Thread Subject: Matlab matrix and plotting help

I have a few problems with some matlab work I have.

I need to count the number of positive values in a matrix
0.5-rand(5,6) and find out which element
in the matrix has the largest value.

3. Write a program that use Matlab ‘function’ to do the
following calculations:
(a) the sum of all elements in a matrix;
(b) the mean of all negative elements in the first column of
a matrix.

4. Write a program that will read in a text string mixed
with numbers and letters from the
keyboard and count how many numbers there are in the string.

5. Plot a 3D graphics described by the following equation:
i = 10sin(0.05 f t)
where i is the current in ampere, the time t changes from 0
to 20 seconds and the
frequency f can change from 10 to 15 Hz.

but I can't work out how.

For 3 (a) I can calculate the sum of each coloumn but not
the sum of all elements.

I can get a 2D graphic for question 5 but can't get my head
around how to get a 3D graphic.

type the following commands to the command-window:

>> help help

>> help find

>> help max

>> help min

>> help size

>> help mesh

>> help sum

>> help mean

>> help input

>> help >

and find out about the colon-operator (:)

Note that you should not use i or j as a variable since they
are reserved. >> help i

LARS

Hi,

You seem to have extended your questions to tags section. So
I'll start from there:

To get help on a particular function, say sum, type 'help
sum' at the command prompt. Also hitting F1 brings out a
nice documentation.

> For 3 (a) I can calculate the sum of each coloumn but not
> the sum of all elements.

Why not repeat the sum operation to get the sum of column
sums. Or convert your matrix to a single vector before
applying sum.
help sum
help colon

> I can get a 2D graphic for question 5 but can't get my head
> around how to get a 3D graphic.

I'm guessing you are using plot. Try surf or mesh.

> 4. Write a program that will read in a text string mixed
> with numbers and letters from the
> keyboard and count how many numbers there are in the string.

Hint: str2double('4') returns 4, whereas str2double('c')
returns NaN.

> (b) the mean of all negative elements in the first column of
> a matrix.

There are so many different approaches to this one, that I
wouldn't want to limit your creativity.

Volkan