How to swap values of two variables?

For instance, if i ask the user to input the values of a and b, in this case they choose a=10 and b=5 how would I be able to switch them so that it'll be a=5 and b=10.

[Sankirna D. Joge]

function [q,r] = swapInputs(a,b)
  [q1,r1] = cart2pol(a,b);
  q1=(pi/2)-q1;
  [q,r]=pol2cart(q1,r1);
end

[Shuvo Kumar Paul]

function [a,b] = swap(a,b)
t = a;
a = b;
b = t;
end

[Subhashini Neelamegam]

This code works

[Jan]

@Subhashini Neelamegam: Yes, but this is more efficient:

function [b, a] = swap(a, b)
end

Answer 1

[David Goodmanson]

Hi Daniel,

Take a look at the 'deal' command, in this case [b a] = deal(a,b)

Answer 2

[Jan]

Cheaper than deal:

function [b, a] = swap(a, b)
% This function has no body!

[Angel Castillo]

It should be:

function [b, a] = swap(b, a)

[Jan]

@Angel Castillo: No, of course not, if you call it like:

[a, b] = swap(a, b)

But if you like to call it like:

[b, a] = swap(a, b)

this is another story. But then the swapping does not happen inside the function, but in the caller. I prefer to let the function do, what its name advertises.

[Steven]

That is really clever

[Yigit Bagci]

how it didnt accept this solution?

[Alec Jacobson]

What's happening under the hood? Does any copy happen?

[Rik]

@Alec Jacobson, There isn't really anything under the hood. Because of the copy-on-change memory management, I would expect no copying to happen anywhere.

This function has two inputs and two outputs. The first input just happens to be the second output (and vice versa).

[lala]

Hello! Which one is better? deal or swap? why?

[Rik]

The advantage of using deal would be that you don't need this function on your path. Other than that, there isn't really much of a difference beyond the order of variables. The two lines below will do the same:

[a,b] = swap(a,b)
[b,a] = deal(a,b)

[Bruno Luong]

swap is faster than deal (more over head).

But in absolute both are pretty fast.

[lala]

@Bruno Luong @Rik Thanks. you are right. In offline machine sawp is faster. But in matlab web, deal is faster, as seen below.

c1 = zeros(100);
c2 = ones(100);

tic
for i = 1:100000
    [c1,c2] = deal(c2,c1);
end
toc

Elapsed time is 0.107589 seconds.

tic
for i = 1:100000
    [c1,c2] = swap(c2,c1);
end
toc

Elapsed time is 0.353585 seconds.

function [a,b] = swap(a,b)
end

[Bruno Luong]

Be awared MATLAB online (web version?) run timing is not very stable due to other factors, not well controlled.

[Rik]

You generally need to warm up function calls if you want to test timings online. This should be reasonably stable. As you can see, they differ by a lot, but in the other direction than your post shows. Using a function instead of a script might also affect the timings (as you can see, the same code runs over 10x faster inside a function).

timeit(@test_deal);timeit(@test_swap);% warm-up round
timeit(@test_deal),timeit(@test_swap)

ans = 0.0798

ans = 0.0017

function test_deal
c1 = zeros(100);
c2 = ones(100);
for i = 1:100000
    [c1,c2] = deal(c2,c1);
end
end
function test_swap
c1 = zeros(100);
c2 = ones(100);
for i = 1:100000
    [c1,c2] = swap(c2,c1);
end
end
function [a,b] = swap(a,b)
end

[lala]

@Rik Thanks for your reply. You are right. :)

Answer 3

[Daniel Afriyie]

[b, a] = deal(a,b)

[Jan]

This was mentioned 2 years ago already.

Answer 4

[AAMIR SHEIKH]

Going Traditionally !!!

a = input("enter a :::");
b = input("enter b :::");
temp = a;
a = b;
b = temp;
[a b]