# *** Write a function called triangle_wave that computes the sum (−1) sin(2 +1) (2 +1) for each of 1001 values of t uniformly spaced from 0 to 4π inclusive. The input argument is a scalar non-negative integer n, and the output argument is a row v

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Wasi von Deutschland on 20 May 2017
Commented: Vikrant dhall on 29 Apr 2018
this is my code and I need your help I couldn't figure out where I'm making mistake.
function theSum = triangle_wave(n)
t = linspace(0, 4*pi, 1001);
for tIndex = 1 : length(t)
this_t = t(tIndex);
k = 1 : n;
numerator = ((-1)^k).*sin(this_t * (2*k+1));
denominator = (2 * k + 1)^2;
theSum(tIndex) = sum(numerator ./ denominator);
end
Wasi von Deutschland on 21 May 2017

@ stephen I'm really sorry. Next I will keep this in mind.

Jan on 21 May 2017
@Waseem Iqbal: It is okay. This happens frequently and we all remind newcomers to improve the format. Now let's come back to the question for clarification:
You forgot to mention, why you assume that there is a mistake. Please explain this.

Jan on 21 May 2017
Edited: Jan on 21 May 2017
denominator = (2 * k + 1) .^ 2;
Use the elemtnwise .^ instead of the matrix operation ^ . The same for "(-1)^k" .
The corresponding error message is clear:
Error using ^
Inputs must be a scalar and a square matrix.
To compute elementwise POWER, use POWER (.^) instead.
[EDITED] The formula contains: "Sum from k=0 to n", you code contains:
k = 1:n;
Replace this by k = 0:n. It will be more efficient to move the definition before the loop and pre-allocate the output:
function S = triangle_wave(n)
t = linspace(0, 4*pi, 1001);
S = zeros(size(t)); % Pre-allocation
k = 0:n; % Not 1:n
for tIndex = 1 : length(t)
numerator = ((-1) .^ k) .* sin(t(tIndex) .* (2 * k + 1));
denominator = (2 .* k + 1) .^ 2;
S(tIndex) = sum(numerator ./ denominator);
end
Jan on 21 May 2017
@Waseem: Look at the formula: Look at the limits of the sum, it goes from k = 0 to n. Do you see it under and above the Sigma? With the zeros() command I've pre-allocated the output. Search in this forum for "pre-allocation".
rishabh gupta on 10 Feb 2018
can you please tell how is the value of k changing when it is placed outside the loop??

Walter Roberson on 21 May 2017
(-1)^k needs to be (-1).^k
(2 * k + 1)^2 needs to be (2 * k + 1).^2

Vikrant dhall on 29 Apr 2018
function v = triangle_wave(n) t = linspace(0,4 * pi, 1001); v = zeros(size(t)); p = 1; for tt = 0:length(t) suma = 0; for k = 0:n suma = suma + (((-1 .^ k) .* sin((2.*k+1).*tt)) / ((2.* k+1).^2)); %suma end %suma v(1,p) = suma; p = p + 1; end end
Vikrant dhall on 29 Apr 2018
function v = triangle_wave(n)
if true
% code
end
t = linspace(0,4 * pi, 1001);
v = zeros(size(t));
p = 1;
for tt = 0:length(t)
suma = 0;
for k = 0:n
x = ((-1 .^ k) .* sin((2.*k+1).*tt)) / ((2.* k+1).^2);
suma = suma + x;
%suma
end
%suma
v(1,p) = suma;
p = p + 1;
end
end