Question

matlab help

Answer 1

%% Problem 9.17
%{
8.10
The Colorado River Drainage Basi covers parts of seven western states. A
series of dams has been constructed on the Colorado River and its
tributaries to store runoff water and to generate low-cost hydroelectric
power (see Figure P8.10). The ability to regulate the flow of water has
made the growth of agriculture and popluation in these arid desert states
possible. Even during the periods of extended drought, a steady, reliable
source of water and electricity has been avaialble to the basin states.
Lake Powell is one of these reservoirs. The file lake_powell.dat containts
data on the water leve in the reservoir for the 8 years from 2000 to 2007.
These data are shown in Table 8.11. Use the data in the file to answer the
following questions:

(a) Determine the average elevation of the water level for each year and for
the 8-year period over which data were collected.

Answer 2

How about this:

while (1) % loop forever
{
input year
if year == 0 then break; end; % quit if you get 0
ix= year - 1999; % assuming the first year is 2000, this gives you ix=1
check that ix is legal
if not legal complain
else
{
do stuff
}
}

Answer 3

clear, clc % clears command workspace window

% loads the mat file into matlab called data
data = importdata('data.mat');

% average elevation of water per year
% asking user what year would you like to find avg elevations of rainfall
elevationWater = input('Please enter a year: ');
yearMax = 7;
k = 0;

while (k <= yearMax)
if (elevationWater == 2000)
avg = mean(data(:, 1));
k = k + 1;
elseif (elevationWater == 2001)
avg = mean(data(:, 2));
k = k + 1;
elseif (elevationWater == 2002)
avg = mean(data(:, 3));
end
k = k + 1;

end
fprintf('average elevation of rainfall is: %0.f \n', avg);

@jim_thompson5910

Answer 4

why are you using an input to get the averages? why not just print out all the averages for each year?

Answer 5

if I did that I wouldn't need a while loop?

Answer 6

I could ask the user for 7 inputs and fprintf out 7 outputs

Answer 7

is your instructions telling you to use input prompts?

Answer 8

no, but I want to use input prompts.

Answer 9

ok let me try something

Answer 10

Why can't I run the loop over multiple times asking the user to input a new year? That is what I was trying to get the program to do.

Answer 11

you'd have to put `elevationWater = input('Please enter a year: ');` somewhere in the "while" loop

Answer 12

Oh, I didn't know that has to go inside the while loop. I thought it would go on the outside.

Answer 13

`I thought it would go on the outside.`
if you want to run it once, then yeah it goes outside the loop

Answer 14

if you want to keep running the input indefinitely, then you'd have

```
while(1)
elevationWater = input('Please enter a year: ');

% if the user types in 0, then that means the user wants
% to break out of the loop and end the program
if(elevationWater == 0)
break;
end

% do other stuff here (like find the avg)
end
```

Answer 15

if you had "while(k < 10)" then the loop will keep running until "k<10" is false

saying "while(1)" will keep the loop running forever since the input "1" is always true. Matlab treats "1" as true and "0" as false.

So the loop will run forever but if the user types in 0, then the command "break" will be given to get out of the loop

Answer 16

I guess the user would have no idea entering 0 breaks out of the loop, so it's always a good idea to inform the user

elevationWater = input('Please enter a year: (type 0 to quit)');

Answer 17

I just want the loop to run until I reach 2007 though.

Answer 18

I want it to ask "Please enter a year?" Enter:2000
break

repeat the question "Please enter a year?" Enter:2001

and so fourth

Answer 19

forth*

Answer 20

that seems way too tedious for both the programmer and the user

Answer 21

lol okay, I will take your word for it.

Answer 22

I am trying something out

Answer 23

why not use a simple for loop?

```
clear, clc % clears command workspace window

% loads the mat file into matlab called data
data = importdata('data.mat');

%go through the years 2000 through 2007
% and compute the averages
% k = 1 corresponds to the year 2000
% k = 8 corresponds to the year 2007
for k = 1:1:8
avg(k) = mean(data(:, k));
end

fprintf('average elevation of rainfall is: %0.f \n', avg);
```

Answer 24

No user input is required. You just run the script m file and it will show you the 8 averages

Answer 25

If we used the while(1) and if (elevationWater == 0) break

how would we go about finding the different columns to average or is this just too complicated since we have to use the columns separately in the matrix? I like the code you have above I am just trying to be more "creative".

Answer 26

say a user types in `2003`

that means elevationWater = 2003

so I'd make

k = elevationWater-2000+1
k = 2003-2000+1
k = 3+1
k = 4

the year 2003 corresponds to data column 4

Answer 27

since `k = 4`, this means
`avg(k) = mean(data(:, k));`
is the same as
`avg(4) = mean(data(:, 4));`

the 4th value in the "avg" vector is equal to the mean of the data in column 4

Answer 28

Okay I am going to play around with that for a bit just for my own benefit.

Answer 29

Honestly, I'd go with the option of not having user input. It seems like too much work. But if you insist on user input, then this is one way to do it

```
while(1)
% get the year from the user
elevationWater = input('Please enter a year: (type 0 to quit)');

% if the user types in 0, then that means the user wants
% to break out of the loop and end the program
if(elevationWater == 0)
break; % get out of the "while" loop
end

% set up the proper index
k = elevationWater-2000+1

% find the average
avg(k) = mean(data(:, k));

% loop repeats back up at the top after this section
% loop will stop if the user types in 0
% otherwise, the loop goes on forever
end
```

Answer 30

sorry I forgot the bit of code after the "while" loop

```
while(1)
% get the year from the user
elevationWater = input('Please enter a year: (type 0 to quit)');

% if the user types in 0, then that means the user wants
% to break out of the loop and end the program
if(elevationWater == 0)
break; % get out of the "while" loop
end

% set up the proper index
k = elevationWater-2000+1

% find the average
avg(k) = mean(data(:, k));

% loop repeats back up at the top after this section
% loop will stop if the user types in 0
% otherwise, the loop goes on forever
end


fprintf('average elevation of rainfall is: %0.f \n', avg);

```

though that will spit out multiple averages and it won't be clear which averages they are

Answer 31

The one thing I struggle the most with when using while loops is finding the correct index. User has to make a proper formula to repeat the correct index.

Answer 32

When programming, always try to make the input instructions as clear as possible. The user may not know what to type in. So when you say "type in the year" expect the user to type in 2000,20001,etc all the way up to 2007

Answer 33

You may need to include extra instructions about only typing in those years. Nothing else

Answer 34

Okay I am going to work on the code above.

Answer 35

I'm realizing that after the "while" loop ends, the vector avg will have multiple values in it. So you may need to go through that vector and find the average

Answer 36

what do you mean?

Answer 37

So you may need to go through that vector and find the average

how would I go about finding the averages?

Answer 38

if "avg" is a vector with all the averages (8 total for the 8 years) of the columns

then you can use the `sum` function to sum up all the elements in the "avg" vector then divide by 8
```
average = sum(avg)/8
```

Answer 39

Okay so when I run my code it just repeats the prompt. It doesn't calculate the average for year 2000

Answer 40

attach your m file for me to look at

Answer 41

I'm curious where you placed the average function @raffle_snaffle

Answer 42

You have to scroll down to the bottom.

Answer 43

Problem 9.17 ?

Answer 44

yes

Answer 45

your 'end' for the 'if' block is located in the wrong spot

Answer 46

see attached

Answer 47

it doesn't want to hook up with the end if I put it back up near the 'break'

Answer 48

what do you mean?

Answer 49

The little line that shows up between the if and end

Answer 50

this is what your code should look like. All I did was move the first "end" up

Answer 51

only one line of code should be in the "if" block

Answer 52

hold on here

Answer 53

for some reason the if can't see the end.

Answer 54

if you had what you initially had, then that other code would only get executed if the user typed in 0. Try it out yourself

Answer 55

it can't see the end? how strange

Answer 56

you may need to restart matlab if it's being buggy

Answer 57

nvm about my last comment, about the line showing up there is no line.

Answer 58

so everything is working now?

Answer 59

no everything is not working

Answer 60

you set up k after you use it. You're supposed to set up k BEFORE you use it

Answer 61

I'm referring to this portion

```
% find the average per year
avg(k) = mean(data(:, k));

% sets up proper index
k = 2000 - 2000 + 1;
```

Answer 62

Okay I actually put the k before and still doesn't run like it should

Answer 63

and k will always be the same. You didn't put any variables in the k equation

Answer 64

k will always be 1 if you put `k = 2000 - 2000 + 1;`

Answer 65

I see what you are saying. thinking

Answer 66

btw, why did you make the input name to be "elevationWater" ? why not call it "year" ?

Answer 67

much better

Answer 68

So does your k equations always relate to your constraint for the "while"?

Answer 69

what do you mean?

Answer 70

So for example while (1) means were are running the loop until user enter '0'

k = year - 2000 + 1;
when I enter: 2000 we get k = 2000 - 2000 + 1 = 1 so this one get thrown... nevermind. It gets thrown into the avg(k) equation

Answer 71

Well, it still doesn't run. It just repeated the loop by asking user to enter a value over ang over again.

Answer 72

`while (1)` means the loop runs as long as "1" is true

"1" is always true, so the loop runs forever

however, we can override this and break out of the loop any time we want. In this case, we break out when the user types in 0

Answer 73

Please enter a year: (type 0 to quit)2000
Please enter a year: (type 0 to quit)2001
Please enter a year: (type 0 to quit)2002
Please enter a year: (type 0 to quit)2003
Please enter a year: (type 0 to quit)

Answer 74

if you did not have the break in there, the loop would run forever. It would run until you force the program to close, the program runs out of memory, or you unplug the computer or something

Answer 75

^That is what it does after i press enter it just keeps on prompting the user over and over again until I pres ctr+c

Answer 76

ctrl+c forces the program to shut down? or it pauses?

Answer 77

it stop the loop.

Answer 78

when you run the loop it propmps the user over and over again. To get out hold ctr and then press c in the command window

Answer 79

alright

Answer 80

Does it run on your end?

Answer 81

yes it works for me

Answer 82

I think when you hit ctrl+c, you are stopping the entire script

which means you skip over the part where you show the averages to the user

you want to type in 0 for the year to properly get out of the loop

Answer 83

Yes, that works. So in order to see results user has to enter 0

Answer 84

yes if you use the cntrl+c method, then the script never finishes to the bottom

Answer 85

I guess when you input multiple years before exiting it averages all the years?

Answer 86

It doesn't average each year individually.

Answer 87

nvm I think I have spent long enough on this problem and need to start working on my lab.

Answer 88

like I said, it's better to make a for loop and forget about user input

Answer 89

this is how I'd do it

```
clear, clc % clears command workspace window

% loads the mat file into matlab called data
data = importdata('data.mat');

%go through the years 2000 through 2007
% and compute the averages
% k = 1 corresponds to the year 2000
% k = 8 corresponds to the year 2007
for k = 1:1:8
avg(k) = mean(data(:, k));
end

fprintf('average elevation of rainfall is: %0.f \n', avg);
```

Answer 90

^yeah I would agree.

Answer 91

wait, I forgot about averaging the averages. Let me fix

Answer 92

```
clear, clc % clears command workspace window

% loads the mat file into matlab called data
data = importdata('data.mat');

%go through the years 2000 through 2007
% and compute the averages
% k = 1 corresponds to the year 2000
% k = 8 corresponds to the year 2007
for k = 1:1:8
avg(k) = mean(data(:, k));
end

average = sum(avg)/8;

fprintf('average elevation of rainfall is: %0.f \n', average);
```