# Calculating the Greatest Common Divisor for two positive integers (simple)

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Answered: Mai Aljneibi on 30 Nov 2018
Hi,
I am very new to Matlab so please excuse my obvious questions. I am trying to construct a simple function that takes two integers as input and returns the GCD. I have used the following functions:
function [A, B] = getData(x,y) %function to get data from the user
x = input('Enter x');
y = input ('Enter y');
A = x;
B = y;
end
function [M, N] = adjust(A, B) %sort the values. N is the bigger value, M is the smaller one
if (A < B)
M = A;
N = B;
end
end
function [GCD] = GCD(M, N) %function to calculate the Greatest Common Divisor
while(true)
R = mod(N, M);
if (R > 0)
N = M;
M = R;
end
if (R == 0)
GCD = M;
end
end
end
function [result] = printData(GCD) %function to print the result
result = GCD;
display(result);
end
I now want to call these functions so that when I open the script, it takes two inputs from me and displays the GCD. But I am stuck on this part. Please guide me on how to compile this jumble of code properly. Thank you!
##### 2 CommentsShowHide 1 older comment
well it is a homework but we have to define these specific functions and then call them. I dont mind rewriting my function using min and max
But i just need a clue on how to put all these separate functions together. Does the logic work? Even if it is not the best way to go about it, i would like to know if it works.

MSP on 12 Aug 2017
This is how u call the function.Calling getData and printData is useless.
A= input('Enter x')
B= input ('Enter y')
[gcd]=GCD_new(M,N)
And you have some problems in your functions.Check the modifications
function [M, N] = adjust(A, B) %sort the values. N is the bigger value, M is the smaller one
if (A < B)
M = A;
N = B;
else
M=B
N=A
end
end
The logic on your GCD doesn't seem right.Make sure to save the functions in the same folder or directory.
function [GCD] = GCD_new(M,N)
R = mod(N, M)
if (R==0)
GCD=M;
else
R = mod(N,M);
N = M;
M = R;
GCD=M;
end
end
And even easier,
type "help GCD in command window,does it all for you"
##### 2 CommentsShowHide 1 older comment
In other language you just do something like
getData();
to call a function and it runs the code. That does not seem to be working here.

Jan on 12 Aug 2017
You need a main function to call the functions:
function main
[A, B] = getData;
G = GCD(A, B); % Do not call the output like the function
printData(G)
end
Now simplify your functions: getData does not get inputs, if you overwrite them immediately. Create A and B directly and omit x and y.
The output printGCD should not return anything. disp(G) is enough.
The sorting is easier by min(A,B) and max(A,B).
The main problem remains your GCD function: It does not work. It is not correct mathematically yet. Take a look at the algorithm again. And currently you have an infinite loop. When do you want to exit it?
Beside Matlab's gcd command, you find the algorithm at WikiPedia also.

How does this look?
function main
[A, B] = getData; %it shows an error here (too many output arguments)
GCD(A, B);
printResult;
function getData %function to get data from the user
A = input('Enter x');
B = input('Enter y');
end
function [M, N] = adjust(A, B) %sort the values.
if (A < B)
M = A;
N = B;
else
M = B;
N = A;
end
end
function [GCD] = GCD(M,N) %i think I get the logic here now, my while loop was unnecessary
R = mod(N, M) %take the mod
if (R==0) %remainder = 0, then M is GCD
GCD=M;
else %otherwise
R = mod(N,M); %take the mod
N = M; %swap the values
M = R;
GCD = M; %GCD is M after loop ends
end
end
function printResult %function to print the result
display(GCD)
end
end
Abdulrehman Samiullah on 12 Aug 2017
Edited: Abdulrehman Samiullah on 12 Aug 2017
Can we write all these functions in one .m file??

HEY IT WORKS NOW. Thank you everyone =)
just had to use the while loop properly.
function [GCD] = GCD(N,M)
while (M > 0)
Q = N / M;
R = mod(N, M);
N = M;
M = R;
end
GCD = N;
end

Mai Aljneibi on 30 Nov 2018
How can we write a recursive function that returns the greatest common divisor of two positive integers.