NMAP: Syntax examples for some common Matlab commands
Nick Teanby 2016
Contents
Defining a vector
Defines a row vector of zeros of length 5
x = zeros(1,5)
x =
0 0 0 0 0
Defines a column vector of zeros of length 5
x = zeros(5,1)
x =
0
0
0
0
0
Defines an evenly spaced row vector from 0 to 2 spaced by 0.5
x = 0:0.5:2
x =
0 0.5000 1.0000 1.5000 2.0000
transpose (swap rows and columns around) to turn into a column vector
x = x'
x =
0
0.5000
1.0000
1.5000
2.0000
define elements by elemet
x = [1,3,6,10,2]
x =
1 3 6 10 2
change and element
x(2) = 10000; x
x =
1 10000 6 10 2
Defining an array
arrays are defined row then column define a square 3x3 array of zeros
x = zeros(3)
x =
0 0 0
0 0 0
0 0 0
define an array 3rows x 4columns array of zeros
x = zeros(3,4)
x =
0 0 0 0
0 0 0 0
0 0 0 0
define elements by hand. Separate elements in different columns by commas, and by ; for next row
x = [1, 2, 3; 4, 5, 6]
x =
1 2 3
4 5 6
change an element in row 2 and column 3
x(2,3) = 100; x
x =
1 2 3
4 5 100
Checking variable definitions
you can see what variables are defined using the 'whos' command
clear all
x=100;
y=zeros(3);
z=zeros(100,2);
whos
Name Size Bytes Class Attributes x 1x1 8 double y 3x3 72 double z 100x2 1600 double
or just check a single variable
whos z
Name Size Bytes Class Attributes z 100x2 1600 double
For Loops
Example to sum the elements of a vector
x = [1,2,3,4,5]; n = length(x); xsum = 0; for j=1:n; xsum = xsum + x(j); end xsum
xsum =
15
Example to multiply all elements by 100
x = [1,2,3,4,5]; n = length(x); for j=1:n; x(j) = x(j)*100; end x
x = 100 200 300 400 500
Example to multiply only first 2 elements by 100
x = [1,2,3,4,5]; for j=1:2; x(j) = x(j)*100; end x
x = 100 200 3 4 5
Example to calculate difference between elements (note that the loop starts at index 2 and the xdiff vector of differences has one less element than the original x vector)
x = [10,20,40,100,300]; n = length(x); xdiff = zeros(1,n-1); for j=2:n; xdiff(j) = x(j)-x(j-1); end xdiff
xdiff =
0 10 20 60 200
Nested for loops
These are often used for two dimensional array processing Example to sum all the elements of the array
x = [1,2,3,4;5,6,7,8;9,10,11,12]; [nrows,ncolumns] = size(x); xsum = 0; for j=1:nrows for k=1:ncolumns xsum = xsum + x(j,k); end end xsum
xsum =
78
If statements and logic
These are used to handle different logical cases within a program. The constructs are:
& (AND), | (OR), == (equal to), ~= (not equal to), > (greater than), >= (greater than or equal to), etc
% Example to check if x is bigger than 3 x = 3; if x >= 3 disp('x is bigger than or equal to 3') else disp('x is not bigger than 3') end
x is bigger than or equal to 3
Example to see if x falls within a range
x = 3; if (x >= 2) & (x <= 4) disp('x is in the range 2 to 4') elseif x > 4 disp('x is greater than 4') elseif x < 2 disp('x is less than 2') else error('Something has gone wrong') end
x is in the range 2 to 4
Example to see if x falls outside a range
x = -1; if (x <= 2) | (x >= 4) disp('x is outside the range 2 to 4') else disp('x is inside the range 2 to 4') end
x is outside the range 2 to 4
Example to make a vector of random numbers and count how many are in the range 0.5-1
n = 7; x = rand(n,1); count = 0; for i=1:n if (x(i) >= 0.5) & (x(i) < 1.0) count = count + 1; end end disp(['x = ' num2str(x')]) disp(['Number in range 0.5-1 is ' num2str(count)])
x = 0.81472 0.90579 0.12699 0.91338 0.63236 0.09754 0.2785 Number in range 0.5-1 is 4
User interaction and messages
print out a message to screen
%>> disp('Computer here, is anyone listening?')
getting user input (\n starts a new line for you to type into)
%>> x = input('Enter value of x\n')
print out an error message and stop program exectution
%>> error('Maximum number of data exceeded')
PLOTTING: line graphs
plot sin(x) between 0 and 2$\pi$ radians.
Important: all angle are in radians in Matlab
x = 0:0.05:2*pi; y = sin(x); plot(x,y)
Add some titles and use a different line colour
x = 0:0.05:2*pi; y = sin(x); plot(x,y,'-r') xlabel('Radians') ylabel('sin(x)') title('A simple plot example 2 ')
plot it in degrees
x = 0:0.05:2*pi; y = sin(x); xdegrees = x*180/pi; plot(xdegrees,y,'+g') xlabel('Degrees') ylabel('sin(x)') title('A simple plot example 3 ')
plot multiple lines on one graph. 'hold on' stops it being replotted
x = 0:0.05:2*pi; y = sin(x); z = cos(x); plot(x,y,'-b') hold on plot(x,z,'-r') hold off xlabel('Radians') ylabel('f(x)') title('A simple plot example 4 ') legend('f(x)=sin(x)','f(x)=cos(x)')
