Index out of bounds error : Python

Question

I have a set of data that is not smooth. Hence I'm trying to interpolate it with the generated mesh.

import numpy as np
data=np.loadtxt('test.txt')
x=data[:,0] #len(x) = 730
y=data[:,1]

Nx= len(x)
Ny=len(y)
del_x= 0.5
xn = np.linspace(0,Nx,2000)
yn = np.linspace(0,Ny,2000)

#loop 
for i in range(0,Nx-1):
 if x[i] > xn[i] and x[i] < xn[i+1]:
  new_x= (i + (xn[i] - x[i])/(xn[i]-xn[i+1]))*del_x
print new_x

I would like to perform the loop that basically does the operation i.e : if my original data x[i] is in between the two grid points xn[i],xn[i+1], then compute new_x.

But I get the following error

Traceback (most recent call last):
 File "new.py", line 27, in <module>
  if x[i] > xn[i] and x[i] < xn[i+1]:
IndexError: index out of bound

Can someone help me out ?

please create a [Minimal, Complete, and Verifiable example](https://stackoverflow.com/help/mcve) — U13-Forward, May 29 '18 at 07:33
You seem to be aware that python ranges are in the form `[0, N[`. What is `len(x)`? From your definition of `xn` I am guessing that the source of the indexing error is that the list `x` is too short. — Sigve Karolius, May 29 '18 at 07:42
what's the size of the array 'data'? Maybe the array ‘’x‘’ don't have 1000 elements. — Hejun, May 29 '18 at 07:48
Ok, so your `for` loop throws an error when it is trying to access `x[730]` (remember that the first item in a Python list is regarded as "element 0"). — Sigve Karolius, May 29 '18 at 07:49
@newstudent Your edited code does not raise any errors --with my data, I do not have yours. — Stop harming Monica, May 29 '18 at 08:21
@Goyo Heres my data https://www.dropbox.com/s/aiuikxlosf6jqi3/profile_nonoisebigd02.txt?dl=0 — newstudent, May 29 '18 at 08:25
@newstudent In a recent comment you describe a different problem. So what is your actual problem? — Stop harming Monica, May 29 '18 at 08:30

score 0 · Answer 1 · edited May 29 '18 at 09:01

0

If len(x) = 730 than its too short for the loop. Your loop iterates 1000 times, i would be at some point bigger than 730 and there are no values like x[730],x[731] etc. You will get an out of bound error because you are accessing not existing values in an array.

edited May 29 '18 at 09:01

Anh Pham

695
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answered May 29 '18 at 07:50

Superluminal

947
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Thanks, So I need to make Nx, Ny = 730? – newstudent May 29 '18 at 07:52
Better do Nx=Len(x), Ny=len(y). Than its dynamic and you will be not supposed to change something if the datafile changes – Superluminal May 29 '18 at 07:53
I did that, but the error persists, Is there any error in the if loop ? – newstudent May 29 '18 at 07:57

score 0 · Answer 2 · answered May 29 '18 at 07:54

0

You can change your for loop like this:

for i in range(0, len(x)):

answered May 29 '18 at 07:54

Hejun

366
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Sigve Karolius · Answer 3 · 2018-05-29T08:13:06.887

First some basics

Python is "zero indexed", meaning that the first element in the list is indexed as 0.
Ranges are given as: [start, stop[, meaning that the last element is not included.

Just see what happens if you try:

a = [1, 2, 3]
print("a has %d elements:" %(len(a)))
for i in range(len(a)):
  print("a[%d] = %d" %(i, a[i]))

The output is:

a has 3 elements:
a[0] = 1
a[1] = 2
a[2] = 3

If you try to access a[3] you will see:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: list index out of range

Now to your problems

Numpy "linspace" takes the following input: numpy.linspace(start, stop, N), resulting in a closed range [start, stop] (notice it includes the "stop") with N elements.

You want the number of elements, or len(xn) to be 730, and I am guessing you want the "stop" value to be max(x):

xn = numpy.linspace(0, max(x), len(x))

for i in range(len(x)):
  ...

Tip: If you are uncertain about the signature of a function you can always ask for help, try:help(numpy.linspace)

Thanks for the detailed ans, but looks like the loop only calculates for a single i , rather than for all in range. — newstudent, May 29 '18 at 08:22
So no `IndexError` at least... Now your problem that the "if statement" is only `true` **once**. — Sigve Karolius, May 29 '18 at 08:28

score 0 · Accepted Answer · answered May 29 '18 at 08:12

numpy linspace official documentation

xn = np.linspace(0,Nx,2000)

in here 0 is start value, Nx is end value and 2000 is the number of values to be generated.

returned object of linspace is something like :

[0, 0.5, 1, ... , ] #2000 elements i.e. a row containing 2000 elements

x=data[:,0] x is something like :

[[1],[2],[3],[4], .... ,[]] #column vector with number of elements in text data

now row size of x can not be greater than 1 ie index has to be less than 1 or equal to 0

and you can't compare these two arrays

[ ... ] and [[], [], [], [], ... ]

are incomparable (you can compare the first element though.)

you could use this : reshape

refer to this answer : What does -1 mean in numpy reshape?

row_x = x.reshape(-1)

will give you a row vector.

and you can convert to column format by:

col_x = row_x.reshape(-1,1)

I would appreciate edits as I am currently at work and thus wrote the answer in hurry.

Index out of bounds error : Python

4 Answers4

First some basics

Now to your problems