Reversing a part of matrix

Hey, I would like to choose two random values in a matrix and reverse from the first random number to the second one.

i=randsample(10,2)

i1=i(1);

i2=i(2);

let's say we got two random numbers - 3 and 7

M=[10 20 30 40 50 60 70 80 90 100];

I would like to get the matrix as following:

M_new=[10 20 70 60 50 40 30 80 90 100];

Could anyone help me to code it?

The method with the flip doesn't help much.

Answer 1

[Star Strider]

A few examples of a robust approach —

M=[10 20 30 40 50 60 70 80 90 100];
i=sort(randsample(10,2))

i = 2×1

5 6

M_new = [M(1:i(1)-1) flip(M(i(1):i(2))) M(i(2)+1:end)]

M_new = 1×10

10 20 30 40 60 50 70 80 90 100

i=sort(randsample(10,2))

i = 2×1

3 9

M_new = [M(1:i(1)-1) flip(M(i(1):i(2))) M(i(2)+1:end)]

M_new = 1×10

10 20 90 80 70 60 50 40 30 100

i=sort(randsample(10,2))

i = 2×1

1 9

M_new = [M(1:i(1)-1) flip(M(i(1):i(2))) M(i(2)+1:end)]

M_new = 1×10

90 80 70 60 50 40 30 20 10 100

i=sort(randsample(10,2))

i = 2×1

2 7

M_new = [M(1:i(1)-1) flip(M(i(1):i(2))) M(i(2)+1:end)]

M_new = 1×10

10 70 60 50 40 30 20 80 90 100

i=sort(randsample(10,2))

i = 2×1

8 9

M_new = [M(1:i(1)-1) flip(M(i(1):i(2))) M(i(2)+1:end)]

M_new = 1×10

10 20 30 40 50 60 70 90 80 100

.

[Bartosz Bagrowski]

Thank you very much for helping me to solve the problem!

[Star Strider]

My pleasure!

If my Answer helped you solve your problem, please Accept it!

.

Answer 2

[Voss]

i=sort(randsample(10,2)) % sort so that i(2) >= i(1)

i = 2×1

3 7

i1=i(1); 
i2=i(2); 

M=[10 20 30 40 50 60 70 80 90 100];

M_new = M;
M_new(i1:i2) = M(i2:-1:i1)

M_new = 1×10

10 20 70 60 50 40 30 80 90 100

Answer 3

[Riccardo Scorretti]

A possible way is to pass through an vector of index (= ind in the code hereafter):

M=[10 20 30 40 50 60 70 80 90 100];
t = randsample(10, 2)   % I reserve i for the imaginary unit

t = 2×1

8 10

ind = 1 : 10;
ind(t) = t([2 1]);
M = M(ind)

M = 1×10

10 20 30 40 50 60 70 100 90 80

Answer 4

[Riccardo Scorretti]

Another way (more efficient, I think) is to do a swp by hand:

M=[10 20 30 40 50 60 70 80 90 100];
t = randsample(10, 2)   % I reserve i for the imaginary unit

t = 2×1

1 10

tmp_ = M(t(1)) ; M(t(1)) = M(t(2)) ; M(t(2)) = tmp_

M = 1×10

100 20 30 40 50 60 70 80 90 10

[Voss]

@Riccardo Scorretti Note that these answers swap the two elements at indices t, but the question asks to reverse the order of all elements between those indices.