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Arrays in Matlab
UC Berkeley
Fall 2004, E77
http://jagger.me.berkeley.edu/~pack/e77
Copyright 2005, Andy Packard. This work is licensed under the Creative Commons Attribution-ShareAlike
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Arrays
A rectangular arrangement of numbers is called
an array.
This is a 4-by-2 array.
It has 4 rows, and 2
columns.
The (3,1) entry is the
entry in the 3rd row,
and 1st column
Later, we will have rectangular arrangement of
more general objects. Those will also be called
arrays.
Scalars, Vectors and Arrays
1-by-1 arrays are also called scalars
1-by-N arrays are also called row vectors
N-by-1 arrays are also called column vectors
Row and Column vectors are also sometimes just called vectors
In Matlab, all arrays of numbers are called double
arrays.
Creating Arrays
Horizontal and Vertical Concatenation (ie., “stacking”)
– Square brackets, [, and ] to define arrays
– Spaces (and/or commas) to separate columns
– Semi-colons to separate rows
Example
3 4 5
>> [ 3 4 5 ; 6 7 8 ] is the 2-by-3 array 

6
7
8


If A and B are arrays with the same number of rows, then
>> C = [ A B ] is the array formed by stacking A “next to” B
Once constructed, C does not “know” that it came from two arrays stacked
next to one another. No partitioning information is maintained.
If A and B are arrays with the same number of columns, then
>> [ A ; B ] is the array formed by stacking A “on top of” B
So, [ [ 3 ; 6 ]
[4 5;7 8] ]
is equal to [ 3 4 5;6 7 8 ]
Creating special arrays
ones(n,m)
– a n-by-m double array, each entry is equal to 1
zeros(n,m)
– a n-by-m double array, each entry is equal to 0
rand(n,m)
– a n-by-m double array, each entry is a random number between 0
and 1.
Examples
>> A = ones(2,3);
>> B = zeros(3,4);
>> C = rand(2,5);
: convention
The “: (colon) convention” is used to create row vectors, whose
entries are evenly spaced.
7:2:18 equals the row vector
[ 7 9 11 13 15 17 ]
If F, J and L are numbers with J>0, F ≤ L, then F:J:L creates a
row vector
[ F F+J F+2*J F+3*J … F+N*J ]
where F+N*J ≤ L, and F+(N+1)*J>L
Many times, the increment is 1. Shorthand for F:1:L is F:L
F:J:L with J<0 (decrementing)
You can also decrement (ie., J<0), in which case L should be
less than F.
Therefore 6.5:-1.5:0 is the row vector
[ 6.5 5.0 3.5 2.0 0.5 ]
The SIZE command
If A is an array, then size(A) is a 1-by-2 array.
– The (1,1) entry is the number of rows of A
– The (1,2) entry is the number of columns of A
If A is an array, then
size(A,1) is the number of rows of A
size(A,2) is the number of columns of A
Example
>> A = rand(5,6);
>> B = size(A)
>> size(A,2)
Accessing single elements of a vector
If A is a vector (ie, a row or column vector), then
A(1) is its first element,
A(2) is its second element,…
Example
>>
>>
>>
>>
A = [ 3 4.2
A(3)
Index = 5;
A(Index)
-7
10.1
0.4
-3.5 ];
This syntax can be used to assign an entry of A. Recall assignment
>> VariableName = Expression
An entry of an array may also be assigned
>> VariableName(Index) = Expression
So, change the 4’th entry of A to the natural logarithm of 3.
>> A(4) = log(3);
Accessing multiple elements of a vector
A(3) refers to the 3rd entry of A. However, the index need not
be a single number.
Example: Make a 1-by-6 row vector, and access multiple
elements, giving back row vectors of various dimensions.
>> A = [ 3 4.2 -7 10.1 0.4 -3.5 ];
>> A([1 4 6]) % 1-by-3, 1st, 4th, 6th entry
>> Index = [3 2 3 5];
>> A(Index) % 1-by-4
Index should contain integers. Regardless of whether A is a
row or column, Index can be a row or a column, and Matlab
will do the same thing in both cases. The expressions below
are the same.
>> A([2 4 3])
>> A([2;4;3])
Assigning multiple elements of a vector
In an assignment to multiple entries of a vector
>> A(Index) = Expression
the right-hand side expression should be a scalar, or the same
size as the array being referenced by A(Index)
Example: Make a 1-by-6 row vector, and access multiple
elements, giving back row vectors of various dimensions.
>> A = [ 3 4.2 -7 10.1 0.4 -3.5 ];
>> A([1 4 6]) = [10 100 1000];
>> Index = [3 2 3 5];
>> A(Index) = pi;
Accessing elements and parts of arrays
If M is an array, then M(3,4) is
– the element in the (3rd row , 4th column) of M
If M is an array, then M([1 4 2],[5 6])
– is a 3-by-2 array, consisting of the entries of M from
• rows [1, 4 and 2]
• columns [5 and 6]
In an assignment
>> M(RIndex,CIndex) = Expression
the right-hand side expression should be a scalar, or the same
size as the array being referenced by M(RIndex,CIndex)
Do some examples
Layout in memory
The entries of a numeric array in Matlab are stored
together in memory in a specific order.
>> A = [ 3 4.2 8 ; -6.7 12 0.75 ];
represents the array
8
 3.4 4.2
 6.7 12 0.75


Name = ‘A’
Size = [2 3];
Data = 3
Somewhere in memory, Matlab has
-6.7
4.2
12
8
0.75
Workspace: Base
RESHAPE
RESHAPE changes the size, but not the values or order of the data in
memory.
>> A = [ 3 4.2 8 ; -6.7 12 0.75 ];
>> B = reshape(A,[3 2]);
The result (in memory is)
Name = ‘A’
Name = ‘B’
Size = [2 3];
Size = [3 2];
Data = 3
Data = 3
-6.7
-6.7
4.2
4.2
12
12
8
8
0.75
0.75
So, A is the array
8
 3.4 4.2
 6.7 12 0.75


while B is the array
3
12

 6.7

8


 4.2 0.75
END
Suppose A is an N-by-M array, and a reference of the form
A(RIndex,CIndex)
Any occurence of the word end in the RIndex is changed
(automatically) to N
Any occurence of the word end in the CIndex is changed
(automatically) to M
Example:
>> M = rand(4,5);
>> M(end,end)
>> M([1 end],[end-2:end])
: as a row or column index
Suppose A is an N-by-M array, and a reference of the form
A(RIndex,CIndex)
If RIndex is a single colon, :, then RIndex is changed
(automatically) to 1:N (every row)
If CIndex is a single colon, :, then CIndex is changed
(automatically) to 1:M (every column)
Example:
>> M = rand(4,5);
>> M(:,[1 3 5])
LINSPACE and LOGSPACE
>> linspace(A,B,N) is a 1-by-N row vector of evenly
spaced numbers, starting at A, and ending at B
>> logspace(A,B,N) is a 1-by-N row vector of
logarithmically spaced numbers, starting at 10A, and ending at
10B.
Examples
>> linspace(1,4,6)
>> logspace(-1,1,5)
>> log10(logspace(-1,1,5))
Unary Numeric Operations on double Arrays
Unary operations involve one input argument. Examples are:
– Negation, using the “minus” sign
– Trig functions, sin, cos, tan, asin, acos, atan,…
– General rounding functions, floor, ceil, fix, round
– Exponential and logs, exp, log, log10, sqrt
– Complex, abs, angle, real, imag
Example: If A is an N1-by-N2-by-N3-by-… array, then
B = sin(A);
is an N1-by-N2-by-N3-by-… array. Every entry of B is
the sin of the corresponding entry of A. The “for”-loop that
cycles the calculation over all array entries is an example of the
vectorized nature of many Matlab builtin functions
Binary (two arguments) operations on Arrays
Addition (and subtraction)
– If A and B are arrays of the same size, then A+B is an array of the same size
whose individual entries are the sum of the corresponding entries of A and B
– If A is an array and B is a scalar, then A+B is an array of the same size as A,
whose individual entries are the sum of the corresponding entries of A and the
scalar B
– If A is a scalar, and B is an array, use same logic as above
Scalar-Array Multiplication
– If A is an array,and B is a scalar, then A*B is an array of the same size as A,
whose individual entries are the product of the corresponding entries of A and
the scalar B.
Element-by-Element Multiplication
– If A and B are arrays of the same size, then A.*B is an array of the same size
whose individual entries are the product of the corresponding entries of A and
B
Matrix multiplication
– If A and B are arrays, then A*B is the matrix multiplication of the two arrays…
More later
Intro to plotting with Matlab
If X is a 1-by-N (or N-by-1) vector, and Y is a 1-by-N (or N-by-1)
vector, then
>> plot(X,Y)
creates a figure window, and plots the data in the axis. The points plotted
are
(X(1),Y(1)), (X(2),Y(2)), … , (X(N),Y(N)).
By default, Matlab will draw straight lines between the data points, and the
points will not be explicitly marked. For more info, do >> help plot
Example:
>> X = linspace(0,3*pi,1000);
>> Y = sin(X);
>> plot(X,Y)
Plotting several lines
If X1 and Y1 are both 1-by-N, and X2 and Y2 are both 1by-M, then
>> plot(X1,Y1,X2,Y2)
will plot both sets of data on the same axis.
Example
>>
>>
>>
>>
X1 = linspace(0,pi,1000);
Y1 = cos(4*X1).*sin(X1);
X2 = [0 1 4 5];
plot(X1,Y1,X2,sqrt(X2))