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I have an array of 10345 bytes, I want to compress the array and then decompress, kindly suggest me the compression algorithm which can reduce the size of array. I am using c language, and the array is of unsigned char type.

Rephrased: Can someone suggest a general-purpose compression algorithm (or library) for C/C++?

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    @Heath, what is the real question? Commented Aug 8, 2010 at 16:35
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    @Heath: You seem a bit close itchy today. Commented Aug 8, 2010 at 16:35
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    @Heath Hinnucutt: Recommended ways to cure bad mood: 0. Shutting down PC 1. Using punching bag 2. Sprinting for a few miles. 3. Weight lifting. Commented Aug 8, 2010 at 16:40
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    This "everybody is a moron" routine is getting old. Commented Aug 8, 2010 at 16:43
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    @Heath If people knew what they were talking about, they wouldn't really have to ask questions about it, now would they? Commented Aug 8, 2010 at 17:01

2 Answers 2

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zlib
Lossless Compression Algorithms

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11 Comments

@SigTerm -- Oh, it's unsigned char. I guess that means the contents of the unsigned char array is compressible by Zlib. Gosh, I had no idea.
@Hans - Oh, yes, it does. Use zlib ever?
@Heath Hunnicutt: Dude, I really recommend spriting/punching bag. Way more exciting.
@SigTerm: What with your username, I would expect you to tell me to "handle myself" for stress relief. :) By the way, in 1960, US government psychiatrist Eric Berne found that people who sublimate via violence are prone to homicide. So I'll pass on the punching bag.
-1 I have to agree with Heath. That's not a response as the question was incomplete, to put it mildly. Maybe the OP shouldn't even consider compression, it's probably the wrong approach. If it's sparsly populated array maybe a list or a tree would be a better data structure, if it's "random" data a generic compressor will not be able to compress anyway or anything else. The question lacks info to give a meaningful answer.
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This post's a community wiki. I don't want any points for this -- I've already voted to close the question.

The number of bytes to compress has very little to do with choice of compression algorithm, although it does affect the implementation. For example, when you have fewer than 2^15 bytes to compress, if you are using ZLib, you will want to specify a compression-level of less than 15. The compression-level in Zlib (one of the two such parameters) controls the depth of the "look-back" dictionary. If your file is shorter than 16k bytes, then a 32k look-back dictionary will never half-fill; in that case, use one less bit of pointer into the look-back for a 1/15th edge on the compression compared to setting ZLib to "max."

The content of the data is what matters. If you are sending images with mostly background, then you might want Run Length Encoding (used by Windows .BMP, for example).

If you are sending mostly English text, than you wish you could use something like ZLib, which implements Huffman encoding and LZW-style look-back dictionary compression.

If your data has been encrypted, then attempting to compress it will not succeed.

If your data is a particular type of signal, and you can tolerate some loss of detail, then you may want to transform it into frequency space and send only the principal components. (e.g., JPEG, MP3)

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@Josh -- My answer is essentially an explanation of why I voted to close. If you read my answer, you'll see that it is very vague and general. As any answer to such a question needs to be. The OP didn't even specify if he wanted lossless, although that's a fair assumption...
@Josh - No, I did not say it's not an answer. It is an answer. And if you read it, you may learn something. But it is not only an answer to the question. It is an answer to the meta-question of "what the heck am [OP] talking about?" So, no, not by my own admission. Only by your "judgment," which is a great admission on your part. Also your judgment is clearly biased by your opinion of me. Objectively, the text above does answer OP. So take your judgment and self-handle it.
@Josh: Look at the two answers here. Which of the two answers is more vague and general? Can you be serious? As for meta, that is for discussion of SO, not discussion of SO questions. Surely, you already know that? Being disingenuous or something?
This is actually a far better answer than the other one - the key part being "The number of bytes to compress has very little to do with choice of compression algorithm ... The content of the data is what matters." . In a sense there are no general purpose compression algorithms - just expansion algorithms that have interesting failure cases.
@Josh: I do realize I criticized my own answer as "very vague and general." ANY answer to this poorly-posed question must be "very vague and general." THAT is why I voted close. But if SO insists this question must live on in the corpus for reference, at least the answer should say something useful.
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