Thread: I have a char * array that i need to remove duplicates

Hi there,

I have something like:

Code:

char * array = { "abc", "def", "ghi", "abc" };

I want to remove duplicates from this array of pointers. So that "abc" is only mentioned once. How can I accomplish this?

Thank you.

To be clear, the code you posted is not an array of pointers, but I get what you're asking.

Assuming it's a given that all elements point to valid C strings, you want to cycle through the array and compare each value with every subsequent value. Look into the "strcmp" function.

You also need to determine how you are going to "remove" duplicates - this will depend on the actual declaration.

Originally Posted by watchintv

Dman, thought I had that right haha. Ok well thanks for the clarification. What would you call that array? for future refernece...

What you posted was just a pointer. GReaper posted a good detailed clarification.

Originally Posted by watchintv

And that sounds about right, I'll give it ago and see how far I get.

Wonderful!

Originally Posted by watchintv

Any more help would be great.

Er, you've received answers to the questions you've asked. Any additional help must be framed from further questions by you.

Originally Posted by watchintv

I just finished making my own reference sheet and theres still stuff believe it or not that I''m wrong on. C is beautiful, but complicated.

Making personal reference sheets is a great thing to do. I still do that to this day when learning something I am not familiar with.

Originally Posted by watchintv

I'd really like to get good with C, so hopefully this board will help me to do that.

You can learn a lot by hanging out here.

Originally Posted by GReaper

What you have there is a compilation error... This is a string array, what you wanted to do:

Code:

char* array[] = { "abc", "def", "ghi", "abc" };

but that also produces a warning, because a string literal( when defined as a pointer and not as a local array ) should always be const.
So here is the correct version of what you want:

Code:

const char* array[] = { "abc", "def", "ghi", "abc" };

Now that that's taken care of, on to your question, I thought of a simple algorithm to get you started:


Try to do something similar to that and come back if you have any problems.

Ok, well I tried. I totally understand what you mean but I'm having problems implementing it.

Code:

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

int main() {


        const char* array[] = { "abc", "def", "ghi", "abc" };

        int c;
        for(c = 0; c < sizeof(*array); c++)
        {

                int t;
                for(t = 0; t < sizeof(*array); t++)
                {
                        if (strcmp(array[c],array[t]) == 0)
                        {
                                array[t] = NULL;
                        }
                }

        }

}

You should explain the problems you're having, along with any errors, warnings, and/or incorrect results.

Here are some observations on the code you posted:

• sizeof(*array) does not give you the number of elements in the array. You can use sizeof(array)/sizeof(array[0]), or alternatively, you can use a named constant.
• "strcmp" expects a valid C string. If your strategy is to "remove" words by setting their pointer to NULL, you should ensure a pointer is not NULL before passing it to "strcmp".
• There's no reason the inner loop should start at zero. Imagine you were doing the comparison on paper, using your fingers to point to the strings. Your left hand ("c") would point to the first word, and your right hand ("t") would point to the second. Then you move your right hand along the list. When you reach the end, your left hand ("c") moves to the second word, and your right hand ("t") would start at the third. Visualizing the logic this way should give you a clue where to start the inner loop.
• You should have another loop to print valid strings at the end of the program, so that you can verify the output.

You should also build your program up gradually, compiling and testing each little bit along the way. A development process

Originally Posted by Matticus

You should explain the problems you're having, along with any errors, warnings, and/or incorrect results.

Here are some observations on the code you posted:

• sizeof(*array) does not give you the number of elements in the array. You can use sizeof(array)/sizeof(array[0]), or alternatively, you can use a named constant.
• "strcmp" expects a valid C string. If your strategy is to "remove" words by setting their pointer to NULL, you should ensure a pointer is not NULL before passing it to "strcmp".
• There's no reason the inner loop should start at zero. Imagine you were doing the comparison on paper, using your fingers to point to the strings. Your left hand ("c") would point to the first word, and your right hand ("t") would point to the second. Then you move your right hand along the list. When you reach the end, your left hand ("c") moves to the second word, and your right hand ("t") would start at the third. Visualizing the logic this way should give you a clue where to start the inner loop.
• You should have another loop to print valid strings at the end of the program, so that you can verify the output.

You should also build your program up gradually, compiling and testing each little bit along the way. A development process

Ok, thanks for the tip on array size. What I'm having trouble with is setting up the loops that are required. I understand the logic mostly. Could you post some examples of the loops in question so i can see what you mean. I know its a lot to ask but I don't think I'm gonna understand any of this with out it. I learn best by reading code. Thhnaks.

Originally Posted by watchintv

Ok, thanks for the tip on array size. What I'm having trouble with is setting up the loops that are required. I understand the logic mostly. Could you post some examples of the loops in question so i can see what you mean. I know its a lot to ask but I don't think I'm gonna understand any of this with out it. I learn best by reading code. Thhnaks.

An "example" of the loop logic would basically be a solution, the posting of which goes against the forum guidelines.

Give it a try and see how far you get. If you get stuck, post your progress and we can go from there.

If you have specific questions about the logic I suggested above, feel free to ask.

Any chance we could sideline the forum rules this one time and just show me the loop, please. Id really appreciate it.

Originally Posted by watchintv

Any chance we could sideline the forum rules this one time and just show me the loop, please. Id really appreciate it.

Doing so would deprive you of a learning experience, so no, I will not hand you a solution.

You need to come up with logic before you start writing any code. Get a pencil and paper, and go through the problem "by hand" a few times. Determine the list of steps you need to perform to arrive at a solution. Then use this logical description to develop your code.

I did not respond to your previous reply because it does not appear to be a genuine attempt - there are some very basic syntax errors in that snippet.

Ok, thanks for the challenge haha. Well, I think I have a solution. Here it is:

Code:

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

int main() {


    const char* array[] = { "abc", "def", "def", "ghi", "abc", "abc" };


    int j;
    for (j = 0; j < sizeof(array)/sizeof(array[0]); j++)
    {
        int k;
        for (k = j + 1; k < sizeof(array)/sizeof(array[0]); k++)
        {        
            if (array[k] != NULL && array[j] != NULL)
            {        
                if (strcmp(array[j],array[k]) == 0)
                {
                    array[k] = NULL;
                }
            }
        }    
    }



    int a;
    for (a = 0; a < sizeof(array)/sizeof(array[0]); a++)
    {
        printf("%s \n",array[a]);
     
    }



}

Thanks Matticus and GReaper.