Operators

Why we need operators

The arithmetic ones

There is a power operator too, written ** in Python and Math.pow in Java, so 7 ** 2 is 49.

Two of these need a proper explanation, and they are the two that cause bugs.

The division trap

In Python 3, 7 / 2 gives 3.5. Sensible.

In C, C++ and Java, 7 / 2 gives 3. Not 3.5, and not rounded to 4. When you divide an integer by an integer in those languages the answer must also be an integer, so the decimal part is simply thrown away.

int result = 7 / 2;      // 3double result = 7 / 2;   // still 3.0, the damage was already donedouble result = 7.0 / 2; // 3.5, now it works

Make one side a decimal and the problem disappears. This one is nasty because nothing crashes: your program keeps running and quietly gives wrong answers.

The remainder, and what it is really for

% gives you what is left over after division. 17 % 5 is 2, because 5 goes into 17 three times with 2 remaining.

That sounds like a school exercise. It is actually one of the most used operators in programming, and the picture below is why.

Three things it does constantly:

• Even or odd. n % 2 == 0 means even.
• Divisible by. n % 5 == 0 means n is a multiple of 5.
• Wrapping round. (current + 1) % size snaps back to 0 when it reaches the end. If you ever build a circular queue, that single line is the whole idea.

Assignment shortcuts

= puts a value in. The rest are pure convenience, and they read well.

count = 10 count += 5    # same as count = count + 5   → 15count -= 3    # same as count = count - 3   → 12count *= 2    # same as count = count * 2   → 24

C, C++, Java and JavaScript also have ++ and -- for adding or subtracting one. There is a difference between i++ and ++i: the first gives the old value and then increments, the second increments first. Use them on their own line and you will never have to think about it. Python has no ++ - write i += 1.

Comparison

These ask a question and always answer with a boolean.

The one to burn in: = puts a value in, == asks whether two things match. In C and older Java, if (x = 5) compiles fine and quietly assigns 5 to x. The condition is then always true and your program misbehaves for reasons you cannot see.

Logical operators, and the one trick worth knowing

and needs both sides true. or needs at least one. not flips it. In C, Java and JavaScript those are written &&, || and !.

The everyday mistake is writing or where you meant and. Read the condition out loud in English: "age is at least 18 or they have an ID" lets a ten-year-old with an ID card through.

Now the trick, which is rarely taught and genuinely useful.

That is not just a speed optimisation. You can lean on it:

if user is not None and user.age > 18:

If user is None, the second half would crash. It never runs, because the first half already failed. Order matters here - swap the two halves and the program breaks.

Precedence

Operators run in a fixed order, the same way 2 + 3 * 4 is 14 and not 20. Roughly: brackets, then powers, then multiply and divide, then add and subtract, then comparisons, then not, and, or.

Do not memorise that list. Memorise the practical rule instead:

Mistakes to watch for

Quick recap

• Operators are the verbs: they act on values and produce new ones
• / truncates when both sides are integers in C and Java
• % is a remainder, and it is how you make anything wrap round
• = assigns, == compares
• and and or short-circuit, which you can rely on deliberately
• When unsure about order, use brackets