The biggest problem with your code is that it's unreadable. Python code rule number one, if it's not readable, no one's gonna look at it for long enough to get any useful information out of it. Always use descriptive variable names. Almost didn't catch the bug in your code, let's see it again with good names, slow-motion replay style:

to_modify = [5,4,3,2,1,0]
indexes = [0,1,3,5]
replacements = [0,0,0,0]

for index in indexes:
    to_modify[indexes[index]] = replacements[index]
    # to_modify[indexes[index]]
    # indexes[index]
    # Yo dawg, I heard you liked indexes, so I put an index inside your indexes
    # so you can go out of bounds while you go out of bounds.

As is obvious when you use descriptive variable names, you're indexing the list of indexes with values from itself, which doesn't make sense in this case.

Also when iterating through 2 lists in parallel I like to use the zip function (or izip if you're worried about memory consumption, but I'm not one of those iteration purists). So try this instead.

for (index, replacement) in zip(indexes, replacements):
    to_modify[index] = replacement

If your problem is only working with lists of numbers then I'd say that @steabert has the answer you were looking for with that numpy stuff. However you can't use sequences or other variable-sized data types as elements of numpy arrays, so if your variable to_modify has anything like that in it, you're probably best off doing it with a for loop.

take is your friend.

a = np.array([[0, 1, 2], [2, 1, 3], [0, 1, 3]])
chg = np.array([3, 0, 2, 1])
inverse_chg=chg.take(chg)
print(inverse_chg.take(a))

gives :

[[1 3 2]
 [2 3 0]
 [1 3 0]]

or more directly with fancy indexing: chg[chg][a], but inverse_chg.take(a) is three times faster.

You can just index your value array with the indices array as they are integers (required for indexing):

vals[ind]

which produces:

array([0.2, 0.2, 0.3, 0.2])

as desired.

If the minimal/maximal elements of a, b form a small range (or at least small enough to fit into RAM), this can be done very quickly using a lookup table:

a = np.array([[51, 30, 20, 10],
              [10, 32, 65, 77],
              [15, 20, 77, 30]])
b = np.array([10, 15, 20, 30, 32, 51, 65, 77])

lo = min(a.min(), b.min())
hi = max(a.max(), b.max())
lut = np.zeros(hi - lo + 1, dtype=np.int64)
lut[b - lo] = np.arange(len(b))

Then:

>>> a_indices = lut[a - lo]
>>> a_indices
array([[5, 3, 2, 0],
       [0, 4, 6, 7],
       [1, 2, 7, 3]])

The list of indices indicating which elements you want to replace should be a Python list (or similar type), not a tuple. Different items in the selection tuple indicate that they should be selected from different axis dimensions.

Therefore, a[(2, 4, 6)] is the same as a[2, 4, 6], which is interpreted as the value at index 2 in the first dimension, index 4 in the second dimension, and index 6 in the third dimension.

The following code works correctly:

indexes = [2, 4, 6]
a[indexes] = newvalues

See also the page on Indexing from the numpy documentation, specifically the second 'Note' block in the introduction as well as the first 'Warning' under Advanced Indexing:

In Python, x[(exp1, exp2, ..., expN)] is equivalent to x[exp1, exp2, ..., expN]; the latter is just syntactic sugar for the former.

The definition of advanced indexing means that x[(1,2,3),] is fundamentally different than x[(1,2,3)]. The latter is equivalent to x[1,2,3] which will trigger basic selection while the former will trigger advanced indexing. Be sure to understand why this occurs.

You should use array[idx] = new_values. This approach is much faster than native python loops. But you will have to convert 'idx' and 'new_values' to numpy arrays as well.

import numpy as np

n = 100000
array = np.random.random(n)
idx = np.random.randint(0, n, n//10)
new_values = np.random.random(n//10)

%time array[idx] = new_values

Wall time: 257 µs

def f():
    for i, v in zip(idx, new_values):
        array[i] = v
%time f()

Wall time: 5.93 ms

Use loc:

df.loc[0:15,'A'] = 16
print (df)
     A   B
0   16  45
1   16   5
2   16  97
3   16  58
4   16  26
5   16  87
6   16  51
7   16  17
8   16  39
9   16  73
10  16  94
11  16  69
12  16  57
13  16  24
14  16  43
15  16  77
16  41   0
17   3  21
18   0  98
19  45  39
20  66  62
21   8  53
22  69  47
23  48  53

Solution with ix is deprecated.

Well, I suppose what you need is

a[a==2] = 10 #replace all 2's with 10's

you can slice your list accordingly!

• That is, with a and b being your initial list and the one that you want to replace from index s, a[:s] will get all elements before from 0 to s that is ([1,2]).

• a[s+len(b):] will get all items from index s to len(b), that is ([6,7,8])

so when you concatenate the first result along with b and then the second result you can get the desired output!

a[:s]+b+a[s+len(b):]

So,

>>> a = [1,2,3,4,5,6,7,8]
>>> b = ['a','b','c']
>>> replace=lambda a,b,s:a[:s]+b+a[s+len(b):]
>>> c = replace(a,b,2)
>>> print c
[1, 2, 'a', 'b', 'c', 6, 7, 8]

Hope this helps!