Community of stackoverflow:

I have a list "rest" which is the following:

rest=[5, 7, 11, 4]

I have another list which is b:

b=[21, 22, 33, 31, 23, 15, 19, 13, 6]

And I have a "last" list:

last=[33, 19, 40, 21, 31, 22, 6, 15, 13, 23]

I have to replace the first 4 elements in b with the elements in rest. How can I replace the elements in last according to the matches with b to get the rest elements?

for example:

     5   7   11   4   #elements from rest
b= [21, 22, 33, 31, 23, 15, 19, 13, 6]

to get last list as the following:

last=[11, 19, 40, 5, 4, 7, 6, 15, 13, 23]  #elements that matched with b were replaced by rest

How can I do this?

CodePudding user response：

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)

CodePudding user response：

You can try to do something like this...

rest_change_index = 0
for i in range(len(b)):
   if b[i] in last:
       last_change_index = last.index(b[i])
       last[last_change_index] = rest[rest_change_index]
       rest_change_index  = 1
print(last)

This iterates through the elements of b, and if an element in last matches the element of b being iterated through in the loop, then it changes that value with the corresponding element of rest (first element of rest for first matching instance, etc.). Let me know if this makes sense.

CodePudding user response：

You can do this as follows:

# Get first 4 items in b
b4 = b[:4]
# Create mapping from b to rest
b_to_rest = dict(zip(b4, rest))
# Use dictionary .get to either replace if in rest or keep if not
last = [b_to_rest.get(x,x) for x in last]

Firstly, I've defined b4 as the first 4 items in b. Then I've used zip and dict to map b values to rest values. Finally I use a list comprehension to replace the items in last. .get(x,x) means try to get x from the dictionary, if it doesn't exist just use x.