Error using cross and not calculating correctly

I am trying to write the code to calculate the area of a trianlge given three vectors. I keep getting an answer that is wrong. Here's the code that I wrote:

input a=['Enter vector a:'];
input b=['Enter vector b:'];
input c=['Enter vector c:'];
A=norm(cross('c-a','b-a'));
Area=A/2

This is what I get when I plug in the following vectors:

>> TAREA

a=['Enter vector a:'][1 2 2]

b=['Enter vector b:'][3 1 4]

c=['Enter vector c:'][5 2 1]

Area =

53.4649

The area should be 5.4083. Why is the code not working properly?

Answer 1

[Walter Roberson]

CA = 'c-a'

CA = 'c-a'

BA = 'b-a'

BA = 'b-a'

dCA = double(CA)

dCA = 1×3

99 45 97

dBA = double(BA)

dBA = 1×3

98 45 97

cross(CA,BA)

ans = 1×3

0 -97 45

cross(dCA,dBA)

ans = 1×3

0 -97 45

From this you can see that cross('c-a','b-a') is the same as cross([99 45 97],[98 45 97]) . You are taking the cross-product of the characters ['c' '-' 'a'] and ['b' '-' 'a'] rather than the cross-product of the difference in values of variables c and a and b.

[Braden]

Rewrote the code to this:

a='Enter vector a';

b='Enter vector b';

c='Enter vector c';

Area=norm(cross(c-a,b-a))/2

Get this error message now:

>> TAREA

A and B must be of length 3 in the dimension in which the cross product is taken.

Error in TAREA (line 4)

Area=norm(cross(c-a,b-a))/2

[Walter Roberson]

a='Enter vector a';
double(a)

ans = 1×14

69 110 116 101 114 32 118 101 99 116 111 114 32 97

You just created a as the vector ['E', 'n', 't', 'e', 'r', ' ', 'v', 'e', 'c', 't', 'o', 'r', ' ', 'a']

Try

a = input('Enter vector a ');
b = input('Enter vector b ');
c = input('Enter vector c ');

[Braden]

Got it now. Thanks

Answer 2

[Dyuman Joshi]

You are using character scalars instead of using variables. The result you get are corresponding to the ascii values of the characters

a=[1 2 2];
b=[3 1 4];
c=[5 2 1];
%Using variables
Area1=norm(cross(c-a,b-a))/2

Area = 5.4083

Area2=norm(cross('c-a','b-a'))/2

A = 53.4649

[Braden]

Rewrote the code to this:

a='Enter vector a';

b='Enter vector b';

c='Enter vector c';

Area=norm(cross(c-a,b-a))/2

Get this error message now:

>> TAREA

A and B must be of length 3 in the dimension in which the cross product is taken.

Error in TAREA (line 4)

Area=norm(cross(c-a,b-a))/2

[Dyuman Joshi]

If you want to ask a user for inputs, try this -

a=input('Enter vector a\n');
b=input('Enter vector b\n');
c=input('Enter vector c\n');
Area1=norm(cross(c-a,b-a))/2

When you run the above code, a prompt will ask to input the values for a, b and c. Provide the values of vectors respectively and you will get the value of Area

[Braden]

Got it now. Thanks