MATLAB Answers

insert element in vector

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Majid Al-Sirafi
Majid Al-Sirafi on 24 Sep 2012
Answered: JMP Phillips on 20 Apr 2021 at 11:51
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
  10 Comments
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 (7)

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

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

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.
  1 Comment
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;
  1 Comment
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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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.

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

JMP Phillips
JMP Phillips on 20 Apr 2021 at 11:51
If you are reading this in 2021 now in MATLAB you can just do something like this (no for loops, no 'cat'). It can also work with inserting more than 1 element into a vector.
y = zeros(1,length(x)+length(b)); %initialise a new vector of the appropriate size
y(a) = b; %insert the values in 'b' at the locations in 'a'
y(y==0) = x; %insert the original values in x into the new vector at their new positions. NOTE: because we use 0 we cannot insert a value of 0, this works for non-zero values only.
where
x is the existing vector to insert values into
a is vector of indices where to put new values
b is vector of new values
Example with inserting multiple values at specified locations into an array:
x = [1,2,3,4,5]
a = [3,5,1,4]
b = [5, nan, 3, 717]
Result:
y = 3 1 5 717 NaN 2 3 4 5
and the original question with inserting 1 element would be solved by
x=[1,2,4,5]
a = 3
b = 3
Result:
y = 1 2 3 4 5

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