Trapezium rule code not work

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Jack Ellis 2019-10-28

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https://ww2.mathworks.cn/matlabcentral/answers/487816-trapezium-rule-code-not-work

编辑： Daniel M 2019-10-28

Hi, guys im new to matlab. Ive written a code for the trapezium rule but it wont work becuase Trapezium_rule is undefined. How do I fix it?

function Trapezium_rule=Trapezium_rule(a,b,m)
% in the line Trapezium_rule = Trapezium_rule + cos(x)*h... cos(x) is the equation so change this
% accoriingy
% m is number of increments
%h is increment size
h=(b-a)/m;
for n=a:h:b
   x = a+(n-0.5)*h;         
   Trapezium_rule = Trapezium_rule + cos(x)*h; 
end
end

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Jack Ellis 2019-10-28

Hi thank you for getting back to me. Ive changed the code to this:

function Trapezium_rule=Trapezium_rule(a,b,m)

% in the line Trapezium_rule = Trapezium_rule + cos(x)*h... cos(x) is the equation so change this

% accoriingy

% m is number of increments

%h is increment size

y=0;

h=(b-a)/m;

for n=a:h:b

x = a+(n-0.5)*h;

y = y + cos(x)*h;

end

Trapezium_rule=y;

end

It worked when I set a=0, b=pi/2 and m=10. Ive tried changing the value of m to 100 but its telling me that Index in position 1 is invalid. Array indices must be positive integers or logical

values. How do I fix this?

Daniel M 2019-10-28

编辑：Daniel M 2019-10-28

在 MATLAB Online 中打开

I can reproduce this error if I call this function in the command window like this:

clear
Trapezium_rule = Trapezium_rule(0,pi/2,100); % this works fine
% call it again
Trapezium_rule = Trapezium_rule(0,pi/2,100);
Error: Index in position 1 is invalid. Array indices must be positive integers or logical values.

So, again, as you've already been told, don't name variables after function names.

You're still doing it within your function too (although in this specific case there is no consequence, but it is still bad practice). Here is an improvement

function output = Trapezium_rule(a,b,m)
% in the line Trapezium_rule = Trapezium_rule + cos(x)*h... cos(x) is the equation so change this
% accoriingy
% m is number of increments
%h is increment size
y = 0;
h = (b-a)/m;
for n = a:h:b
   x = a+(n-0.5)*h;         
   y = y + cos(x)*h; 
end
output = y;
end