CODEWITHSUNDEEP
pythonlistpython-3.xreplacesequence
R.O.S.S
R.O.S.S

Reputation: 615

Replace sequence in list with a value in Python

I want to replace example two specific one-after-one going elements in a list with another element (elements).
For example - replace ["+", "="] with ["+="]:

Input: 

[3, "blah", "+", "foo", "=", "+", "="]

Output: 

[3, "blah", "+", "foo", "=", "+="]

Upvotes: 4

Views: 2782

Answers (3)

Mutantoe
Mutantoe

Reputation: 697

The following, though very inefficient for long lists this will work:

loop=[3, "blah", "+", "foo", "=", "+", "="]
out=[]
prevdone=False
for i in range(len(loop)):
    if loop[i]=="+" and loop[i+1]=="=":
        out.append("+=")
        prevdone=True
    elif not(prevdone):
        out.append(loop[i])
    else:
        prevdone=False
print(out)

It iterates through the list and checks if the current and following characters meet conditions. If they do, it will add += and skip the next item.

I have considered using "".join(list) and string.split("") but that wouldn't (I don't think) work for multiple-character elements.

As for a general function, it could be modified as such:

def loopReplace(loopList, item1, item2):
    out=[]
    prevdone=False
    for i in range(len(loopList)):
        if loopList[i]==item1 and loopList[i+1]==item2:
            out.append(str(item1)+str(item2))
            prevdone=True
        elif not(prevdone):
            out.append(loopList[i])
        else:
            prevdone=False
    return out

Upvotes: 2

R.O.S.S
R.O.S.S

Reputation: 615

Ok, as I see these answers I think I'll post my solution. I've done a little wrapper function. 

def replace(sequence, replacement, lst, expand=False):
    out = list(lst)
    for i, e in enumerate(lst):
        if e == sequence[0]:
            i1 = i
            f = 1
            for e1, e2 in zip(sequence, lst[i:]):
                if e1 != e2:
                    f = 0
                    break
                i1 += 1
            if f == 1:
                del out[i:i1]
                if expand:
                    for x in list(replacement):
                        out.insert(i, x)
                else:
                    out.insert(i, replacement)
    return out

Upvotes: 1

Monodeep
Monodeep

Reputation: 1392

list = [3, "blah", "+", "foo", "=", "+", "="]

for index, item in enumerate(list):
    if item =='+' and list[index +1]=='=':
        list[index] = "+="
        del list[index + 1]

print(list)

Upvotes: 1

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