how to understand "Decision tree for classification (text description)"

The "Decision tree for classification (text description)" is difficult to understand. Who can give detail explanation.

Thank you

Answer 1

[Roberto Herrera-Lara]

The philosophy of operation of any algorithm based on decision trees is quite simple. In fact, although sometimes containing important differences in the way to do this or that step, any algorithm of this category is based on the strategy of divide and conquer. In general, this philosophy is based on the successive division of the problem into several subproblems with a smaller number of dimensions, until a solution for each of the simpler problems can be found. Based on this principle, the classifiers based on decision trees try to find ways to divide the universe into successively more subgroups (creating nodes containing the respective tests) until each addressing only one class or until one of the classes shows a clear majority do not justifying further divisions, generating in this situation a leaf containing the class majority....

more details here,

http://en.wikibooks.org/wiki/Data_Mining_Algorithms_In_R/Classification/Decision_Trees

http://ocw.mit.edu/courses/sloan-school-of-management/15-097-prediction-machine-learning-and-statistics-spring-2012/lecture-notes/MIT15_097S12_lec08.pdf

Answer 2

[Amend]

The text description for "Decision tree for classification" has some differences from the "if-then-else-end" statement.

For example (from matlab), decision tree for classification

1. if x3<2.45 then node 2 elseif x3>=2.45 then node 3 else setosa
2. class = setosa
3. if x4<1.75 then node 4 elseif x4>=1.75 then node 5 else versicolor
4. if x3<4.95 then node 6 elseif x3>=4.95 then node 7 else versicolor
5. class = virginica
6. if x4<1.65 then node 8 elseif x4>=1.65 then node 9 else versicolor
7. class = virginica
8. class = versicolor
9. class = virginica

from above text, I know "class" represents a leaf. but how can I know leaf "setosa" goes to left branch not right branch from line 1. What does it mean that the word "else" in line 1,3,4 and 6 because it has no sense to tell me left branch or right branch.

[marwa lagha]

it is easier for you to view the decision tree rather that thinking wether it is left or right branch

 view(Mdl,'mode','graph');