Array indices must be positive integers or logical values

Hey guys,

I'm trying to complete a homeowrk problem and I keep running into the error "Array indices must be positive integers or logical values." I don't know what I am doing wrong since my professor told us how to do most of the problem. Any help would be greatly appreciated. Thank you!

clear;clc

x = [0.888: 0.01: 1.178]; %x values
for n = 1: 1: length(x)
    %must have different vector because variable index must be an integer
    f(x) = -5*x(n)^3+17*x(n)^2+8*x(n)+9; %first function
    g(x) = -13*x(n)^3+16*x(n)^2+9*x(n)+18; %secon function
    if f(x) == g(x)   
        %if the two functions outputs equal each other then they intersect
        disp(x)
        %displays the x value of the intersection
    end
end

figure(1)
plot(x, f(x), x, g(x))
%creates a plot with two lines on the y-axis
title('WA06B by Tiffany Tabor')
xlabel('x values')
ylabel('f(x) and g(x)')

Answer 1

[Alan Stevens]

The following works

x = [0.888: 0.01: 1.178]; %x values
for n = 1: 1: length(x)
    %must have different vector because variable index must be an integer
    f(n) = -5*x(n)^3+17*x(n)^2+8*x(n)+9; %first function  %%%%%%%%% f(n) not f(x)
    g(n) = -13*x(n)^3+16*x(n)^2+9*x(n)+18; %secon function  %%%%%%%%%%%  g(n) not g(x)
    if f(n) == g(n)   
        %if the two functions outputs equal each other then they intersect
        disp(x)
        %displays the x value of the intersection
    end
end

figure(1)
plot(x, f, x, g)  %%%%%%%%%%%  Just f and g
%creates a plot with two lines on the y-axis
title('WA06B by Tiffany Tabor')
xlabel('x values')
ylabel('f(x) and g(x)')

Answer 2

[Star Strider]

See the documentation sections on Anonymous Functions and also Array vs. Matrix Operations.

You will need to understand those in order to complete this.

Answer 3

[Image Analyst]

This is one of the most FAQed of the FAQs, so see the FAQ for a thorough discussion.