you can't change part of an array

Tinsaw

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15-04-2025

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Arrays in many programming languages, including JavaScript, Python, and Java, behave in a way that might surprise newcomers. While you can't directly modify a portion of an array in the same way you might change a single element, you can create a new array based on modifications to the original. This subtle difference is crucial to understanding how arrays function. This article will clarify this concept and provide examples to illustrate the point.

Understanding Array Immutability (or Lack Thereof)

The key concept here is that arrays themselves are often mutable (changeable) in many common programming languages. However, the methods used to appear to change a section of the array often create a new array with the desired changes. Let's explore this with some examples.

JavaScript Example: The Slice and Splice Misconception

In JavaScript, functions like slice() and splice() are often used to work with portions of arrays. However, these don't modify the original array in place. Instead, they return new arrays containing the modified sections.

let myArray = [1, 2, 3, 4, 5];

// Slice creates a new array; myArray remains unchanged
let newArray = myArray.slice(1, 4); // newArray = [2, 3, 4]
console.log(myArray); // Output: [1, 2, 3, 4, 5]
console.log(newArray); // Output: [2, 3, 4]


// Splice modifies the original array and *returns* the removed elements. 
let removedElements = myArray.splice(1, 2); //removedElements = [2,3]
console.log(myArray); // Output: [1, 4, 5]
console.log(removedElements); // Output: [2, 3]

As you see, slice() creates a new array without altering the original. splice() modifies the original array in place but returns a new array containing the elements that were removed.

Python Example: List Slicing and Copying

Python lists also demonstrate this behavior. Slicing a list creates a new list, leaving the original untouched.

my_list = [10, 20, 30, 40, 50]

# Slicing creates a new list
new_list = my_list[1:4]  # new_list = [20, 30, 40]
print(my_list)  # Output: [10, 20, 30, 40, 50]
print(new_list)  # Output: [20, 30, 40]

#To modify in place requires explicit assignment
my_list[1:3] = [55,66]
print(my_list) # Output: [10, 55, 66, 40, 50]

Again, slicing creates a copy. Directly assigning a new section to an existing slice is one of the few ways to modify part of the list, in place.

Why This Matters

Understanding this immutable nature (or more precisely, the way modifications create new data structures) is crucial for preventing unexpected behavior in your code. Modifying a slice or section of an array without understanding that a new array is being generated can lead to errors, especially when working with functions that accept arrays as arguments.

Best Practices

• Always be aware of the return value: When using array manipulation functions, carefully examine the documentation to understand if they modify the original array or return a new one.
• Use appropriate methods: Choose methods that create new arrays when you don't want to change the original.
• Explicit Copying: If you need to modify a copy of the array, explicitly create a copy before performing operations on it. This helps avoid side effects.

By understanding that modifying parts of an array usually results in a new array, you can write more robust and predictable code. Remember to always check the documentation of the specific array functions in your chosen programming language to understand exactly how they handle changes.