Finding and Replacing Elements in a List

Finding and replacing elements in a list

You can use the built-in enumerate to get both index and value while iterating the list. Then, use the value to test for a condition and the index to replace that value in the original list:

>>> a = [1, 2, 3, 4, 5, 1, 2, 3, 4, 5, 1]
>>> for i, n in enumerate(a):
...   if n == 1:
...      a[i] = 10
...
>>> a
[10, 2, 3, 4, 5, 10, 2, 3, 4, 5, 10]

Replace elements in lists based on indexes python

Try this:

rest=[5, 7, 11, 4]
b=[21, 22, 33, 31, 23, 15, 19, 13, 6]
last=[33, 19, 40, 21, 31, 22, 6, 15, 13, 23]

for i, l in enumerate(last):
    if l in b:
        if b.index(l) < len(rest):
            last[i] = rest[b.index(l)]
            
print(last)

Finding an element in nested python list and then replacing it

Using numpy:

NL = np.array(NL)

mask = np.where(NL == 5)
NL[mask] = 10

---

array([[ 1,  2,  3],
       [ 4, 10,  6],
       [ 7,  8,  9]])

---

Solution2:

def get_index(num, List):
    for row, i in enumerate(List):
        if num in i:
            return row, i.index(num)
    return -1

idx = get_index(5,NL)
if idx>0:
    NL[idx[0]][idx[1]] = 7

---

[[1, 2, 3], [4, 7, 6], [7, 8, 9]]

How to replace elements in python list by values from dictionary if list-element = dict-key?

Use dict.get inside a list comprehension:

lst = ["yellow", "red", "green",  "blue"]
dic = {"green":"go", "yellow":"attention", "red":"stay"}
res = [dic.get(e, e) for e in lst]
print(res)

Output

['attention', 'stay', 'go', 'blue']

Having trouble replacing elements in a 2d list with a 1d list using for-loops

Here are some comments on your code and examples of how to make it do what the question describes:

(1.) References vs copies: list1 = [[0]*4]*10 creates a single 4-element list and populates list1 with 10 references to it (NOT 10 copies of it).

For an example of what this implies, watch this:

list1 = [[0]*4]*10
list1[1][0] = 33
list1[1][1] = 39
list1[1][2] = 45
list1[1][3] = 51
print(list1)

... gives this:

[[33, 39, 45, 51], [33, 39, 45, 51], [33, 39, 45, 51], [33, 39, 45, 51], [33, 39, 45, 51], [33, 39, 45, 51], [33, 39, 45, 51], [33, 39, 45, 51], [33, 39, 45, 51], [33, 39, 45, 51]]

In other words, updating one list element within list1 updates them all, since each element of list1 is just a reference to the same list.

If this is not what you want, you can use list1 = [[0]*4 for _ in range(10)] instead to give you a distinct list (10 in total) for each index in list1:

list1 = [[0]*4 for _ in range(10)]
list1[1][0] = 33
list1[1][1] = 39
list1[1][2] = 45
list1[1][3] = 51
print(list1)

... gives:

[[0, 0, 0, 0], [33, 39, 45, 51], [0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0]]

Your code as written would tend to imply that the second approach above is what is needed.

(2.) Code fix assuming nested lists cannot be replaced: It's unclear from the question whether whether you are allowed to replace each list in list1 to get the desired values, or if you are expected to leave the nested lists in place and simply modify their numerical contents.

If list replacement is not allowed, then your code can be rewritten like this:

list1 = [[0]*4 for _ in range(10)]
list2 = [11,13,15,17]
list1[0][:] = list2

for i in range(9):
    for j in range(4):
        list1[i + 1][j] = list2[j]*(3**(i + 1))
print(list1)

... giving:

[[11, 13, 15, 17], [33, 39, 45, 51], [99, 117, 135, 153], [297, 351, 405, 459], [891, 1053, 1215, 1377], [2673, 3159, 3645, 4131], [8019, 9477, 10935, 12393], [24057, 28431, 32805, 37179], [72171, 85293, 98415, 111537], [216513, 255879, 295245, 334611]]

Note that these changes were made to your code:

• Changed the initialization of list1 to allocate 10 distinct nested lists.
• Eliminated i = 1 (not needed because the for loop takes care of initializing i), j = 0 (similar reason), j+=1 (not needed because the for loop takes care of updating j) and i+=1 (similar reason).
• Changed list1[i][j] to list1[i + 1][j] to fix the indexing.
• Changed 3**i to 3**(i + 1) to calculate the correct multiplier.

(3.) Code fix assuming nested lists can be replaced: In this case, your looping logic can be simplified and you don't need to use nested lists when initializing list1.

Here is a long-hand way to do what you ask which will overwrite the contents of list1 with new nested lists that have the desired values:

list1 = [None] * 10
list2 = [11,13,15,17]
list1[0] = list2
mul = 3
for i in range(1, len(list1)):
    temp = [0] * len(list2)
    for j in range(len(list2)):
        temp[j] = mul * list2[j]
    list1[i] = temp
    mul *= 3
print(list1)

Here is a way that uses a list comprehension inside a loop:

list1 = [None] * 10
list2 = [11,13,15,17]
list1[0] = list2
mul = 3
for i in range(1, len(list1)):
    list1[i] = [mul * v for v in list2]
    mul *= 3
print(list1)

And finally, here is a very compact nested list comprehension approach:

list1 = [None] * 10
list2 = [11,13,15,17]
list1 = [[(3 ** i) * v for v in list2] for i in range(len(list1))]

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