14:440:127– Introduction to Computers for Engineers
Notes for Lecture 3
Rutgers University, Fall 2008
Instructor Blase E. Ur
0.1
Input
So far in this class, the main variability in the programs you’ve written has come from you, the
programmers, by changing the value of variables you’ve set. In the real world, you’ll usually want
to get input from the user, the person who is interacting with your program. For example, if you
designed and built an ATM in Matlab, you’d ask the customer how much money they’d like to
withdraw.
Getting user input in Matlab uses the
input
function.
There are two pieces of information you
need to provide along with the input function: what variable to store the information in, as well as
what information should be displayed to the screen in order to prompt the user (give them infor
mation about what to enter). Here’s the syntax:
money = input(’Please enter how much to withdraw: ’);
0.2
Creating Vectors and Matrices
0.2.1
Vectors
Creating matrices or vectors (matrices with only 1 row OR 1 column) is quite easy in Matlab. You
simply enclose a list of elements in square brackets. Here’s our first row vector (1 row, 4 columns):
x = [ 1 5 8 9 ]
To instead create column vectors, you need to indicate to Matlab to ”skip to the next row” in
the vector. You use the semicolon ( ; ) to do this:
y = [ 1; 5; 8; 9 ]
0.2.2
Matrices
As you might guess, creating a Matrix is essentially the same as a vector. Don’t forget that the
semicolon skips to the next row:
z = [ 1 2 3; 4 5 6 ]
This creates a 2x3 (2 rows by 3 columns) Matrix.
Whenever we talk about the size of Matrices,
the number of rows always comes first.
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0.2.3
The Length/Size of a Vector/Matrix
To find the size of a Matrix, you can use the
size
function. It returns a 1x2 vector containing the
number of rows, and then the number of columns, in that matrix. You can either store the whole
matrix in one variable, or use a special way of writing the command to store the number of rows
and number of columns separately:
% assume M is a 2x3 matrix
z = size(M);
[r c] = size(M);
z would contain the matrix [2 3], r would contain the value 2, and c would contain the value 3.
A vector only has one meaningful dimension, although you don’t always know whether it has only
1 row or instead only 1 column. Additionally, sometimes you only care about the largest dimension
of a matrix rather than both the number of rows and columns. In this case, you can use the
length
function, which returns the larger of the number of rows and number of columns, and thus the
length of a vector:
% assume X is a vector, or a Matrix where only one dimension matters
l = length(X)
0.3
Special Ways to Create Vectors/Matrices
Matlab presents a number of more efficient ways to create certain types of vectors and matrices.
0.3.1
Colon
For cases where you need to create a vector of, say, the integers from 1 to 1000, you can do so by
using the colon operator, which in Matlab essentially means ”from X to Y” when you write X:Y.
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 Fall '08
 Finch
 Linear Algebra, matlab, Dot Product

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