Matching values for a list in python

Question

Beginner question: I am trying to evaluate whether two values in a row match.

a = ['sl','sl','cr','cl']

This is my current code:

while (j+1) < len(a):
    if a[j] == a[j+1]:
        num = num + 1
    else: 
        num = num

However, when I do it this way it doesn't count the first value in the list - this is due to (j+1), however, if I remove it, get an error telling me my index is out of range - any advice would be appreciated.

`num = len(a) - len(set(a))`? Note that the `else` can be dropped, and your approach only checks immediate neighbours. — Moses Koledoye, Jun 07 '17 at 10:02
Start from 0, count up to 1 less than the last index. @MosesKoledoye are you missing another len there? :) — Ilja Everilä, Jun 07 '17 at 10:02
What if a value is present 3 times? Do you want to increment the counter once or twice? — cs95, Jun 07 '17 at 10:03
That also cannot be your current code, as you never really increment j. — Ilja Everilä, Jun 07 '17 at 10:05
@MosesKoledoye: Do you mean `num = len(a) - len(set(a))`? Also, OP's question is about "two values in a row", i.e. immediate neighbors. — Tim Pietzcker, Jun 07 '17 at 10:05
@TimPietzcker I assumed `a` is the row which is supposedly part of a larger list of lists (like those from a `csv.reader`). I think OP needs to confirm this — Moses Koledoye, Jun 07 '17 at 10:08
What would the result of `a = ['sl','sl','sl','cr','cl', 'bob']` be? — Jon Clements, Jun 07 '17 at 10:08

score 1 · Accepted Answer · answered Jun 07 '17 at 10:09

1

There are a few things you should improve in your code. First, loop over enumerate(list) to easily compare items in the list. Second, IndexError can be used to quit the loop after the last index is reached. Here an --easy-to-read-- example.

for i,item in enumerate(a):
    try:
        if item = a[i+1]:
            print(item, 'and', a[i+1], 'are the same')
            #increment your counter here
    except IndexError:
        break

answered Jun 07 '17 at 10:09

alec_djinn

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You could always loop over `enumerate(a[:-1])` to avoid the `IndexErrror` in the first place. Or `a[1:]` and use the previous value instead of the next... (also you might want to use `==` in the `if`) – Baldrickk Jun 07 '17 at 10:17
@Baldrickk Indeed, that is a good suggestion for a beginner. I prefer my way, I find it more explicit, but thanks! – alec_djinn Jun 07 '17 at 10:20

Matching values for a list in python

1 Answers1