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insert element in vector

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Majid Al-Sirafi
Majid Al-Sirafi on 24 Sep 2012
Hi everyone
now, how can I insert element in vector .... for example
a=[1,2,4,5]
how can I embed 3 between 2 and 4 in above vector to be
a=[1,2,3,4,5]
thank you
majid

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Walter Roberson
Walter Roberson on 28 Jan 2018
Sami Belhareth commented to Jan Simon:
This isn't a homework problem, they are looking for how to do a specific thing that may or may not be difficult to find in the documentation due to its specificity. Rude and unnecessary comment
Walter Roberson
Walter Roberson on 28 Jan 2018
Sami Belhareth:
Back in September 2012 when Majid Al-Sirafi asked the question, Majid was a student asking student questions. At the time we responded, we knew that from previous interactions. There was a significant chance that it was indeed a homework related question. People were justified in asking whether it was homework or not, as we answer differently if it is: for homework we explain a lot more and teach the student how to think about questions to increase their understanding of the language and of programming in general, where-as for non-homework we are more likely to give short code under the assumption that the person already understands MATLAB and programming.
Jan
Jan on 29 Jan 2018
@Walter: Thanks for your comment concerning homework. I think Sami's point is more my "rudeness".
The admins had cleaned up this thread in Nov-2017 already. I think it is strange, that the first activity from Sami's account is flagging a 3 year old comment in a 6 year old thread as being rude.

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Answers (6)

Wayne King
Wayne King on 24 Sep 2012
a = [1,2,4,5];
b = [a(1:2) 3 a(3:end)];

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Jonathan Campelli
Jonathan Campelli on 12 Mar 2015
Here is an application specific solution:
a=[1 2 4 5] %Your predefined vector.
a=sort([a 3]) %The "[a 3]" operation adds the element 3 to the end of vector "a", creating vector [1 2 4 5 3]. "Sort()" then alligns the new vector's elements in order from least to greatest.

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Maximilian Sgodda
Maximilian Sgodda on 7 Aug 2020
Thanks Jonathan!
I had a slightly different problem, like the question was asked and could solve it with your code :)

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Andrei Bobrov
Andrei Bobrov on 24 Sep 2012
Edited: Andrei Bobrov on 24 Sep 2012
b = 3;
i1 = 3;
a = [a(1:i1-1),b,a(i1:end)];
or
a = [1 2 4 5];
i1 = 3;
b = 3000;
n = numel(b);
out = ones(size(a) + [0 n]);
out(i1 + (0:n-1)) = 0;
out(out > 0) = a;
out(out == 0) = b;

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Bernard
Bernard on 29 Jul 2019
a = [1 2 4 5];
b = 3;
idx = 3;
a = [a(1:length(a) < idx), b, a(1:length(a) >= idx)];
Simpler version of your second example, which I prefer because it works with idx == 1 or idx == length(a).

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Daniel Shub
Daniel Shub on 24 Sep 2012
While I think this is a homework problem ...
Function handles and cat are your friends
insert = @(a, x, n)cat(2, x(1:n), a, x(n+1:end));
insert(3, [1,2,4,5], 2)
ans = 1 2 3 4 5

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Walter Roberson
Walter Roberson on 7 Mar 2018
There is no MATLAB operator for inserting into a MATLAB vector. Concatenating elements as described by Wayne King, Andrei Bobrov, and Daniel Shub is the natural MATLAB solution to this task.
The only exception to this is MATLAB String objects (R2016b and later), which have insertBefore and insertAfter operations defined for them that search the input strings to match a given text and insert at that point.
I have attached code to implement insertion after a given point into generalized vectors. Generalized vectors here refers to the fact that the vectors might have 3 or more dimensions, and that the code is not restricted to numeric or char.
There are some important differences between this code and the ones posted above:
  1. this code handles vectors of any dimension, not just row vectors
  2. the output is the same class as the initial vector even if different data types are involved. For example the other implementations if asked to insert 'a' after (double) 50 would produce '2a' because [50 'a'] automatically converts the double to char because of MATLAB rules about converting to the most restrictive data type when cat() is used. The code I attached will produce [50 97] instead -- but if you ask to insert (double) 50 before 'a' then you will get '2a' because the data type of the original vector is retained
  3. However, an empty original array will cause the output to be in the type of the new data so that you can always use [] to indicate empty array no matter what type you are appending
There are a number of design decisions explicitly documented in the code -- this seemingly simple operation is surprisingly complex.

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Elena Fiermonte
Elena Fiermonte on 30 Sep 2018
Edited: Elena Fiermonte on 6 Oct 2018

Hi everyone. I know it's a bit long, this should work with every kind of sorted vector. Feel free to refine it. Here it is:

 %code
A=[1 2 4 5]; % you must predefined a sorted vector.
B=zeros(1,length(A)+1);
L=length(A);
n=input('ins num: ')
for i=1:L
  for j=i:L
    if n>A(i)
    B(i+1)=n;
    B(i)=A(i);
    B(j+1)=A(j);
    elseif n==A(1)
    B(1:2)=A(1);
    B(i+1)=A(i);
    end
  end
end
B

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