How Arrays Work — API Cheat Sheet

This is a reference page, not a graded exercise. Arrays are the one data structure that shows up in nearly every interview problem, so fumbling the basic syntax costs you time you don't have.

Java — int[] / T[]

Want to...	Syntax	Example
Declare fixed-size	new int[n]	int[] arr = new int[5]; (zero-filled)
Declare with values	array literal	int[] arr = {1, 2, 3};
Get length	.length (field, no parens)	arr.length
Read/write an index	[]	arr[0] = 5;
Sort	Arrays.sort(arr)	ascending, in place
Copy	Arrays.copyOf(arr, n)	arrays.copyOfRange(arr, from, to) for a slice
Fill with a value	Arrays.fill(arr, v)	Arrays.fill(arr, -1)
Print for debugging	Arrays.toString(arr)	arr.toString() alone prints garbage
2D grid	new int[rows][cols]	zero-filled; access grid[r][c]
Note	Fixed size — no push/pop. Use ArrayList (see the Lists cheat sheet) when size changes.

C# — int[] / T[]

Want to...	Syntax	Example
Declare fixed-size	new int[n]	int[] arr = new int[5];
Declare with values	array literal	int[] arr = {1, 2, 3};
Get length	.Length (property, no parens)	arr.Length
Read/write an index	[]	arr[0] = 5;
Sort	Array.Sort(arr)	ascending, in place
Copy	(int[])arr.Clone()	or Array.Copy(src, dst, n)
Fill with a value	Array.Fill(arr, v)	Array.Fill(arr, -1)
Print for debugging	string.Join(", ", arr)
2D grid	new int[rows, cols] (rectangular) or int[rows][] (jagged)	grid[r, c] vs grid[r][c] — don't mix these up
Note	Fixed size. Use List<T> when size changes.

Python — list (doubles as the array type)

Want to...	Syntax	Example
Declare fixed-size, zero-filled	list multiplication	arr = [0] * n
Declare with values	list literal	arr = [1, 2, 3]
Get length	len(arr)	builtin, not a method
Read/write an index	[]	arr[0] = 5
Sort	arr.sort() (in place) / sorted(arr) (new list)	arr.sort(reverse=True)
Slice / copy	slicing	arr[1:3], arr[:] copies the whole list
Grow / shrink	.append(v), .pop(), .insert(i, v), .remove(v)	Python lists are dynamic, not fixed-size like Java/C# arrays
2D grid	list comprehension	grid = [[0] * cols for _ in range(rows)] — never [[0]*cols]*rows, that repeats the same inner list

JavaScript / TypeScript — Array / T[]

Want to...	Syntax	Example
Declare, zero-filled	Array(n).fill(0)	new Array(5).fill(0)
Declare with values	array literal	const arr = [1, 2, 3];
Get length	.length (property, no parens)	arr.length
Read/write an index	[]	arr[0] = 5;
Sort	.sort() (in place!)	arr.sort((a, b) => a - b) — default sort is lexicographic, always pass a comparator for numbers
Slice / copy	.slice(start, end)	arr.slice() copies the whole array
Grow / shrink	.push(v), .pop(), .unshift(v), .shift(), .splice(i, count, ...items)	push/pop are O(1); shift/unshift are O(n)
Transform	.map(), .filter(), .reduce()	return new arrays, don't mutate
2D grid	nested Array.from	Array.from({ length: rows }, () => Array(cols).fill(0))

The gotcha that costs people the most time: in Java and C#, arrays are fixed-size — there's no push/pop, you'd need a List/ArrayList for that. In Python and JS, what's called an "array" in interview-speak is actually a dynamic list — resizing is free. Know which one your language gives you before you reach for .append/.push and it doesn't exist.