Problem 11. Back and Forth Rows

Created by Cody Team

Solve Later

Given a number n, create an n-by-n matrix in which the integers from 1 to n^2 wind back and forth along the rows as shown in the examples below.

Examples:

 Input  n = 3
 Output a = [ 1 2 3
              6 5 4
              7 8 9 ]

 Input  n = 4
 Output a = [ 1     2     3     4
              8     7     6     5
              9    10    11    12
             16    15    14    13 ]

Solve

Solution Stats

Last solution submitted on Oct 18, 2019

Last 200 Solutions

Problem Comments

15 Comments

15 Comments

Show 12 older comments

Mandeep Singh on 14 Jun 2013

nice one

André on 24 Jun 2014

good one

ABDELRAHMAN on 12 Dec 2015

i got same solution on matlab, but i don't know why i didn't here

Samarth Shivaswamy on 28 Dec 2015

I am no expert, but does anyone else think that the first test case is flawed? "%%n = 4;" instead of " %% (new line) n = 4;" So, "n" is never defined for the first case. Am I right? If not, I apologize!

shahin rmz on 29 Oct 2016

that seems legit

Aniket Patil on 7 Mar 2017

nice one

UnCroissant on 7 May 2017

Really enjoyed this one.

Loris Chavée on 17 Jul 2017

Can someone help me? My solution works on my matlab but gives me an error during the test! What am I supposed to do ? :(

Kyle Chappell on 8 Nov 2017

I think the answer to test suite 5 should be c = 2 since the first row is 0.21 and the second is 0.26. Please correct me if I'm wrong.

Lányi György on 13 Mar 2018

Awesome!

christopher Minix on 24 Apr 2018

Awesome

Majid Farzaneh on 17 May 2018

Great

Shizun Li on 31 Oct 2018

Check this out.
function b = back_and_forth(n)
b=1:n^2;
b=reshape(b,n,n)';
for i=2:2:n
b(i,:)=fliplr(b(i,:));
end
end

Sharon Spelt on 8 Nov 2018

It appears a '\r' is needed after the %% in the first test of the solution to make it process properly.

goc3 on 9 Nov 2018

The problem with the first test case has been fixed here.

Solution Comments

Solution 1663519

1 Comment

1 Comment

Shizun Li on 31 Oct 2018

Check this out.
function b = back_and_forth(n)
b=1:n^2;
b=reshape(b,n,n)';
for i=2:2:n
b(i,:)=fliplr(b(i,:));
end
end

Solution 1344664

1 Comment

1 Comment

Thomas Pingnam on 15 Nov 2017

my code works on my PC but not on this..

a = [1:(n^2)];
a = reshape(a,[],n)'

Solution 1258843

2 Comments

2 Comments

Peng Wang on 29 Aug 2017

How can u do it in 10

Maruf Ahamed on 21 Apr 2018

I also want to know.

Solution 1104383

1 Comment

1 Comment

Yoshimasa Nakatani on 15 Jan 2017

I can't write shorter program.

(^.^;)

Solution 1101531

1 Comment

1 Comment

SIVAPRAKASH DANABALASINGAM on 10 Jan 2017

very easy

Solution 1094249

1 Comment

1 Comment

TAYALAN SIVALINGAM on 29 Dec 2016

Solution 989219

2 Comments

2 Comments

Tomas Macha on 2 Oct 2016

Why is this not working please?

a = ones(n,n);
for i = 1:n^2;
a(i) = i;
end
a = a'

I've tried this in school and it worked fine... :O Could you please help and explain? :)

Allen on 10 Oct 2017

Look closer at the requirements. Even rows should be flipped, such that their values are in descending order when reading from left-to-right.

Solution 888538

1 Comment

1 Comment

pranju chakrapani on 11 May 2016

Why is this problem showing errors in line 6 when I have a code of 4 line?

Solution 720471

2 Comments

2 Comments

qloreyze ueyzzyyc on 24 Aug 2015

It works on matlab

ABDELRAHMAN on 12 Dec 2015

i got same solution on matlab, but i don't know why i didn't here

Solution 364752

1 Comment

1 Comment

Gnaneswar Nadh satapathi on 5 Dec 2013

good Question

Solution 301505

1 Comment

1 Comment

dariodazio on 11 Aug 2013

really clever :)

Solution 237221

4 Comments

4 Comments

Show 1 older comment

Pedro on 28 Apr 2013

Sorry for the nasty solution. I'm hoping the use of str2num and anonymous functions will eventually be limited, since most simple problems can be solved with this cheat.

Florent Hennart on 22 May 2013

Hi,
Could you comment how using str2num and anonymous functions decreases the score from 46 to 13? Is it a glinch in the notation system?
Regards

Pedro on 23 May 2013

Hi, Florent.

The scoring in Cody is based on the number of nodes in the parse tree, and strings only require one node.

You can have a look for yourself by typing:
'>> mtree('back_and_forth.m','-file').show'.

Try to do it for the same code, without str2num, and you'll see that even small things like parenthesis or plus signs require their own node in the tree.

In terms of performance, the two versions are not so different. The problem here is that every solution, that use str2num in this way, will have the same Cody score, regardless of how elegant or efficient the anonymous function is.

arcturus on 24 May 2013

I really like how you've split the matrix into two. That's a clever insight! And I am really interested in knowing WHY str2num requires only one node.

Solution 208665

2 Comments

2 Comments

Shuang-Wei on 6 Jan 2014

brilliant!

niu on 28 Feb 2014

clever!
abs(sort(-cumprod(-ones(n)).*reshape(1:n^2,n,n)',2));
This one is more clear.what do you think?

Solution 176360

1 Comment

1 Comment

sea knowledge on 13 Dec 2012

well done,why can't come up with this method

Solution 122786

1 Comment

1 Comment

Nigel Davies on 3 Aug 2012

Sorry, I just applied a slightly involved ans trick to the previous best solution. I hurts me though that the best solution involves a for loop.

Solution 41664

1 Comment

1 Comment

prithvi on 15 Feb 2012

This should work

Solution 2339

1 Comment

1 Comment

Jan on 27 Jan 2012

This is faster and nicer than calling an external function to flip the direction.

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Problem 11. Back and Forth Rows

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