I am trying to create 3 numpy arrays/lists using data from another array called mean_data as follows:

```
---> 39 R = np.array(mean_data[:,0])
40 P = np.array(mean_data[:,1])
41 Z = np.array(mean_data[:,2])
```

When I try run the program I get the error:

```
TypeError: list indices must be integers, not tuple
```

The mean_data list looks like this sample…

```
[6.0, 315.0, 4.8123788544375692e-06],
[6.5, 0.0, 2.259217450023793e-06],
[6.5, 45.0, 9.2823565008402673e-06],
[6.5, 90.0, 8.309270169336028e-06],
[6.5, 135.0, 6.4709418114245381e-05],
[6.5, 180.0, 1.7227922423558414e-05],
[6.5, 225.0, 1.2308522579848724e-05],
[6.5, 270.0, 2.6905672894824344e-05],
[6.5, 315.0, 2.2727114437176048e-05]]
```

I don’t know how to prevent this error, I have tried creating mean_data as a np.array and using np.append to add values to it but that doesn’t solve the problem either.

Here’s the traceback (was using ipython before)

```
Traceback (most recent call last):
File "polarplot.py", line 36, in <module>
R = np.array(mean_data[:,0])
TypeError: list indices must be integers, not tuple
```

And the other way I tried to create an array was:

```
mean_data = np.array([])
for ur, ua in it.product(uradius, uangle):
samepoints = (data[:,0]==ur) & (data[:,1]==ua)
if samepoints.sum() > 1: # check if there is more than one match
np.append(mean_data[ur, ua, np.mean(data[samepoints,-1])])
elif samepoints.sum() == 1:
np.append(mean_data, [ur, ua, data[samepoints,-1]])
```

The traceback on that is:

```
IndexError Traceback (most recent call last)
<ipython-input-3-5268bc25e75e> in <module>()
31 samepoints = (data[:,0]==ur) & (data[:,1]==ua)
32 if samepoints.sum() > 1: # check if there is more than one match
---> 33 np.append(mean_data[ur, ua, np.mean(data[samepoints,-1])])
34 elif samepoints.sum() == 1:
35 np.append(mean_data, [ur, ua, data[samepoints,-1]])
IndexError: invalid index
```

### 6 Answers

The variable `mean_data`

is a nested list, in Python accessing a nested list cannot be done by multi-dimensional slicing, i.e.: `mean_data[1,2]`

, instead one would write `mean_data[1][2]`

.

This is because`mean_data[2]`

is a list. Further indexing is done recursively – since `mean_data[2]`

is a list, `mean_data[2][0]`

is the first index of that list.

Additionally, `mean_data[:][0]`

does not work because `mean_data[:]`

returns `mean_data`

.

The solution is to replace the array ,or import the original data, as follows:

```
mean_data = np.array(mean_data)
```

numpy arrays (like MATLAB arrays and unlike nested lists) support multi-dimensional slicing with tuples.

You probably do not need to be making lists and appending them to make your array. You can likely just do it all at once, which is faster since you can use numpy to do your loops instead of doing them yourself in pure python.

To answer your question, as others have said, you cannot access a nested list with two indices like you did. You can if you convert `mean_data`

to an array before not after you try to slice it:

```
R = np.array(mean_data)[:,0]
```

instead of

```
R = np.array(mean_data[:,0])
```

But, assuming mean_data has a shape `nx3`

, instead of

```
R = np.array(mean_data)[:,0]
P = np.array(mean_data)[:,1]
Z = np.array(mean_data)[:,2]
```

You can simply do

```
A = np.array(mean_data).mean(axis=0)
```

which averages over the `0`

th axis and returns a length-`n`

array

But to my original point, I will make up some data to try to illustrate how you can do this without building any lists one item at a time:

```
import numpy as np
mean_data = np.array([
[6.0, 315.0, 4.8123788544375692e-06],
[6.5, 0.0, 2.259217450023793e-06],
[6.5, 45.0, 9.2823565008402673e-06],
[6.5, 90.0, 8.309270169336028e-06],
[6.5, 135.0, 6.4709418114245381e-05],
[6.5, 180.0, 1.7227922423558414e-05],
[6.5, 225.0, 1.2308522579848724e-05],
[6.5, 270.0, 2.6905672894824344e-05],
[6.5, 315.0, 2.2727114437176048e-05]])
R = mean_data[:,0]
print R
print R.shape
```

EDIT

The reason why you had an `invalid index`

error is the lack of a comma between `mean_data`

and the values you wanted to add.

Also, np.append returns a copy of the array, and does not change the original array. From the documentation :

Returns : append : ndarray

A copy of arr with values appended to axis. Note that append does not occur in-place: a new array is allocated and filled. If axis is None, out is a flattened array.

So you have to assign the `np.append`

result to an array (could be `mean_data`

itself, I think), **and**, since you don’t want a flattened array, you must also specify the axis on which you want to append.

With that in mind, I think you could try something like

```
mean_data = np.append(mean_data, [[ur, ua, np.mean(data[samepoints,-1])]], axis=0)
```

Do have a look at the doubled `[[`

and `]]`

: I think they are necessary since both arrays must have the same shape.

np.append needs the array as the first argument and the list you want to append as the second:

```
mean_data = np.append(mean_data, [ur, ua, np.mean(data[samepoints,-1])])
```

Just if someone is having this issue and hadn’t done `list[index, sub-index]`

, you could be having the problem because you’re missing a comma between two arrays in an array of arrays (It happened to me).

Seriously this took my much valuable time but not found solution. Basically if you are taking data from sql then you are converting then you can follow these steps.

```
rows = cursor.fetchall()---- only tried on single column
dataset=[]
for x in rows:
dataset.append(float(x[0]))
```