Edge Cases and Testing

The usual traps (n = 1, all equal, no answer, overflow, negative numbers, off-by-one, several test cases, output format), testing by hand with tiny inputs, debugging with cerr, brute force, and a stress test inside one C++ program.

Назария

Дарси 3 аз 6

Хониш: 16 дақиқа

Саволҳо: 6

Your program passes both samples. You submit — Wrong answer on test 7. The samples are written to explain the statement, not to try every corner of it; the hidden tests are written to break programs. This lesson is about breaking yours first: knowing where bugs hide, testing by hand, and letting the computer test for you.

The usual traps

Most wrong answers from correct-looking programs come from a short list. Before you submit, go through it with the problem's limits in front of you:

Test by hand with tiny inputs

For each trap that applies, write a test small enough to solve on paper. Work out the answer before you run the program, and only then compare. Three numbers are usually enough, and a tiny test is easy to read when something goes wrong.

Here is a program that prints the largest of n numbers. It passes every sample with a positive number in it. Try the trap "negative numbers" on it — three negatives:

A tiny test that catches a bug

C++

1#include <bits/stdc++.h>

2using namespace std;

4int main() {

5 ios_base::sync_with_stdio(false);

6 cin.tie(NULL);

8 int n;

9 cin >> n;

10 vector<int> a(n);

11 for (int i = 0; i < n; i++) {

12 cin >> a[i];

13 }

14 int best = 0;

15 for (int i = 0; i < n; i++) {

16 if (a[i] > best) {

17 best = a[i];

18 }

19 }

20 cout << best << "\n";

21 return 0;

22}

Read n

сатр 9

Three numbers will follow.

a3 катак

—

вуруд

3 -7 -2 -5

n3best—i—

1 / 11

The fix is to start from a value that really is in the input, int best = a[0];, or from something smaller than every allowed value.

Now hunt for bugs yourself. Each program below passes its sample; build an input that makes it print a wrong answer:

Break the program

The usual traps

Test by hand with tiny inputs

Practice: Edge Cases

Print what the program is doing

A brute force you can trust

Stress testing

Before you submit

Check your understanding