != argv[i])
        a[i] = argv[i-1];
}

you can easily get things wrong by adding a line like so:

for (i = 2; i < size; i++) {
    if (argv[i-1] != argv[i])
        printf("argv[%i] and argv[%i] are identical\n", i-1, i);
        a[i] = argv[i-1];
}

because there you may forget to add another curly brake. So, I suggest to always add it even for one-liners like so:

for (i = 2; i < size; i++) {
    if (argv[i-1] != argv[i]) {
        a[i] = argv[i-1];
    }
}

even thou they are not really necessary here.

edited Nov 21, 2016 at 20:42

answered Nov 21, 2016 at 20:26

Bodo Thiesen

2,47719 silver badges34 bronze badges

3 Comments

iMataMi Over a year ago

I did for(i = 2; i < size; i++){ if (!strcmp(argv[i-1], argv[i])) a[i] = argv[i-1]; } and i am getting segmentation fault

Bodo Thiesen Over a year ago

Yea, check the start and end values of i in the loop and the array indices you're accessing. (i.e. put some printf("line %i: i=%i\n", __LINE__, i); in the code and analyze, what's going on ;)

Bodo Thiesen Over a year ago

Oh, and one recommendation about one-liners in if statements - I'll put in my answer, as it involves code formatting ...

Collectives™ on Stack Overflow

Remove duplicate elements from pointer array C

2 Answers 2

7 Comments

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

7 Comments

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related