Lagrange Interpolation Code Error/Help
显示 更早的评论
Hello! I am to graph an interpolating polynomial and the data points. The data points are from a file which I will provide. I keep running into an error which is "Array indices must be positive integers or logical values." for my variable ys. I would love to use polyval however I am not allowed in this assignment. Any idea how to fix this or what I need to change?? I know that p is not the right thing to do...Thanks in advance!
data file (first line are the x values, second line is the y values, and third line is N which will be the number of points to use to graph the polynomial which is found in xs)
1 2 3 4 5 6 7
1 4 9 16 25 36 49
101
code:
clear;
clf;
%reading from dot_data file
fid = fopen('data_file.m','r');
x = str2num(fgetl(fid)); %reads in first line of data
n = length(x); %length of x
tline = fgets(fid); %reads in the second line of data
y = str2num(tline); %converts to numbers
tline = fgets(fid, 3); %reads in last line of data
N = str2num(tline); %N, also converts to number
fclose(fid); %close file
xx = [x];
yy = [y];
deg = n - 1;
%lagrange
sum=0;
for i=1:length(x)
p=1;
for j=1:length(x)
if j~=i
c = poly(x(j))/(x(i)-x(j));
p = conv(p,c);
end
end
term = p*y(i);
sum= sum + term;
end
%max x value
x_n = max(x);
%plotting the polynomial and data points
xs = linspace(x(1), x_n, N);
ys = p(xs); %this is where I have a mistake when using p
plot(xx,ys, 'r'); %plotting the curve/line
hold on;
plot(x, y, '-o');%plotting the points
采纳的回答
KSSV
2021-3-9
You can evaluate polyval using:
deg = length(p)-1 ; % degree of he polynomial
% get powers of x till deg
X = xs'.^(deg:-1:0) ;
y = sum(p.*X,2) ; % assuming p has coefficients are in decresing powers of n
9 个评论
Manny
2021-3-9
Walter Roberson
2021-3-9
You would replace your
ys = p(xs); %this is where I have a mistake when using p
with
deg = length(p)-1 ; % degree of he polynomial
% get powers of x till deg
X = xs'.^(deg:-1:0) ;
ys = sum(p.*X,2) ; % assuming p has coefficients are in decresing powers of n
Manny
2021-3-9
KSSV
2021-3-9
Show the whole code.....don't include file reading.....give the values of inputs instead.
Manny
2021-3-9
Walter Roberson
2021-3-9
Is your p a row or a column?
sum= sum + term;
What are you doing with that variable named sum ?
p = conv(p,c);
You are overwriting p for each x value, so it is going to end up holding the value it was last assigned, the one for i = length(x). Are you sure that is what you want?
KSSV
2021-3-9
You should not use the variable names as inbuilt functions. Check the variable sum = 0, this is not allowed.
clear all ;
x = [1 2 3 4 5 6 7];
y = [1 4 9 16 25 36 49];
n = length(x); %length of x
deg = n - 1; %deg of polynomial
thesum=0;
for i=1:length(x)
p=1;
for j=1:length(x)
if j~=i
c = poly(x(j))/(x(i)-x(j));
p = conv(p,c);
end
end
term = p*y(i);
thesum= thesum + term;
end
x_n = max(x); %max x value
%plotting the polynomial and data points
N = 100 ;
xs = linspace(x(1), x_n,N);
%deg = n-1 ; % degree of he polynomial
% get powers of x till deg
X = xs'.^(deg:-1:0);
ys = sum(x.*X,2) ; % assuming p has coefficients are in decresing powers of n
plot(xs, ys, 'r');
hold on;
plot(x, y, '-o');
Walter Roberson
2021-3-9
Using sum as a variable name leads to problems more often than not, as it is very common to want to use sum as a function in the same section of code. We advise to never use sum as the name of a variable.
Manny
2021-3-9
That makes so much sense! I didn't realize sum was already a variable name so now understand why there were so many issues.
Now the only issue left that I can see is that my data points are being plotted on a straight line and not on the curve. I notice the x axis with 10^4 on the top.
I apologize for asking so many questions and I appreciate all of the help!!
更多回答（0 个）
类别
在 帮助中心 和 File Exchange 中查找有关 Discrete Data Plots 的更多信息
Translated by 
