What should I use? Or are there special occasion where I should use one over the other?

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It's been years but I'll guess :) I think a Karnaugh map helps lead to minimal required logic for the problem. It's a special case of a truth table. – nielsbot Jul 22 '12 at 01:16
4 Answers
The K map does not necessarily fail for higher dimensions. The problem is that it is so difficult to visualize for more than five variables. A 4 variable K-map is 2 dimensional and easy to visualize. A 5 variable is three dimensional, but is still manageable from a visualization standpoint, because the 2 states of the 5th variable only require visually moving from one plane to the next, without moving in the x or y directions of either plane. Just getting equations correct with more than 5 variables is difficult enough using the K map, much less considering an optimum set of terms ("core" prime implicants and "choice" prime implicants).
A truth table is a list of every possible output for every possible input. As you can imagine, these can get quite large. From the truth table, you can directly get a boolean expression via sum of products, but this is also a rather large expression. Karnaugh maps take truth tables and provide a visual way to produce a much simpler formula for expressing the same logic. That said, any time you produce a truth table, you can and probably should produce a K-Map to simplify the logic.

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A k map use reduces the number of logic gates to be used to solve an logic expression and hence reduces the error

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The disadvantage of k map :
- It is not suitable for computer reduction.
- It is not suitable when the number of variables involved exceed four.
- Care must be taken to field in every cell with the relevant entry, such as a 0, 1 (or) don't care terms.

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