PART 3: structures

Contents

structures

Structures are a useful way of grouping MATLAB variables that belong together. For example, imagine that you're simulating a ball being dropped from a building. You might have a scalar representing the mass of the ball, a vector representing its position, another vector representing its velocity, etc. You can group all the ball properties together into a MATLAB structure called ball:

clear
ball.mass = 10;
ball.position = [0, 0, 100];
ball.velocity = [0, 0, 0];

examining structures

To see the contents of a structure, just type it's name at the command prompt >>

ball

ball = 

        mass: 10
    position: [0 0 100]
    velocity: [0 0 0]

The individual components of the structure are called fields. You can also examine the content of an individual field:

ball.position

ans =

     0     0   100

adding fields

You can operate on elements of the structure just like any other MATLAB variable. For example to move the current XYZ-position of the ball by +10 in the y direction, you could type:

ball.position = ball.position + [0, 10, 0]

ball = 

        mass: 10
    position: [0 10 100]
    velocity: [0 0 0]

You can add a new field to the data structure at any time. For example, we'll add a new field for the radius and set its value:

ball.radius = 2.0

ball = 

        mass: 10
    position: [0 10 100]
    velocity: [0 0 0]
      radius: 2

arrays of structures

You can have an array of structures to represent multiple objects. For example, let's create two more "balls" by making a copy of the original "ball" structure:

ball(2:3,1) = ball;

now let's see what has happened:

ball

ball = 

3x1 struct array with fields:
    mass
    position
    velocity
    radius

MATLAB tells us that "ball" is now a 3x1 struct array. Each of the three elements of the struct array is a "ball" structure. Let's say we wanted each of the three balls to have a different mass:

ball(1).mass = 1.0;
ball(2).mass = 5.0;
ball(3).mass = 10.0;

Let's look at ball(2) now:

ball(2)

ans = 

        mass: 5
    position: [0 10 100]
    velocity: [0 0 0]
      radius: 2

accessing struct array contents

Now let's examine the masses of all three balls:

ball.mass

ans =

     1


ans =

     5


ans =

    10

Notice that MATLAB returned 3 different answers, one for each structure in the struct array.

Usually, we'd like all of these answers returned together in a single array. This can be accomplished by enclosing everything in square brackets.

[ball.mass]

ans =

     1     5    10

So, if we wanted to compute the total mass of all three balls, we could type:

totalMass = sum([ball.mass])

totalMass =

    16

However, if we tried it without the square brackets, we would get an obscure error message:

try
    totalMass = sum(ball.mass)
catch
    lasterr
end

ans =

Error using ==> sum
Trailing string input must be 'double' or 'native'.

If you see strange error messages like the one above when using struct arrays, it should make you think about square brackets!

Published with MATLAB® 7.0