2.5.5: Finding, Deleting, Adding or Replacing Data
- Page ID
- 370473
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)The following examples will illustrate how to find, change, add or delete data in a vector. The same principles apply to manipulation of data in matrix form.
(a) Finding Data:
Suppose we wish to find the elements of vector V which are greater than or equal to 7:
V = [-2 3 7 -26 48 10 -8]', i = find(V>=7)
results in
V =
-2
3
7
-26
48
10
-8
i =
3
5
6
The statement, i = find(V>=7), returns a vector, i, containing the index of the elements of V which are greater than or equal to 7. The third, fifth and sixth elements of V are ≥ 7.
There are six relational operators that can be used in Matlab:
Relational Operators
< less than
<= less than or equal to
> greater than
>= greater than or equal to
== equal to
~= not equal to
(b) Deleting Data:
Specific elements in a matrix or vector can be deleted using the empty matrix (or vector), []. The statement
x = []
assigns a matrix of dimension zero-by-zero to x. To delete the elements of V which are greater than or equal to seven, use the command
V(i) = []
which returns
V =
-2
3
-26
-8
The elements of V which have indices given by the elements of i, are removed by assigning them to an empty matrix. Single elements may be removed in similar fashion. The command
V(3) = []
deletes the third element of V and returns
V =
-2
3
-8
An efficient way of removing rows and columns of a matrix is to assign them to an empty matrix. The command, A(:,3)=[], deletes column 3 of matrix A.
(c) Adding Data:
Suppose we wish to combine the elements of vectors V and U:
U = [3:2:10]', W = [V; U]
This returns
U =
3
5
7
9
W =
-2
3
-8
3
5
7
9
If we wish to add a single data point (-17) to the end of vector W:
W = [W; -17]
This returns
W =
-2
3
-8
3
5
7
9
-17
(d) Replacing Data:
To replace the fourth element of W with -12, enter the following
W(4) = -12
This returns
W =
-2
3
-8
-12
5
7
9
-17