Locate the position of number
Contents:I tried this but with no avail. Sign in to comment.
- Throws if element not present in list.
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This gives the position of total number of data points which are greater than Not the position within each row. If you absolutely must do it separately for each row for reasons you do not explain here , then you could do.
Locate the position (index) of an element in an array
To put it simpler. What I expect is for each row is something like this. Sign in to answer this question. Unable to complete the action because of changes made to the page.
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Linear time-complexity in list length
Asked by sisay sisay view profile. Tags find for loop data. If the value isn't there, catching the ValueError is rather verbose - and I prefer to avoid that. I'm usually iterating over the list anyways, so I'll usually keep a pointer to any interesting information, getting the index with enumerate.
If you're munging data, you should probably be using pandas - which has far more elegant tools than the pure Python workarounds I've shown. I do not recall needing list. However, I have looked through the Python standard library, and I see some excellent uses for it. The keyword module uses it to find comment markers in the module to automatically regenerate the list of keywords in it via metaprogramming. What these usages seem to have in common is that they seem to operate on lists of constrained sizes important because of O n lookup time for list.
While there are use-cases for it, they are fairly uncommon.
Get address of lookup result
If you find yourself looking for this answer, ask yourself if what you're doing is the most direct usage of the tools provided by the language for your use-case. All indexes with the zip function:. A variant on the answer from FMc and user will give a dict that can return all indices for any entry:.
You could also use this as a one liner to get all indices for a single entry. There are no guarantees for efficiency, though I did use set a to reduce the number of times the lambda is called. With enumerate alist you can store the first element n that is the index of the list when the element x is equal to what you look for. This function takes the item and the list as arguments and return the position of the item in the list, like we saw before. The nice thing about this approach is the function always returns a list of indices -- even if it is an empty list.
It works with strings as well. After another year of heads-down python development, I'm a bit embarrassed by my original answer, so to set the record straight, one can certainly use the above code; however, the much more idiomatic way to get the same behavior would be to use list comprehension, along with the enumerate function. And now, after reviewing this question and all the answers, I realize that this is exactly what FMc suggested in his earlier answer.
At the time I originally answered this question, I didn't even see that answer, because I didn't understand it. I hope that my somewhat more verbose example will aid understanding. If the single line of code above still doesn't make sense to you, I highly recommend you Google 'python list comprehension' and take a few minutes to familiarize yourself. It's just one of the many powerful features that make it a joy to use Python to develop code. This solution is not as powerful as others, but if you're a beginner and only know about for loops it's still possible to find the first index of an item while avoiding the ValueError:.
Here we are iterating using i to get the indexes, but if we need rather to focus on the items we can switch to j.
Locate the position (index) of an element in an array
So instead you can make it similar to the indexOf function of JavaScript which returns -1 if the item was not found:. For those coming from another language like me, maybe with a simple loop it's easier to understand and use it:.
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- Finding the index of an item given a list containing it in Python - Stack Overflow!
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I am thankful for So what exactly does enumerate do? That helped me to understand. Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site the association bonus does not count. Would you like to answer one of these unanswered questions instead? Finding the index of an item given a list containing it in Python Ask Question.
It is probably worth initially taking a look at the docstring for it: Raises ValueError if the value is not present. Linear time-complexity in list length An index call checks every element of the list in order, until it finds a match. Throws if element not present in list A call to index results in a ValueError if the item's not present.
Alex Taylor 4, 2 11 If "bar" exists twice at list, you'll never find the key for the second "bar". How to do this for numpy array? It's stipulated in the question that the item is in the list. One thing that is really helpful in learning Python is to use the interactive help function: As a list comprehension: Enumeration works better than the index-based methods for me, since I'm looking to gather the indices of strings using 'startswith" , and I need to gather multiple occurrences.
In my hands, the enumerate version is consistently slightly faster. Some implementation details may have changed since the measurement above was posted. AlexCoventry Highly depends on computer specifications, but almost five years on wow time flies , I'd tend to agree with you. To get all indexes: HongboZhu 2, 2 19 And if doesn't exist in the list? A problem will arise if the element is not in the list. This function handles the issue: Mar 17 '17 at KeyWeeUsr 5, 8 18 Graham Giller 3 2. The 3rd method iterates twice over the list, right? At the time of writing perhaps, but you ought to check newer answers to see if it is still true.
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