Convert List to Integer in Python

Using for Loop

To convert the entire list into one integer in Python:

• Create a list having integer-type elements.
• Declare and initialize a variable with 0; it will be used to hold the final integer value.
• Use the for loop to iterate over the list items.
• In each iteration:
  • multiply the variable created in the second step with 10 and add the current list item.
  • Assign the value (produced in the previous step) to the variable created in step two.

First, we created a list named my_list with three elements of integer type. Then, our job is to convert that from a list of integers to one integer value; for that, we defined and initialized the variable named number with 0. Then, we used the for loop to go through all the list items.

For every current_digit in the my_list, we multiplied the number with 10 and added current_digit. This will have the effect of shifting digits of the number variable to the left by one place and adding the upcoming digit to the right. For instance, we have a [5, 6, 7] list in the above example, which will have the following iterations:

1. number = 0*10+5 = 5
2. number = 5*10+6 = 56
3. number = 56*10+7 = 567

Once the loop is over, we will have 567 as one integer type value; to check its type, you can use the type() method as type(number).

Using List Comprehension

To convert the entire list into one integer in Python:

• Create a list having integer-type elements.
• Use list comprehension to convert each list item from integer type value to string type value; for instance, from [5, 6, 7] to ['5', '6', '7'].
• Use the join() method to join all the list items to make a single string value, such as from ['5', '6', '7'] to '567'.
• Use the int() method for changing data type from string to integer.

After creating a list named my_list having integer type elements, we used list comprehension to loop over all the list items and converted every item to a string by wrapping around with the str() method. The str() method converts the specified value to a string type value. At this point, we had my_list as ['5', '6', '7'], which we stored in the new_list variable.

Next, we used the join() method to join all the elements of new_list to make it one string value and stored it in the string_value variable. Using the join() method, we transformed ['5', '6', '7'] to '567'.

Now, we can use the int() method to dynamically cast string_value to an integer value which we saved in number. Finally, we used the print() method to print the value of the number variable; we can also print the type of number by printing as print(type(number)).

Using map() and join() Methods

To convert the entire list into one integer in Python:

• Create a list having integer-type elements.
• Use the map() method to convert every list item’s type from integer to string.
• Use the join() method to join all the list items to make it one string-type value.
• Use the int() method to cast the type from string to integer dynamically.

This code fence is similar to the previous example, but we used functions instead of list comprehension. Here, we used the map() method to transform [5, 6, 7] to ['5', '6', '7'] and saved it in new_list variable. If you are curious how the map() function works, then let’s understand that below.

We passed two arguments to map(); the first was the function (str), and the second was the iterable (my_list). The map() method iterated over the my_list items where every list item was passed to the str function as an argument; for instance, the first list item 5 was sent to the str function as str(5), which returned string type value as '5'. By using the map() method, we transformed the [5, 6, 7] to ['5', '6', '7'], which we saved in the new_list variable.

This new_list variable was then passed to the join() method to join all the list items and form one string type value as '567' that we stored in string_value. Finally, we performed typecasting using the int() method. This int() method took string_value as a parameter and converted its type from string to integer that we saved in the number variable.

Using the reduce() Method with lambda Expression

To convert the entire list into one integer in Python:

• Create a list having integer-type elements.
• Use the reduce() method with the lambda function to convert the entire list to one integer.

This code example is the first example’s more concise and professional form. Here, we did the same, multiplied the number by 10, added the current digit, and used this result for the next integer. But this time, number is x and current_digit is y, and we used the reduce() function with the lambda function to achieve this.

The reduce() method took two arguments: first was the function (lambda), and second was the iterable (my_list). The lambda expression took two arguments, x & y, and produced x*10+y. The reduce() method’s job was to apply the lambda function to the first two list items of my_list, then apply the same function to the outcome and next item, and so on, till all items have been processed.

Once the reduce() method is done with its processing, we will get the answer which would be an integer-type value.

That’s all about how to convert List to Integer in Python.