Homework help - need help debugging it

The Question Given

Use the data below to perform the following:

x = [1 2 3 4 5 6 7 8 9 10];

y = [0.841 0.909 0.141 -0.756 -0.958 -0.279 0.656 0.989 0.412 -0.544];

1. Plot the (x, y) data points as red squares
2. Determine the sign of each y data point using the sign() function. You must use the sign() function. If the sign command returns a positive value, square the value of y. If the sign command returns a negative value, square root the absolute value of y.
3. Plot the result as a blue solid line on the same plot produced in part A.

My Solution

%Data
x = [1 2 3 4 5 6 7 8 9 10];
y = [0.841 0.909 0.141 -0.756 -0.958 -0.279 0.656 0.989 0.412 -0.544];

%Plot the (x,y) data points as red squares
plot (x,y, 'rs')
title ('graph')
hold on

%Determine the sign of each y data point using the sign() function
a=sign(y)

%If statements (Part B)
if a==1;
y=(y^.2);
elseif a==-1;
y=sqrt(abs (y));
end

%Plot the second result on the same plot
plot (x,y, 'bl')
hold off

The Problem

It does seem that my graph is not showing as it should. All the values are positive, but from my understanding, they shouldn't.

Answer 1

[madhan ravi]

Use logical index instead of if and elseif:

x = [1 2 3 4 5 6 7 8 9 10];
y = [0.841 0.909 0.141 -0.756 -0.958 -0.279 0.656 0.989 0.412 -0.544];
%Plot the (x,y) data points as red squares
plot (x,y, 'rs')
title ('graph')
hold on
%Determine the sign of each y data point using the sign() function
a=sign(y);
%If statements (Part B)
y(a==1)=y(a==1).^2;
y(a==-1)=sqrt(abs(y(a==-1)));
%Plot the second result on the same plot
plot (x,y, 'bl')
hold off

[Lin Jun Lee]

Thanks! Just to check, if my understanding is correct, this code means

y(a==1)=y(a==1).^2;

"When y is positive, square y"

and

y(a==-1)=sqrt(abs(y(a==-1)));

"When y is negative, square root the absolute value of y"

Is there a reason why if statements and elseif statements can't be used? Is it because there are two variables or, what is the reason?

[madhan ravi]

Anytime :) , Exactly! you understood it , if you want to use if and elseif statements you have to use a loop which is a bit more work. This is matlab! so learn to vectorize.

Answer 2

[MUHAMMED IRFAN]

Hey,

In the part B of your code, you might need a for loop to look through each of the element in a and check whether it is +1 or -1. Something like:

for i = 1:length(a)
    Element = a(i);
    % code to check for sign goes here.
end