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I am writing a function, which takes some char array as an input from the main(). This function returns pointer to char. There are three problems:

  1. If everything goes as it should, this function should return a pointer to the subarray of the passed array. Since main() needs to be able to access this subarray, I can use malloc in the function to create this global subarray.
  2. I need the function to pass an empty char array {} to main().
  3. To do the error checking, I need to pass NULL pointer. I only have question about the second issue. How do I pass pointer to the empty character array {} to the main() ?

I don't know if malloc will be helpful here.

**EDIT**:

Ok, I think I caused some confusion. Let me specify the exact problem. I am writing a function of the form char * test(char * a, int start, int end, int len) . Now a typical input to this would be an array like {'a', 'b', 'c'} for the array, start = 0 , end = 3 , len = 3. len here is the length of the input array. start is the starting position. In this example, its 0, which is beginning of the array. Now the end value says that start at the start value and take end no of elements from the start value. In this example, it will mean , start at 'a' and take 3 elements from there, i.e. 'a' , 'b', and 'c'. And form a new array with such a selection. Here we get original array back. So let a[] = {'a', 'b', 'c'}. Then test(a , 0 , 3 , 3) will return a pointer to {'a', 'b', 'c'}. Likewise, test(a , 0 , 2 , 3) will return a pointer to {'a', 'b'} and test(a , 1 , 2 , 3) will return a pointer to {'b', 'c'}. Now I want the function to return a NULL pointer if (start < 0) || (end < 0) || (0 == len) || (start+ end > len). So in the following situations the function returns a NULL pointer. test(a , 0 , 4 , 3) test(a , 1 , 3 , 3) test(a , 2 , 2 , 3) test(a , 3 , 1 , 3) test(a , -1 , 2 , 3) test(a , -1 , -2 , 3)

and if a[] = {} then test(a , 0 , 1 , 3) should return NULL. I got this figured. Now if the end value is 0, I want to function to return the pointer to the empty array {}. Function should not modify the original array.

EDIT NO 2:

I am doing sample problem 3 from http://admin.cs.mum.edu/Pretest/sampletest.htm. Hope that helps...

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  • can u post some code for simplicity!!! Commented Feb 5, 2013 at 12:58
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    How will the main function know that the array you return is empty? Commented Feb 5, 2013 at 13:00
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    Couldn't you return the subarray by index reference instead? In other words, return the start and end indices from the original array, instead of creating a brand new one. Commented Feb 5, 2013 at 13:01
  • How do I pass pointer to the empty character array - how do you define "empty"? Is it uninitialized? Is it full of 0's? Commented Feb 5, 2013 at 13:06
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    There are no "empty arrays" in C, the notation {} doesn't mean anything that can be desribed using those words. In other words, you're not making a lot of sense, I'm afraid. Commented Feb 5, 2013 at 13:09

1 Answer 1

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You can return structure which will contain the pointer to the address & length which will be filled inside your function.

struct ret_me
{
    char *p;
    int len;
}

.....

main()
{
    .......
    struct ret_me xxx;
    xxx=your_fun(...);
    if(xxx.len == 0)
    ...array is empty...
    ..........

}
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1 Comment

+1 for adding sense to the discussion of array lengths, and also for using struct return.

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