Как найти объект в массиве python

What is a good way to find the index of an element in a list in Python?
Note that the list may not be sorted.

Is there a way to specify what comparison operator to use?

Benyamin Jafari's user avatar

asked Mar 3, 2009 at 1:45

Himadri Choudhury's user avatar

2

From Dive Into Python:

>>> li
['a', 'b', 'new', 'mpilgrim', 'z', 'example', 'new', 'two', 'elements']
>>> li.index("example")
5

stivlo's user avatar

stivlo

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answered Mar 3, 2009 at 1:52

Matt Howell's user avatar

3

If you just want to find out if an element is contained in the list or not:

>>> li
['a', 'b', 'new', 'mpilgrim', 'z', 'example', 'new', 'two', 'elements']
>>> 'example' in li
True
>>> 'damn' in li
False

answered Feb 17, 2011 at 10:00

Eduardo's user avatar

EduardoEduardo

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0

The best way is probably to use the list method .index.

For the objects in the list, you can do something like:

def __eq__(self, other):
    return self.Value == other.Value

with any special processing you need.

You can also use a for/in statement with enumerate(arr)

Example of finding the index of an item that has value > 100.

for index, item in enumerate(arr):
    if item > 100:
        return index, item

Source

tedder42's user avatar

tedder42

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answered Mar 3, 2009 at 1:51

Brian R. Bondy's user avatar

Brian R. BondyBrian R. Bondy

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Here is another way using list comprehension (some people might find it debatable). It is very approachable for simple tests, e.g. comparisons on object attributes (which I need a lot):

el = [x for x in mylist if x.attr == "foo"][0]

Of course this assumes the existence (and, actually, uniqueness) of a suitable element in the list.

answered Sep 24, 2010 at 9:35

ThomasH's user avatar

ThomasHThomasH

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3

assuming you want to find a value in a numpy array,
I guess something like this might work:

Numpy.where(arr=="value")[0]

Jorgesys's user avatar

Jorgesys

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answered Jan 27, 2011 at 15:03

Sebastian Gurovich's user avatar

2

There is the index method, i = array.index(value), but I don’t think you can specify a custom comparison operator. It wouldn’t be hard to write your own function to do so, though:

def custom_index(array, compare_function):
    for i, v in enumerate(array):
        if compare_function(v):
            return i

answered Mar 3, 2009 at 1:50

David Z's user avatar

David ZDavid Z

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I use function for returning index for the matching element (Python 2.6):

def index(l, f):
     return next((i for i in xrange(len(l)) if f(l[i])), None)

Then use it via lambda function for retrieving needed element by any required equation e.g. by using element name.

element = mylist[index(mylist, lambda item: item["name"] == "my name")]

If i need to use it in several places in my code i just define specific find function e.g. for finding element by name:

def find_name(l, name):
     return l[index(l, lambda item: item["name"] == name)]

And then it is quite easy and readable:

element = find_name(mylist,"my name")

answered Oct 20, 2011 at 12:30

jki's user avatar

jkijki

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0

The index method of a list will do this for you. If you want to guarantee order, sort the list first using sorted(). Sorted accepts a cmp or key parameter to dictate how the sorting will happen:

a = [5, 4, 3]
print sorted(a).index(5)

Or:

a = ['one', 'aardvark', 'a']
print sorted(a, key=len).index('a')

answered Mar 3, 2009 at 1:52

Jarret Hardie's user avatar

Jarret HardieJarret Hardie

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how’s this one?

def global_index(lst, test):
    return ( pair[0] for pair in zip(range(len(lst)), lst) if test(pair[1]) )

Usage:

>>> global_index([1, 2, 3, 4, 5, 6], lambda x: x>3)
<generator object <genexpr> at ...>
>>> list(_)
[3, 4, 5]

answered Mar 3, 2009 at 2:06

SingleNegationElimination's user avatar

2

I found this by adapting some tutos. Thanks to google, and to all of you 😉

def findall(L, test):
    i=0
    indices = []
    while(True):
        try:
            # next value in list passing the test
            nextvalue = filter(test, L[i:])[0]

            # add index of this value in the index list,
            # by searching the value in L[i:] 
            indices.append(L.index(nextvalue, i))

            # iterate i, that is the next index from where to search
            i=indices[-1]+1
        #when there is no further "good value", filter returns [],
        # hence there is an out of range exeption
        except IndexError:
            return indices

A very simple use:

a = [0,0,2,1]
ind = findall(a, lambda x:x>0))

[2, 3]

P.S. scuse my english

answered Oct 16, 2011 at 11:41

Gael's user avatar

На чтение 4 мин Просмотров 4.7к. Опубликовано 03.03.2023

Содержание

  1. Введение
  2. Поиск методом count
  3. Поиск при помощи цикла for
  4. Поиск с использованием оператора in
  5. В одну строку
  6. Поиск с помощью лямбда функции
  7. Поиск с помощью функции any()
  8. Заключение

Введение

В ходе статьи рассмотрим 5 способов поиска элемента в списке Python.

Поиск методом count

Метод count() возвращает вхождение указанного элемента в последовательность. Создадим список разных цветов, чтобы в нём производить поиск:

colors = ['black', 'yellow', 'grey', 'brown']

Зададим условие, что если в списке colors присутствует элемент ‘yellow’, то в консоль будет выведено сообщение, что элемент присутствует. Если же условие не сработало, то сработает else, и будет выведена надпись, что элемента отсутствует в списке:

colors = ['black', 'yellow', 'grey', 'brown']

if colors.count('yellow'):
    print('Элемент присутствует в списке!')
else:
    print('Элемент отсутствует в списке!')

# Вывод: Элемент присутствует в списке!

Поиск при помощи цикла for

Создадим цикл, в котором будем перебирать элементы из списка colors. Внутри цикла зададим условие, что если во время итерации color приняла значение ‘yellow’, то элемент присутствует:

colors = ['black', 'yellow', 'grey', 'brown']

for color in colors:
    if color == 'yellow':
         print('Элемент присутствует в списке!')

# Вывод: Элемент присутствует в списке!

Поиск с использованием оператора in

Оператор in предназначен для проверки наличия элемента в последовательности, и возвращает либо True, либо False.

Зададим условие, в котором если ‘yellow’ присутствует в списке, то выводится соответствующее сообщение:

colors = ['black', 'yellow', 'grey', 'brown']

if 'yellow' in colors:
    print('Элемент присутствует в списке!')
else:
    print('Элемент отсутствует в списке!')

# Вывод: Элемент присутствует в списке!

В одну строку

Также можно найти элемент в списке при помощи оператора in всего в одну строку:

colors = ['black', 'yellow', 'grey', 'brown']

print('Элемент присутствует в списке!') if 'yellow' in colors else print('Элемент отсутствует в списке!')

# Вывод: Элемент присутствует в списке!

Или можно ещё вот так:

colors = ['black', 'yellow', 'grey', 'brown']

if 'yellow' in colors: print('Элемент присутствует в списке!')

# Вывод: Элемент присутствует в списке!

Поиск с помощью лямбда функции

В переменную filtering будет сохранён итоговый результат. Обернём результат в список (list()), т.к. метода filter() возвращает объект filter. Отфильтруем все элементы списка, и оставим только искомый, если он конечно присутствует:

colors = ['black', 'yellow', 'grey', 'brown']

filtering = list(filter(lambda x: 'yellow' in x, colors))

Итак, если искомый элемент находился в списке, то он сохранился в переменную filtering. Создадим условие, что если переменная filtering не пустая, то выведем сообщение о присутствии элемента в списке. Иначе – отсутствии:

colors = ['black', 'yellow', 'grey', 'brown']

filtering = list(filter(lambda x: 'yellow' in x, colors))

if filtering:
    print('Элемент присутствует в списке!')
else:
    print('Элемент отсутствует в списке!')

# Вывод: Элемент присутствует в списке!

Поиск с помощью функции any()

Функция any принимает в качестве аргумента итерабельный объект, и возвращает True, если хотя бы один элемент равен True, иначе будет возвращено False.

Создадим условие, что если функция any() вернёт True, то элемент присутствует:

colors = ['black', 'yellow', 'grey', 'brown']

if any(color in 'yellow' for color in colors):
    print('Элемент присутствует в списке!')
else:
    print('Элемент отсутствует в списке!')

# Вывод: Элемент присутствует в списке!

Внутри функции any() при помощи цикла производится проверка присутствия элемента в списке.

Заключение

В ходе статьи мы с Вами разобрали целых 5 способов поиска элемента в списке Python. Надеюсь Вам понравилась статья, желаю удачи и успехов! 🙂

Python Find in List – How to Find the Index of an Item or Element in a List

In this article you will learn how to find the index of an element contained in a list in the Python programming language.

There are a few ways to achieve this, and in this article you will learn three of the different techniques used to find the index of a list element in Python.

The three techniques used are:

  • finding the index using the index() list method,
  • using a for-loop,
  • and finally, using list comprehension and the enumerate() function.

Specifically, here is what we will cover in depth:

  1. An overview of lists in Python
    1. How indexing works
  2. Use the index() method to find the index of an item
    1.Use optional parameters with the index() method
  3. Get the indices of all occurrences of an item in a list
    1. Use a for-loop to get indices of all occurrences of an item in a list
    2. Use list comprehension and the enumerate() function to get indices of all occurrences of an item in a list

What are Lists in Python?

Lists are a built-in data type in Python, and one of the most powerful data structures.

They act as containers and store multiple, typically related, items under the same variable name.

Items are placed and enclosed inside square brackets, []. Each item inside the square brackets is separated by a comma, ,.

# a list called 'my_information' that contains strings and numbers
my_information = ["John Doe", 34, "London", 1.76]

From the example above, you can see that lists can contain items that are of any data type, meaning list elements can be heterogeneous.

Unlike arrays that only store items that are of the same type, lists allow for more flexibility.

Lists are also mutable, which means they are changeable and dynamic. List items can be updated, new items can be added to the list, and any item can be removed at any time throughout the life of the program.

An Overview of Indexing in Python

As mentioned, lists are a collection of items. Specifically, they are an ordered collection of items and they preserve that set and defined order for the most part.

Each element inside a list will have a unique position that identifies it.

That position is called the element’s index.

Indices in Python, and in all programming languages, start at 0 and not 1.

Let’s take a look at the list that was used in the previous section:

my_information = ["John Doe", 34, "London", 1.76]

The list is zero-indexed and counting starts at 0.

The first list element, "John Doe", has an index of 0.
The second list element, 34, has an index of 1.
The third list element, "London", has an index of 2.
The forth list element, 1.76, has an index of 3.

Indices come in useful for accessing specific list items whose position (index) you know.

So, you can grab any list element you want by using its index.

To access an item, first include the name of the list and then in square brackets include the integer that corresponds to the index for the item you want to access.

Here is how you would access each item using its index:

my_information = ["John Doe", 34, "London", 1.76]

print(my_information[0])
print(my_information[1])
print(my_information[2])
print(my_information[3])

#output

#John Doe
#34
#London
#1.76

But what about finding the index of a list item in Python?

In the sections that follow you will see some of the ways you can find the index of list elements.

So far you’ve seen how to access a value by referencing its index number.

What happens though when you don’t know the index number and you’re working with a large list?

You can give a value and find its index and in that way check the position it has within the list.

For that, Python’s built-in index() method is used as a search tool.

The syntax of the index() method looks like this:

my_list.index(item, start, end)

Let’s break it down:

  • my_list is the name of the list you are searching through.
  • .index() is the search method which takes three parameters. One parameter is required and the other two are optional.
  • item is the required parameter. It’s the element whose index you are searching for.
  • start is the first optional parameter. It’s the index where you will start your search from.
  • end the second optional parameter. It’s the index where you will end your search.

Let’s see an example using only the required parameter:

programming_languages = ["JavaScript","Python","Java","C++"]

print(programming_languages.index("Python"))

#output
#1

In the example above, the index() method only takes one argument which is the element whose index you are looking for.

Keep in mind that the argument you pass is case-sensitive. This means that if you had passed “python”, and not “Python”, you would have received an error as “python” with a lowercase “p” is not part of the list.

The return value is an integer, which is the index number of the list item that was passed as an argument to the method.

Let’s look at another example:

programming_languages = ["JavaScript","Python","Java","C++"]

print(programming_languages.index("React"))

#output
#line 3, in <module>
#    print(programming_languages.index("React"))
#ValueError: 'React' is not in list

If you try and search for an item but there is no match in the list you’re searching through, Python will throw an error as the return value – specifically it will return a ValueError.

This means that the item you’re searching for doesn’t exist in the list.

A way to prevent this from happening, is to wrap the call to the index() method in a try/except block.

If the value does not exist, there will be a message to the console saying it is not stored in the list and therefore doesn’t exist.

programming_languages = ["JavaScript","Python","Java","Python","C++","Python"]

try:
    print(programming_languages.index("React"))
except ValueError:
    print("That item does not exist")
    
#output
#That item does not exist

Another way would be to check to see if the item is inside the list in the first place, before looking for its index number. The output will be a Boolean value – it will be either True or False.

programming_languages = ["JavaScript","Python","Java","Python","C++","Python"]

print("React" in programming_languages)

#output
#False

How to Use the Optional Parameters with the index() Method

Let’s take a look at the following example:

programming_languages = ["JavaScript","Python","Java","Python","C++","Python"]

print(programming_languages.index("Python"))

#output
#1

In the list programming_languages there are three instances of the “Python” string that is being searched.

As a way to test, you could work backwards as in this case the list is small.

You could count and figure out their index numbers and then reference them like you’ve seen in previous sections:

programming_languages = ["JavaScript","Python","Java","Python","C++","Python"]

print(programming_languages[1])
print(programming_languages[3])
print(programming_languages[5])

#output
#Python
#Python
#Python

There is one at position 1, another one at position 3 and the last one is at position 5.

Why aren’t they showing in the output when the index() method is used?

When the index() method is used, the return value is only the first occurence of the item in the list. The rest of the occurrences are not returned.

The index() method returns only the index of the position where the item appears the first time.

You could try passing the optional start and end parameters to the index() method.

You already know that the first occurence starts at index 1, so that could be the value of the start parameter.

For the end parameter you could first find the length of the list.

To find the length, use the len() function:

print(len(programming_languages)) 

#output is 6

The value for end parameter would then be the length of the list minus 1. The index of the last item in a list is always one less than the length of the list.

So, putting all that together, here is how you could try to get all three instances of the item:

programming_languages = ["JavaScript","Python","Java","Python","C++","Python"]

print(programming_languages.index("Python",1,5))

#output
#1

The output still returns only the first instance!

Although the start and end parameters provide a range of positions for your search, the return value when using the index() method is still only the first occurence of the item in the list.

How to Get the Indices of All Occurrences of an Item in A List

Use a for-loop to Get the Indices of All Occurrences of an Item in A List

Let’s take the same example that we’ve used so far.

That list has three occurrences of the string “Python”.

programming_languages = ["JavaScript","Python","Java","Python","C++","Python"]

First, create a new, empty list.

This will be the list where all indices of “Python” will be stored.

programming_languages = ["JavaScript","Python","Java","Python","C++","Python"]

python_indices = []

Next, use a for-loop. This is a way to iterate (or loop) through the list, and get each item in the original list. Specifically, we loop over each item’s index number.

programming_languages = ["JavaScript","Python","Java","Python","C++","Python"]

python_indices = []

for programming_language in range(len(programming_languages)):

You first use the for keyword.

Then create a variable, in this case programming_language, which will act as a placeholder for the position of each item in the original list, during the iterating process.

Next, you need to specify the set amount of iterations the for-loop should perform.

In this case, the loop will iterate through the full length of the list, from start to finish. The syntax range(len(programming_languages)) is a way to access all items in the list programming_languages.

The range() function takes a sequence of numbers that specify the number it should start counting from and the number it should end the counting with.

The len() function calculates the length of the list, so in this case counting would start at 0 and end at – but not include – 6, which is the end of the list.

Lastly, you need to set a logical condition.

Essentially, you want to say: “If during the iteration, the value at the given position is equal to ‘Python’, add that position to the new list I created earlier”.

You use the append() method for adding an element to a list.

programming_languages = ["JavaScript","Python","Java","Python","C++","Python"]

python_indices = []

for programming_language in range(len(programming_languages)):
    if programming_languages[programming_language] == "Python":
      python_indices.append(programming_language)

print(python_indices)

#output

#[1, 3, 5]

Use List Comprehension and the enumerate() Function to Get the Indices of All Occurrences of an Item in A List

Another way to find the indices of all the occurrences of a particular item is to use list comprehension.

List comprehension is a way to create a new list based on an existing list.

Here is how you would get all indices of each occurrence of the string “Python”, using list comprehension:

programming_languages = ["JavaScript","Python","Java","Python","C++","Python"]

python_indices  = [index for (index, item) in enumerate(programming_languages) if item == "Python"]

print(python_indices)

#[1, 3, 5]

With the enumerate() function you can store the indices of the items that meet the condition you set.

It first provides a pair (index, item) for each element in the list (programming_languages) that is passed as the argument to the function.

index is for the index number of the list item and item is for the list item itself.

Then, it acts as a counter which starts counting from 0 and increments each time the condition you set is met, selecting and moving the indices of the items that meet your criteria.

Paired with the list comprehension, a new list is created with all indices of the string “Python”.

Conclusion

And there you have it! You now know some of the ways to find the index of an item, and ways to find the indices of multiple occurrences of an item, in a list in Python.

I hope you found this article useful.

To learn more about the Python programming language, check out freeCodeCamp’s Scientific Computing with Python Certification.

You’ll start from the basics and learn in an interacitve and beginner-friendly way. You’ll also build five projects at the end to put into practice and help reinforce what you’ve learned.

Thanks for reading and happy coding!



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Python hosting: Host, run, and code Python in the cloud!

Arrays are usually referred to as lists. For convience, lets call them arrays in this article.

Python has a method to search for an element in an array, known as index().
We can find an index using:


x = ['p','y','t','h','o','n']
print(x.index('o'))

Arrays start with the index zero (0) in Python:

python-string

Python character array

If you would run x.index(‘p’) you would get zero as output (first index).

Related course:

  • Python Crash Course: Master Python Programming

Array duplicates: If the array contains duplicates, the index() method will only return the first element.

Find multiple occurences

If you want multiple to find multiple occurrences of an element, use the lambda function below.


get_indexes = lambda x, xs: [i for (y, i) in zip(xs, range(len(xs))) if x == y]

Find in string arrays

To find an element in a string array use:


x = ["Moon","Earth","Jupiter"]
print(x.index("Earth"))

You could use this code if you want to find multiple occurrences:



x = ["Moon","Earth","Jupiter","Neptune","Earth","Venus"]
get_indexes = lambda x, xs: [i for (y, i) in zip(xs, range(len(xs))) if x == y]
print(get_indexes("Earth",x))


Find in numeric array

The index method works on numeric arrays too:


x = [5,1,7,0,3,4]
print(x.index(7))

To find multiple occurrences you can use this lambda function:



x = [5,1,7,0,3,4,5,3,2,6,7,3,6]
get_indexes = lambda x, xs: [i for (y, i) in zip(xs, range(len(xs))) if x == y]
print(get_indexes(7,x))



Время чтения 3 мин.

Существует несколько способов проверки наличия элемента в списке в Python:

  1. Использование метода index() для поиска индекса элемента в списке.
  2. Использование оператора in для проверки наличия элемента в списке.
  3. Использование метода count() для подсчета количества вхождений элемента.
  4. Использование функции any().
  5. Функция filter() создает новый список элементов на основе условий.
  6. Применение цикла for.

Содержание

  1. Способ 1: Использование метода index()
  2. Способ 2: Использование «оператора in»
  3. Способ 3: Использование функции count()
  4. Синтаксис
  5. Пример
  6. Способ 4: использование понимания списка с any()
  7. Способ 5: Использование метода filter()
  8. Способ 6: Использование цикла for

Способ 1: Использование метода index()

Чтобы найти элемент в списке Python, вы можете использовать метод list index(). Список index() — это встроенный метод, который ищет элемент в списке и возвращает его индекс.

Если один и тот же элемент присутствует более одного раза, метод возвращает индекс первого вхождения элемента.

Индекс в Python начинается с 0, а не с 1. Таким образом, через индекс мы можем найти позицию элемента в списке.

streaming = [‘netflix’, ‘hulu’, ‘disney+’, ‘appletv+’]

index = streaming.index(‘disney+’)

print(‘The index of disney+ is:’, index)

Выход

The index of disney+ is: 2

Метод list.index() принимает единственный аргумент, элемент, и возвращает его позицию в списке.

Способ 2: Использование «оператора in»

Используйте оператор in, чтобы проверить, есть ли элемент в списке.

main_list = [11, 21, 19, 46]

if 19 in main_list:

  print(“Element is in the list”)

else:

  print(“Element is not in the list”)

Выход

Вы можете видеть, что элемент «19» находится в списке. Вот почему оператор in возвращает True.

Если вы проверите элемент «50», то оператор in вернет False и выполнит оператор else.

Способ 3: Использование функции count()

Метод list.count() возвращает количество вхождений данного элемента в списке.

Синтаксис

Метод count() принимает единственный элемент аргумента: элемент, который будет подсчитан.

Пример

main_list = [11, 21, 19, 46]

count = main_list.count(21)

if count > 0:

  print(“Element is in the list”)

else:

  print(“Element is not in the list”)

Выход

Мы подсчитываем элемент «21», используя список в этой функции example.count(), и если он больше 0, это означает, что элемент существует; в противном случае это не так.

Способ 4: использование понимания списка с any()

Any() — это встроенная функция Python, которая возвращает True, если какой-либо элемент в итерируемом объекте имеет значение True. В противном случае возвращается False.

main_list = [11, 21, 19, 46]

output = any(item in main_list for item in main_list if item == 22)

print(str(bool(output)))

Выход

Вы можете видеть, что в списке нет «22». Таким образом, нахождение «22» в списке вернет False функцией any(). Если функция any() возвращает True, элемент в списке существует.

Способ 5: Использование метода filter()

Метод filter() перебирает элементы списка, применяя функцию к каждому из них.

Функция filter() возвращает итератор, который перебирает элементы, когда функция возвращает значение True.

main_list = [11, 21, 19, 46]

filtered = filter(lambda element: element == 19, main_list)

print(list(filtered))

Выход

В этом примере мы используем функцию filter(), которая принимает функцию и перечисляет ее в качестве аргумента.

Мы использовали лямбда-функцию, чтобы проверить, совпадает ли входной элемент с любым элементом из списка, и если это так, он вернет итератор. Чтобы преобразовать итератор в список в Python, используйте функцию list().

Мы использовали функцию list() для преобразования итератора, возвращаемого функцией filter(), в список.

Способ 6: Использование цикла for

Вы можете узнать, находится ли элемент в списке, используя цикл for в Python.

main_list = [11, 21, 19, 46]

for i in main_list:

  if(i == 46):

    print(“Element Exists”)

Выход

В этом примере мы прошли список элемент за элементом, используя цикл for, и если элемент списка совпадает с входным элементом, он напечатает «Element exists».

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