what is the meaning/output of this piece of MATLAB code? - matlab

Question

Can someone explain to me the meaning/significance of this code line by line? I am a beginner in MATLAB/programming so please use simple terminology particularly when explaining the fprintf function. (I tried to use the 'help' function in MATLAB to understand the 'fprintf' function and I still dont understand it... also if its simple enough, what is the expected output?
A = zeros(1,3); %pre-allocate space
k = 1; %count loop iterations
valueMatrix = [1 2 3; 5 6 7];
for jj = valueMatrix
fprintf('iteration %d:\n', k)
A(k) = jj(1) + jj(2);
jj, A %display variables on terminal
k = k + 1;
end

Answer 1

Sure. Let's go through each line of code separately. Bear in mind that I'm answering mainly because the for loop uses a matrix instead of a vector, and not many people know what that actually does.
A = zeros(1,3); %pre-allocate space
This creates an empty vector of three elements and this is stored in A.
k = 1; %count loop iterations
This I will explain later.
valueMatrix = [1 2 3; 5 6 7];
Declare a matrix of size 2 x 3, which looks like this:
1 2 3
5 6 7
for jj = valueMatrix
This is a for loop. I'm assuming you know what that is. for loops allow you to execute a piece of code inside the for syntax a given number of times. You specify a vector or a matrix and the loop executes the piece of code within the for construct again and again... and each time it executes, it pulls each value from the vector or matrix and stores it into a loop counter variable. In this case, that variable is jj. For example, if we did this:
for jj = 1 : 8
disp(jj);
end
This loop will start with jj = 1, and we will display this in the console. Next, we go jj = 2 and we display this in the console and we will keep going until jj = 8 then we stop. Now with a matrix this is slightly different. How it works is that at each iteration, we pull out a column at a time. For example, if we did:
for jj = [1 2 3; 4 5 6; 7 8 9]
disp(jj);
end
We would display:
1
4
7
... at the first iteration, then:
2
5
8
... then finally at the last iteration:
3
6
9
As such, if we did:
for jj = valueMatrix
... this means that we will run this loop three times. At the first iteration, jj would be:
1
5
The next iteration:
2
6
The last iteration:
3
7
What's important is that at each iteration, jj is a 2 element vector.
fprintf('iteration %d:\n', k)
fprintf stands for formatted print to file. However, the way it is being called prints to the screen instead of a text file. This may look unfamiliar to you because this is essentially the same way you'd call printf in C. The first parameter to this function is a string that you want to display to the screen. What you can also do is display the contents of a variable by putting the right specifiers inside the string. You use what are known as formatting strings. In this case, %d specifies an integer you want to print out, and \n means to move to the next line. Each formatting string is accompanied by the variable you want to print. In our case, %d is paired with the variable k, so you want to print out the variable k at each iteration. It would look like this at each iteration:
iteration 1:
iteration 2:
iteration 3:
A(k) = jj(1) + jj(2);
You'll notice that k starts at 1 and increments by 1 each time the loop iterates (the k = k + 1 statement two lines later). Therefore, at each iteration, we populate each entry in A with the addition of the two elements in jj.
jj, A %display variables on terminal
Self-explanatory. We show these variables to the screen, on top of the iteration print out statements... a bit messy IMHO.
k = k + 1;
Talked about this already. Increments at each iteration.
end
End the loop.
Therefore, the expected result that the three element vector stored in A has its contents changed, where each element sums from a column from valueMatrix. As such, for A you should get:
A = [6 8 10];