Как заменить значения в списке в Python
Часто вас может заинтересовать замена одного или нескольких значений в списке в Python.
К счастью, это легко сделать в Python, и в этом руководстве дается несколько различных примеров того, как это сделать.
Пример 1. Замена одного значения в списке
Следующий синтаксис показывает, как заменить одно значение в списке в Python:
Пример 2. Замена нескольких значений в списке
Следующий синтаксис показывает, как заменить несколько значений в списке в Python:
Пример 3. Замена определенных значений в списке
Следующий синтаксис показывает, как заменить определенные значения в списке в Python:
Вы также можете использовать следующий синтаксис для замены значений, превышающих определенный порог:
Точно так же вы можете заменить значения, которые меньше или равны некоторому порогу:
7 Efficient Ways to Replace Item in List in Python
Lists in python are data structures used to store items of multiple types together under the same list. Using a list, we can store several data types such as string values, integer, float, sets, nested lists, etc. The items are stored inside square brackets ‘[ ]’. They are mutable data types, i.e., they can be changed even after they have been created. We can access list elements using indexing. In this article, we shall look into 7 efficient ways to replace items in a python list.
How to replace item in list python
Lists are compelling and versatile data structures. As mentioned above, lists are mutable data types. Even after a list has been created, we can make changes in the list items. This property of lists makes them very easy to work with. Here, we shall be looking into 7 different ways in order to replace item in a list in python.
- Using list indexing
- Looping using for loop
- Using list comprehension
- With map and lambda function
- Executing a while loop
- Using list slicing
- Replacing list item using numpy
1. Using list indexing
The list elements can be easily accessed with the help of indexing. This is the most basic and the easiest method of accessing list elements. Since all the elements inside a list are stored in an ordered fashion, we can sequentially retrieve its elements. The first element is stored at index 0, and the last element is stored at index ‘len(list)-1’.
Let us take a list named ‘my_list’, which stores names of colors. Now, if we want to replace the first item inside the list from ‘Red’ to ‘Black’, we can do that using indexing. We assign the element stored at the 0th index to a new value. Then the list printed would contain the replaced item.
2. Looping using for loop
We can also execute a for loop to iterate over the list items. When a certain condition is fulfilled inside the for loop, we will then replace that list item using indexing.
We shall take the same ‘my_list’ as in the above example. Then, we shall run the for loop for the length of the list ‘my_list’. Inside the loop, we have placed the condition that when the list element equals ‘Orange’, we will replace the item at that index with ‘Black’. Then after the for loop ends, we shall print that list.
The output prints the list with the replaced value.
3. Using list comprehension
List Comprehension in python is a compact piece of code. Using list comprehension, we can generate new sequences from already existing sequences. Instead of writing an entire block of code for executing a for loop or an if-else loop, we can write a single line code for the same.
The syntax for list comprehension is:
It consists of the expression which has to be printed into the new list, the loop statement, and the original list from which the new values will be obtained.
We shall use list comprehension to append the string ‘ color’ to all the list elements of my_list. We shall replace the old values with the updated values.
The list with replaced values is:
4. With map and lambda function
Map() is a built-in function in python using which we can iterate over an iterable sequence without having to write a loop statement.
The syntax of the map is:
Here, every value inside the iterable will be passed into a function, and the map() function will output the inside function’s return value. Inside the map() function, we have taken a lambda function as the first argument.
A lambda function is an anonymous function containing a single line expression. It can have any number of arguments, but only one expression can be evaluated. Here, the lambda function will append the string ‘ color’ for all the items present in the list and return the new value. Then using the list() function, we shall convert the map object returned by the map() function into a list.
The output is:
If we want to replace a particular item in the list, then the code for that will be:
We have used the replace() method to replace the value ‘Orange’ from my_list to ‘Black.’ The output is:
5. Executing a while loop
We can also have a while loop in order to replace item in a list in python. We shall take a variable ‘i’ which will be initially set to zero. The while loop shall execute while the value of ‘i’ is less than the length of the list my_list. We shall use the replace() method to replace the list item ‘Gray’ with ‘Green.’ The variable ‘i’ shall be incremented at the end of the loop.
The updated list is:
6. Using list slicing
We can also perform slicing inside a list. Using slicing enables us to access only a certain part of a list.
The syntax of the list is:
Using list slicing, we can replace a given item inside a list. First, we shall store the index of the item to be replaced into a variable named ‘index.’ Then, using list slicing, we will replace that item with a new value, ‘Green.’
The output is:
7. Replacing list item using numpy
We can also replace a list item using python’s numpy library. First, we will convert the list into a numpy array. Then using numpy’s where() function, we specify a condition according to which we will replace the value. We shall replace the value ‘Gray’ with ‘Green.’ Else, the value will be the same.
The output is:
FAQ’s Related to Replace item in list in Python
Using list comprehension, map-lambda function, replace function, etc., we can perform in-place replacement of list items.
This sums up five different ways of replacing an item in a list. If you have any questions in your mind, then let us know in the comments below.
Простые и эффективные способы замены элемента в списке Python
В данной статье мы обсудим, как заменить элемент в списке Python. Поехали!
Замена элемента списка
Python позволяет нам легко заменить элемент в списке. Просто укажите индекс элемента, который вы хотите заменить, и присвойте новое значение.
Этот код заменяет третий элемент списка (помните, индексация начинается с нуля!) на число 8. Конечный список будет выглядеть так: [1, 2, 8, 4, 5] .
Замена нескольких элементов
Для замены нескольких элементов вы можете использовать срезы:
Этот код заменяет второй и третий элементы на числа 8 и 9 соответственно. Конечный список будет выглядеть так: [1, 8, 9, 4, 5] .
Замена элемента по значению
Иногда вы можете захотеть заменить элемент по значению, а не по индексу. В этом случае вам нужно использовать метод index() для поиска индекса элемента, а затем заменить элемент по этому индексу.
Этот код находит индекс числа 3 в списке и заменяет этот элемент на 8. Конечный список будет выглядеть так: [1, 2, 8, 4, 5] .
Обратите внимание, что метод index() возвращает индекс первого найденного элемента. Если в списке есть несколько одинаковых элементов и вы хотите заменить их все, вам придется использовать цикл или генератор списка.
Замена всех элементов списка с определенным значением
В этом коде мы используем генератор списка, чтобы создать новый список. Элементы, равные 3, заменяются на 8, все остальные элементы остаются неизменными. Конечный список будет выглядеть так: [1, 2, 8, 4, 5, 8, 6, 8] .
Замена элементов, используя lambda и map
В Python вы также можете использовать функции lambda и map() для замены элементов в списке. Функция map() применяет заданную функцию ко всем элементам итерируемого объекта (например, списка).
В этом коде lambda функция заменяет все 3 на 8. Конечный список будет выглядеть так: [1, 2, 8, 4, 5, 8, 6, 8] .
Замена элементов на основе условий
Иногда может потребоваться заменить элементы на основе определенных условий. В таком случае можно использовать генераторы списков или функцию map() в сочетании с функцией lambda.
В этом коде мы заменили все нечетные числа в списке на 8. Конечный список будет выглядеть так: [8, 2, 8, 4, 8, 8, 6, 8] .
Заключение
Python предоставляет множество удобных и эффективных способов замены элементов в списках. Вы можете использовать прямую индексацию для замены конкретного элемента, срезы для замены диапазона элементов, генераторы списков или функцию map() для замены элементов на основе определенных условий.
Выбор конкретного метода зависит от конкретной задачи, которую вы пытаетесь решить, и от вашего предпочтения стиля программирования.
Replace Item in List in Python: A Complete Guide
There are three ways to replace an item in a Python list. You can use list indexing or a for loop to replace an item. If you want to create a new list based on an existing list and make a change, you can use a list comprehension.
You may decide that you want to change a value in a list. Suppose you’re building a menu for a restaurant. You may notice that you have misspelled one of the menu items. To fix this mistake, you will need to change an existing element in the list.
Python Replace Item in List
You can replace an item in a Python list using a for loop, list indexing, or a list comprehension. The first two methods modify an existing list whereas a list comprehension creates a new list with the specified changes.
Let’s summarize each method:
- List indexing: We use the index number of a list item to modify the value associated with that item. The equals sign is used to change a value in a list.
- List comprehension: The list comprehension syntax creates a new list from an existing one. You can specify conditions in your list comprehension to determine the values in the new list.
- For Loop: The loop iterates over the items in a list. You use indexing to make the actual change to the list. We use the enumerate() method to create two lists of index numbers and values over which we can iterate.
In this guide, we walk through each of these methods. We’ll refer to an example of each to help you get started.
Python Replace Item in List: Using List Indexing
The easiest way to replace an item in a list is to use the Python indexing syntax. Indexing allows you to choose an element or range of elements in a list. With the assignment operator, you can change a value at a given position in a list.
We’re building a program that stores information on prices at a clothing store. The price of the first item in our list should be increased by $5. To do this, we use list indexing.
Let’s start by creating a list which contains our product prices:
We use indexing to select and change the first item in our list, 99.95. This value has the index position zero. This is because lists are indexed starting from zero:
Our code selects the item at position zero and sets its value to 104.95. This is a $5 increase on the old value. Our code returns our list of items with the first price changed:
We can change our list by adding five to the current value of prices[0]:
prices[0] corresponds with the first item in our list (the one at index position 0).
Our code returns a list with the same values as our first example:
Python Replace Item in List: Using a List Comprehension
A Python list comprehension may be the most Pythonic way of finding and replacing an item in a list. This method is useful if you want to create a new list based on the values of an existing one.
Using a list comprehension lets you iterate through items in an existing list and create a new list based on a certain criterion. You can generate a new list that only contains items beginning with “C” from an existing list, for instance.
Here, we build a program that calculates a 10% discount on all the products in a clothing store that are worth more than $50. We use our list of product prices from earlier:
Next, we define a list comprehension to replace the items in our list:
This list comprehension iterates through the “prices” list and searches for values worth more than $50. A discount of 10% is applied to those items. We round the discounted values to two decimal places using the round() method.
Our code returns our list of new prices:
A 10% discount has been successfully applied to every item.
Python Replace Item in List: Using a For Loop
You can replace items in a list using a Python for loop. To do so, we need to the Python enumerate() function. This function returns two lists: the index numbers in a list and the values in a list. We iterate over these two lists with a single for loop.
In this example, we’re going to use the same list of prices in our code:
We then define a for loop that iterates over this list with the enumerate() function:
The value “index” stores the index position of an item. “Item” is the value that correlates with that index position. The comma separates the two list values returned by the enumerate() method.
Retrieving two or more values from a method or another value is called unpacking. We have “unpacked” two lists from the enumerate() method.
We use the same formula from earlier to calculate a 10% discount on items worth over $50. Let’s run our code and see what happens:
Our code successfully changes the items in our “prices” list based on our discount.
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Conclusion
You can replace items in a Python list using list indexing, a list comprehension, or a for loop.
If you want to replace one value in a list, the indexing syntax is most appropriate. To replace multiple items in a list that meet a criterion, using a list comprehension is a good solution. While for loops are functional, they are less Pythonic than list comprehensions.
Are you interested in more resources to help you learn Python? If so, you should check out our How to Learn Python guide. This guide contains a list of top courses, books, and learning resources that will help you master the language.
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