Loop Analysis in MATLAB: What is the Code Doing?

[gfd43tg]

Hello,

I am trying to decipher loop codes and I am having a lot of difficulty doing so

Here is an example code

A = 0;
while A < 1
    for k=1:5
        A = A+(-1)^k;
    end
    A = A+k;
end

So I thought the while function does something an infinite amount of times. This is what I think this code is doing

It starts with A = 0, (not sure what the while A < 1 is doing), then the for statement does this

A = 0 + (-1)^1 = -1
A = -1 + 1 = 0

for k = 1, then it must repeat this statement (k=2). The A = 0 in this line is not from the first line of code, it comes from the result for k=1,
A = 0 + (-1)^2 = 1
A = 1 + 2 = 3

well I guess somewhere here the A < 1 is violated, thus the code should stop? I run it and I get 4. I don't understand how this code is working.

 

[jedishrfu]

This code is pretty much nonsensical and can be simplified:

the for k=1:5 loop

A = A -1 + 1 -1 +1 -1

which basically subtracts one from A which means the outer while loop will run forever except

the statement A = A + k is outside the loop and so k is undefined at this point ie you should get a compile error

Where did you get this code?

 

[gfd43tg]

It was an example code problem in the lesson

 

[kreil]

Hello,

I am trying to decipher loop codes and I am having a lot of difficulty doing so

Here is an example code

A = 0;
while A < 1
    for k=1:5
        A = A+(-1)^k;
    end
    A = A+k;
end

<snipped>

I run it and I get 4. I don't understand how this code is working.

Step through it line by line and you can see how it gets a result of 4.

1. A = 0.
2. while A<1 is true, so continue.
3. Enter the for loop, and start with k=1.
4. A =-1
5. now k=2.
6. A = 0
7. now k=3.
8. A = -1
9. now k=4.
10. A = 0
11. now k=5, the last time through the for loop.
12. A =-1.
13. A = A+k, so A = -1+5 = 4.
14. Check if A<1. It evaluates to false so exit the while loop.
15. Return the answer of 4.

Basically, a while loop executes as long as the condition is true. In this case, it executes as long as A<1 is true.

A for loop executes for each specified value. So for k=1:5 means the loop executes for each separate value of k. Notice that the A=A+k line doesn't execute until the for loop is completely done.

 

[alphy]

Hello,

Thank you for reaching out about your difficulties with deciphering loop codes in MATLAB. I understand that it can be challenging to understand the logic behind a code, especially when it involves loops. Let me explain what this code is doing step by step to help you better understand it.

First, let's look at the while loop. This loop will continue to run as long as the condition "A < 1" is true. In this case, A is initially set to 0, so the loop will run at least once.

Next, we have the for loop. This loop will run 5 times, as specified by the statement "for k=1:5". Inside the loop, A is being updated with each iteration. Let's break down the code inside the for loop:

1. k=1: This sets the value of k to 1 for the first iteration.
2. (-1)^k: This is a mathematical operation that raises -1 to the power of k. For k=1, this results in -1, and for k=2, it results in 1.
3. A = A+(-1)^k: This updates the value of A by adding the result of the previous operation (-1 or 1) to the current value of A.
4. A = A+k: This updates the value of A again by adding the value of k (which is currently 1) to the current value of A.

So, for the first iteration of the for loop, A will be updated to 0 + (-1) + 1 = 0. For the second iteration, A will be updated to 0 + 1 + 2 = 3. This process will continue for 5 iterations, resulting in the final value of A being 3.

Now, back to the while loop. After the for loop has completed, A will be updated again by adding the value of k (which is currently 5) to the current value of A, resulting in A = 3 + 5 = 8. Since 8 is not less than 1, the condition of the while loop is now false, and the loop will stop running.

To summarize, this code is using a while loop to control the overall flow of the code, and a for loop to repeatedly update the value of A. In this case, the code is not running an infinite number of times, but rather 6 times