How to get value at particular index from a vector?

This is a discussion on How to get value at particular index from a vector? within the C++ Programming forums, part of the General Programming Boards category; Hello evervbody How to get value at particular index from a vector? Thanks in advance Ketu...

  1. #1
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    Question How to get value at particular index from a vector?

    Hello evervbody

    How to get value at particular index from a vector?

    Thanks in advance
    Ketu

  2. #2
    Jack of many languages Dino's Avatar
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    I wrote a member method to do this just last week. Iterate through the vector and keep a counter as you go. When you hit the "nTH" element, return the vector element. Not too efficient with real large vectors, but it works.

    Todd

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    Thanks a lot Todd burch.
    Is this means that there is no such method in STL library?
    I mean predefined method in STL...

  4. #4
    Jack of many languages Dino's Avatar
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    I don't know. I'm new to C++ myself, and I'm still reading the books. I created a get_element() method - here's the function I came up with: ("sentences" is a vector<Sentence *> vector.

    (oops - the method I described was my first implementation - this one is better - I forgot I changed it. Glad I looked!! I return NULL if the requested element does not exist.)

    Code:
    Sentence * Sentences::get_element(unsigned int n) { 
    	Sentence * temp = NULL ;
    	vector<Sentence *> * s ; 
    
    	s = &sentences ; 
    	
    	if (n >= s->size()) return temp ; 
    	return (*s)[n] ; 
    }

  5. #5
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    If you're using an STL vector, remember it is random access, so you may access any element using the index operator

    Code:
    std::vector<int> a;
    
    a.push_back(1);
    a.push_back(2);
    a.push_back(3);
    
    std::cout << a[0] << " " << a[1] << " " << a[2] << std::endl;
    Prints out

    Code:
    1 2 3
    Edit: if you want to search for a particular index where a value is stored, you'd do something like

    Code:
    int index(const vector<int>& a, int key)
    {
    
      for(int i = 0; i < a.size(); ++i)
      {  
          if(a[i] == key)   //key found at index i
             return i;
      }
     
        return -1;   //key not found
    }
    
    std::vector<int> a;
    
    a.push_back(1);
    a.push_back(2);
    a.push_back(3);
    
    std::cout << index(a, 2) << std::endl;
    prints out

    Code:
    1
    Last edited by indigo0086; 01-02-2008 at 08:35 AM.

  6. #6
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    You can use std::distance() from the <iterator> header.
    Code:
    std::vector<int> vec;
    std::vector<int>::iterator it;
    ...
    std::vector<int>::difference_type diff = std::distance( vec.begin(), it );

  7. #7
    The superhaterodyne twomers's Avatar
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    the random access operator ([]), doesn't throw, so if you want something that does use .at(). Works the same as [] otherwise, to the best of my knowledge.

  8. #8
    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by Todd Burch View Post
    I wrote a member method to do this just last week. Iterate through the vector and keep a counter as you go. When you hit the "nTH" element, return the vector element. Not too efficient with real large vectors, but it works.

    Todd
    Are you serious?!

  9. #9
    Jack of many languages Dino's Avatar
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    Yep! Well, that was my first hack. See subsequent post. Obviously, I figured it out.

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    Did I see a joke in here somwhere...this is the C board

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    C++ Witch laserlight's Avatar
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    Did I see a joke in here somwhere...this is the C board
    It's in the C++ Programming board, and I do not recall it being in the C programming board otherwise I would have moved it.
    C + C++ Compiler: MinGW port of GCC
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    Look up a C++ Reference and learn How To Ask Questions The Smart Way

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    >> Yep! Well, that was my first hack. See subsequent post. Obviously, I figured it out.

    Todd Burch, I hope that you are not actually doing that in your code. If you are, please say it so that we can help educate you about containers and vectors in particular.

  13. #13
    Jack of many languages Dino's Avatar
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    I had it in there for about 2 hours, and now it's long gone.

    I'm Just learning C++ from books, while writing a commissioned application at the same time! !!!

    Trust me - if I can't find an answer from the books when I run across my next stumbling block - I'll certainly let you all educate me! I'm not too proud to ask for help when it's warranted.

    Todd

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    Sounds good. These things can be quite confusing if you get unlucky and don't stumble across the text that explains them very well.

    Here's how I might re-do your posted code:
    Code:
    Sentence * Sentences::get_element(unsigned int n) { 
    	if (n >= sentences.size())
    		return 0;
    	return sentences[n]; 
    }
    Actually, I think I misread your code the first time. I guess you already figured out that you shouldn't be iterating through the vector. What you posted still had a bunch of unnecessary stuff in it, but it wasn't as bad as I made it sound.

  15. #15
    Jack of many languages Dino's Avatar
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    Thanks for the tweak - I will implement that "return 0" instead of allocating an empty pointer. I'm new to C as well - only been coding in it a few months now.

    While some code in that method appears superfluous for the example at hand (and it is), in my actual code, based on the state of processing, get_element() might return a pointer from one of two different vectors. That's why I set up variable s with the address of one vector. I removed some of my code to post, but didn't rewrite it to be as simple as it could be.

    Todd

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