C# Fast way to find value in a jagged Array

Question

I have a jagged Array String[][]. Now I need to find the array with a particular value in String[n][0] what i have a the moment is a simple

foreach foo in bar{
 if(foo[0]==needle){
  return foo;
 }
}

As you may see this is very slow for obvious reasons. I'm new to C# and just saw the indexOf, but how can i use indexOf in a jagged array?

Another way i tought of is Sorting the Array by String[n][0], going to the record in the middle, check if my value is larger or bigger, jump in the half of the upper/lower area and so on, maybe 3 or 4 times so i can find the record faster.

So whats the fastest way of getting the Array in a jagged Array where I only know [][0]?

What do you mean by "slow"? How many 1D-arrays does "bar" contain, how often do you have to search like this, what performance do you get and what performance do you want to achieve? — Doc Brown
– Doc Brown, Commented Mar 3, 2011 at 12:18
The sorting and jumping method you describe is called en.wikipedia.org/wiki/Binary_search_algorithm — TimCodes.NET
– TimCodes.NET, Commented Mar 3, 2011 at 12:21
Well i have about 25k foo and 25k needles. I don't know how long exactly it takes, but i guess about 30 Minutes. I need to get it down to maybe 5 Minutes. — lawl0r
– lawl0r, Commented Mar 3, 2011 at 12:23
When foo[0] does not change between searches, creating a Dictionary like suggested by Elian Ebbing will do the trick in a few seconds. — Doc Brown
– Doc Brown, Commented Mar 3, 2011 at 12:26

Elian Ebbing · Accepted Answer · 2011-03-03 12:18:27Z

4

I would use a Dictionary<int, int[]> where the key is the first item of the array. Dictionaries have constant time access, and are very fast for random access if the entire data fits in memory.

answered Mar 3, 2011 at 12:18

Elian Ebbing

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1 Comment

icn Over a year ago

Dictionary does not allow you to have the same keys. What happens if the first integer element of two arrays are the same? You have two different arrays in your dictionary with the same key!

Andy Holt · Accepted Answer · 2011-03-03 12:25:03Z

3

Perhaps rather than a jagged array, you'd be better suited with a Dictionary, where the key is the first item of each of your arrays. Since dictionaries are hash tables, look ups should be fast.

// Some test data.    
var dictionary = new Dictionary<string, string[]>
{
    { "foo1", new string[] { "foo1", "bar1" }},
    { "foo2", new string[] { "foo2", "bleh" }},
    { "foo3", new string[] { "foo3", "bar3", "hello", "world" }},
    { "foo4", new string[] { "foo4",  }},
    { "foo5", new string[] { "foo5", "bar5", "test" }},
};

string[] item = dictionary["foo3"]; // Fast look up.

answered Mar 3, 2011 at 12:25

Andy Holt

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Comments

Stecya · Accepted Answer · 2011-03-03 20:19:11Z

2

Well, you are searching for a value than can be on array[i][0] for every i from 0 to n, right? Then, it has nothing to do with jagged arrays, but with finding a value in a collection (which in your case is the collection of {array[i][0], for each 0 <= i < n )

If you're going to search only once, then the best thing that can be done is your own solution, that runs on linear time: O(n)

If your're going to search many times (without changind array[i][0] for any i) then you can sort the array by String[n][0] and do a "binary search", which is exactly the algorithm you described. Sorting will take O(n lgn) of time. But each search will be very fast O(lg n).

edited Mar 3, 2011 at 20:19

Stecya

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answered Mar 3, 2011 at 12:25

name

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