Common mistakes and how to avoid them

Every programmer makes these mistakes. The difference between a beginner and an experienced engineer is mostly that the experienced one expects them, and checks for them before they cause trouble. Here are the ones that bite most often with arrays.

1. Off-by-one: stepping past the end

An array of length 5 has indexes 0 to 4. Index 5, drawn as a dashed red slot, is outside the array.

An array of length n has indexes 0 to n - 1. Writing <= in the loop condition visits index n, which doesn't exist. What happens next depends on the language — and none of the outcomes are good:

JavaScript quietly returns undefined, and the bug surfaces later as NaN or a strange comparison.
Python and Java crash with an IndexError / ArrayIndexOutOfBoundsException — loud, but at least honest.
C++ reads whatever memory happens to be next. The program may seem to work, then fail somewhere else entirely.

Habit: when you write a loop bound or an index like i + 1, ask "what happens on the last element?" If you read nums[i + 1], the loop must stop at i < n - 1.

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Still to come

2. Forgetting the empty array
3. Two names, one array (aliasing)
4. Changing an array while looping over it
5. Integer overflow in sums and products
6. Language gotchas worth memorising
A 20-second checklist before you run your code

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Common mistakes and how to avoid them

1. Off-by-one: stepping past the end

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