Developing by testing

We have been asked to write a C++ function that checks if the passed integer is prime. Let's develop it using Google Test as testing environment.

Step 1

The function should have this prototype:

extern bool isPrime(int value);

Let's write a first implementation, any value we pass in, the function says it is not a prime:

bool isPrime(int value)
{
    return false;
}

We know it is not working properly, but at least it works.

Step 2

We setup the Google Test environment, so we are going to call RUN_ALL_TEST on a few tests we are about to write.

Negative numbers are not prime, so here is a first lot of tests:

TEST(IsPrimeTest, Negative) {
  EXPECT_FALSE(isPrime(-1));
  EXPECT_FALSE(isPrime(-2));
  EXPECT_FALSE(isPrime(INT_MIN));
}

Let's consider the first positive numbers:

TEST(IsPrimeTest, Trivial) {
  EXPECT_FALSE(isPrime(0));
  EXPECT_FALSE(isPrime(1));
  EXPECT_TRUE(isPrime(2));
  EXPECT_TRUE(isPrime(3));
}

And then do some testing on other positive numbers:

TEST(IsPrimeTest, Positive) {
  EXPECT_FALSE(isPrime(4));
  EXPECT_TRUE(isPrime(5));
  EXPECT_FALSE(isPrime(6));
  EXPECT_TRUE(isPrime(23));
}

Step 3

We compile and run our application. If we correctly setup Google Test, we are going to find out that the first lot of testing succeed (hurray!) but we have a few errors in the other two ones:

IsPrimeTest.Trivial:
(...)\test.cpp(94): error: Value of: isPrime(2)
  Actual: false
Expected: true
(...)\test.cpp(95): error: Value of: isPrime(3)
  Actual: false
Expected: true

IsPrimeTest.Trivial:
(...)\test.cpp(101): error: Value of: isPrime(5)
  Actual: false
Expected: true
(...)\test.cpp(103): error: Value of: isPrime(23)
  Actual: false
Expected: true

Looks like the algorithm we applied in our function is not perfect.

Step 4

We change of function in this way:

bool isPrime(int value)
{
    if(value < 2)
        return false;
    // 2 is prime, all other even numbers not
    if((value != 2) && (value % 2 == 0))
        return false;
    return true;
}

Wow - no errors detected. But have we done enough testing?

Step 4

Let's add some more tests:

TEST(IsPrimeTest, LessThan100) {
  EXPECT_FALSE(isPrime(87));
  EXPECT_TRUE(isPrime(89));
  EXPECT_FALSE(isPrime(95));
  EXPECT_TRUE(isPrime(97));
}

Looks like we still have problems:

(...)\test.cpp(107): error: Value of: isPrime(87)
  Actual: true
Expected: false
(...)\test.cpp(109): error: Value of: isPrime(95)
  Actual: true
Expected: false

And so on ...