creating 30 digits numbers and finding sum

this is the question basically: add two 30 digit numbers and print the result. This is how it wants me to do it: store the integers in vectors, where each element in the vector stores a digit of the integer. Your script should initialize two 30 digit integers, storing each in a vector and then add these integers, also storing the result in a vector. Create the original numbers using the randi function.

This is what I have done so far:

clear all
num1 = zeros(1,30);
for i = 1:30
    num1(i) = randi(9);
end
fprintf('\n')
num1;
fprintf('%d',num1); fprintf('\n')
num2 = zeros(1,30);
for j = 1:30
    num2(j) = randi(9);
end
fprintf('\n')
num2;
fprintf('%d',num2); fprintf('\n')

This gives me two integers. I need to add them and display the result. Am I doing good so far? Any idea on how to approach the sum now?

Answer 1

[Stephen23]

Two useful tips for learning MATLAB:

• Get comfortable using the documentation : it has lots of examples and information that will make your own life easier! Learn to navigate the Contents (on the left-hand side of the webpage).
• Learn about vectorization and how to write efficient code right from the beginning, rather than learning poor coding practices first and trying to unlearn them later...

Some examples from your code of how doing these can be useful. These four lines

num1 = zeros(1,30);
for i = 1:30
    num1(i) = randi(9);
end

can be entirely replaced with one randi call

num1 = randi(9,1,30);

which is faster, neater and much easier to read!

But there is still a mistake here which we can fix by reading the randi documentation. It states quite clearly that randi(imax) returns a pseudorandom scalar integer between 1 and imax., so your code

randi(9)

will return any of 1,2,3,4,5,6,7,8,9 but excludes 0 because the lower bound of randi is one, not zero. Did you notice that zero is missing? How would you include zero in this list? (Hint: try a different upper bound, and subtract one). This brings us onto one more tip:

• Always check your code as you are writing it! Check that each line does exactly what you want it to do. Do not write a massive amount of code, try to run it, and then get confused because you don't know where the mistake occurs.

To perform the sum you should consider:

• preallocate a vector of zeros to put the sum in.
• use a for-loop to iterate over the length of your vectors.
• start at the ones (the right-end) and add the numbers one at a time.
• take care of any remainders!

Answer 2

[James Tursa]

>> "Any idea on how to approach the sum now?"

Well, if you had two 30-digit numbers written on a piece of paper, how would you go about adding them? You would line up the columns and add them one column at a time with carry-over. So code up the same thing in MATLAB for these two numbers. There are ways to vectorize some of this, but I would suggest you simply code up a simple loop to do it the way you would do it on paper first.