C Test Cases

What is a Test Case?

A test case is one input, together with the output you expect. For example, the input 7 should give the output Odd.

One test case is not enough to trust a program. You need many: normal input, and the edge cases from the previous chapter.

In this chapter, you will learn how to:

• Let your program test itself automatically.
• Check your code with test cases on W3Schools Practice Problems.

Test Your Code Automatically

Instead of changing the input and running the program again and again, let the program check itself.

Write a small test() function. It calls the function you want to test, compares the result with the expected answer, and prints PASS or FAIL. Here, we test an isEven() function with the test cases from the previous chapter:

#include <stdio.h>

// Returns 1 if n is even, and 0 if not
int isEven(int n) {
  return n % 2 == 0;
}

void test(int n, int expected) {
  int result = isEven(n);
  if (result == expected) {
    printf("PASS: %d\n", n);
  } else {
    printf("FAIL: %d (expected %d, got %d)\n", n, expected, result);
  }
}

int main() {
  test(7, 0);
  test(4, 1);
  test(0, 1);
  test(-7, 0);
  return 0;
}

PASS: 7
PASS: 4
PASS: 0
PASS: -7

All four tests pass. Now let's break the function on purpose, and run the same tests again. This version makes the same mistake as in the previous chapter: it checks if the remainder is 1 (odd), and says "even" for everything else:

int isEven(int n) {
  return n % 2 != 1;  // The buggy check
}

PASS: 7
PASS: 4
PASS: 0
FAIL: -7 (expected 0, got 1)

The FAIL line tells you exactly which test case broke: -7.

Switch between the two versions to compare the results of all the test cases:

Tip: When you fix a bug, keep the test case that found it. Then you will notice right away if the bug ever comes back.

W3Schools Practice Problems

On W3Schools, you can solve C Practice Problems in our online editor. Each problem is checked with test cases, just like the ones in this chapter. Your program runs once for each test case, and you see how many test cases it passed.

Even or Odd is one of our practice problems: read a number, and print Even if it can be divided by 2, or Odd if it cannot. It is tested with these test cases:

Notice the test case 0. It is an edge case: is 0 even? Yes, because 0 % 2 is 0, so the program must print Even.

In the practice problem, your program does not get the numbers from an array. The starter code reads one number with scanf() (see User Input). Your program must then print exactly Even or Odd.

Now it is your turn! Solve the problem in our practice editor:

Summary

• A test case is an input, together with the output you expect.
• A test() function can check many test cases at once, and tell you which ones fail.
• W3Schools Practice Problems run your program once for each test case, and tell you how many pass.

You have now learned how to understand a problem, build the logic, find edge cases and logical errors, and test your code with many test cases. Use these skills on our C Practice Problems!