Command Prompt and Expressions

At its heart, MATLAB® is a big calculator. To calculate something simply type it in at the "command prompt" and press Enter. Thus, to calculate 1 + 1 we type it in and press Enter. The screen should show:

>> 1+1
ans =
2 

meaning that the answer is 2.

Exercise 1. Run MATLAB, find the command window and the blinking cursor. Find the answer to the following arithmetic problems:

• $$1234+4321=?$$
• $$104-765=?$$
• $$47*33=?$$
• $$3^4=?$$ (The operator for "power" is the circumflex ^, usually found by pressing Shift ⇑ 6
• How far is $$19^2$$ from its approximation $$20^2-2*20$$ (Remember that $$(a-b)^2=a^2-2ab+b^2$$, thus the answer should be ±1)
• Find an approximation to 1/73
• Find an approximation to $$\sqrt{31}$$ (while you can of course use the fact that $$\sqrt{x}=x^{0.5}$$, you can also "look for" a dedicated function square root by learning how to use the lookfor command....)
• If you get 5% interest-rate (yearly) on a loan, compounded monthly, and you start with \$1000, how much money will you have after 20 years? (don't be confused by an answer of the form 2.7e3 which simply means $$2.7\times10^3$$)
• If two sides of a right triangle have lengths 31 and 45, what is the length of the hypotenuse?

You may have noticed in the exercises that the answer is only given with 5 digits of accuracy (at most). For example, we can ask MATLAB for the value of $$\pi$$ and get:

>> pi
ans =
3.1416

Internally, MATLAB keeps a 16 (more-or-less) digit version of the number it shows us, but to keep things orderly, it only displays the answer rounded to show 5 digits (by default). We can change this by issuing a command:

>> format long
>> pi
ans =
3.141592653589793

We can see this, by subtracting part of $$\pi$$ from ans, which always holds the full, unrounded answer to the previous, unassigned expression:

>> format short
>> pi
ans =
3.1416
>> ans-3.1415
ans =
9.2654e-05
>> ans - 9.2653e-5
ans =
5.8979e-10

Exercise 2. Remember the cosine rule? $$c^2=a^2+b^2-2a b\, \cos(\theta)$$. Find the length of the hypotenuse of a triangle with angle 30ο, and sides with lengths 10 and 20. The MATLAB trigono­metric functions (cos, sin, tan) use radians, so you will need to convert using $$\pi$$.

For guided practice and further exploration of how to use the command prompt, watch Video Lecture 2: The Command Prompt.