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lessons/lesson04/for_loops.m

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+%% Lesson 2b: Control Sequence - For loops
+% Similar to other languages, MATLAB have if, while and for control
+% sequences. For loops are one of the commonly used control sequences in
+% MATLAB. We will continue the discussion of if and while in the next
+% lesson.
+
+clear;
+n = 1e7;
+% D = zeros(1,n);   % This is called pre-allocation
+                    % try uncommenting this line to see the
+                    % difference. Memory allocations are slow!
+
+% Calculate fib(n). Hard to do vectorized unless we know Binet's formula.
+% (Note that this will quickly overflow, but that's not our objective
+% here.)
+D(1) = 1;
+D(2) = 2;
+tic                 % start timer
+for i = 3:n
+    D(i) = D(i-1) + D(i-2);
+end
+toc                 % print elapsed time since last tic
+
+%% Be careful!
+% For loops are considered the more inefficient type of operation in
+% MATLAB. Performing operations on vectors is faster because your hardware
+% can optimize vectorized instructions, and because the "for" construct
+% exists in MATLAB's runtime, which is essentially a scripting language and
+% thus is slow.
+%
+% However, things are changing, and things like parfor (parallel-for) and
+% advanced JIT (just-in-time) compilation are improving the performance of
+% loops. But vectorized operations are almost always faster and should
+% be used where possible.