I keep getting "Array indices must be positive integers or logical values."

clear;clc

x = .993 : 0.01 : 1.283;

for n = 1 :1: length(x)

f(x) = 7*x(n)^3 - 2*x(n)^2 + 7*x(n) - 14;

g(x) = -19*x(n)^3 + 3*x(n)^2 + 16*x(n) + 8;

end

plot(x,f,'+r',x,g,'-.b')

xlabel('x values')

ylabel('f(x) and g(x) values')

legend('f(x)','g(x)')

I keep getting:

Array indices must be positive integers or logical values.

Error in (line 9)

f(x) = 7*x(n)^3 - 2*x(n)^2 + 7*x(n) -14;

Please help

Answer 1

[Sulaymon Eshkabilov]

clearvars;clc
x = .993 : 0.01 : 1.283;
for n = 1 :length(x)
 f(n) = 7*x(n)^3 - 2*x(n)^2 + 7*x(n) - 14;    % Index has to be n
 g(n) = -19*x(n)^3 + 3*x(n)^2 + 16*x(n) + 8;  % Index has to be n
end
plot(x,f,'+r',x,g,'-.b')
xlabel('x values')
ylabel('f(x) and g(x) values')
legend('f(x)','g(x)')

[Tracy]

thanks!!

Answer 2

[VBBV]

%true
  clearvars;clc
x = .993 : 0.01 : 1.283;
f = 7*x.^3 - 2*x.^2 + 7*x - 14;  
g = -19*x.^3 + 3*x.^2 + 16*x + 8; 
plot(x,f,'+r',x,g,'-.b')
xlabel('x values')
ylabel('f(x) and g(x) values')
legend('f(x)','g(x)')

for loop is not necessary in your case.

Answer 3

[Andrei Bobrov]

x = (.993 : 0.01 : 1.283)';
f = polyval([7;-2;7;-14],x);
g = polyval([-19; 3; 16; 8],x);

Answer 4

[Image Analyst]

A thorough discussion is in the FAQ:

https://matlab.fandom.com/wiki/FAQ#.22Subscript_indices_must_either_be_real_positive_integers_or_logicals..22