Algorithms: Conditionals

In the previous post we learned variables — the boxes that store information. Near the end there was a small line: adult ← age ≥ 18. That line produced a true/false value — but we never actually did anything with it.

That is exactly what this post is about: looking at a true/false value and choosing a path. A program being able to say “if this is true do that, otherwise do this.” We call it a conditional, and it is the only way an algorithm can make a decision. Everything we have written so far flowed straight down; with conditionals, our program starts making its first real choices.

How does a computer “decide”?

It doesn’t, really. A computer never thinks “hmm, should I turn on the light now that it’s dark?” What it does is far simpler and more mechanical: it asks a question you wrote for it in advance, the answer is only ever true or false, and based on that answer it takes one of two paths — again, the ones you laid out.

That is all there is to it. You do the same thing all day long:

• If it’s raining, take an umbrella.
• If the light is red, stop, otherwise go.
• If there’s enough money in the account, pay; otherwise cancel.

A conditional is how you teach this “if … then” pattern to a computer. The decision isn’t the computer’s — it’s yours; you write the rule and it applies it to the letter. Recall the rule from the first post: a computer isn’t smart, it’s obedient.

Single-branch conditional: “if this, do that”

The simplest conditional asks a single question; if the answer is true it does an extra step, if it’s false it does nothing and carries on. It has just one branch.

The shape: start with IF, write the question, say THEN, write the work on an indented line below, and close the block with ENDIF.

IF isRaining THEN
    PRINT "Take your umbrella"
ENDIF

PRINT "Go outside"

Read it top to bottom: “If it’s raining, take an umbrella. (Close the block.) Then, either way, go outside.” If it isn’t raining, the PRINT "Take your umbrella" line is skipped entirely and the program goes straight to Go outside. Remember this flowchart from the flowcharts post? What you see below is its written form:

flowchart TD
    A([Start]) --> B{is it raining?}
    B -- Yes --> C[Take your umbrella]
    B -- No --> D[Go outside]
    C --> D
    D --> E([End])

The diamond is IF, the “Yes” arrow is the indented block, and the point where the two paths meet is everything after ENDIF. The drawing and the text are two faces of one idea.

What’s inside a condition? A true/false question

What you write between IF and THEN is a question, and its answer must be either true or false. So where does that true/false come from? From something familiar from the variables post: a comparison.

We know the comparison operators from earlier posts; they are the fuel of conditionals:

Each line produces a true/false, and IF looks at that value to pick a path. For example, IF age ≥ 18 THEN really asks “is age ≥ 18 true?” and acts on the answer.

Double-branch conditional: “either this or that”

Often one path isn’t enough: if the condition is true we do one thing, if it’s false we do something else. This is where ELSE comes in. The block splits in two; exactly one of them runs, never both.

IF age ≥ 18 THEN
    PRINT "Ticket issued, enjoy the film"
ELSE
    PRINT "Sorry, this film is 18+"
ENDIF

If age is 20 the top line runs, if it’s 15 the bottom line runs — never both. It is a fork in the road; you take one branch only:

flowchart TD
    A([Start]) --> B{age ≥ 18?}
    B -- Yes --> C[/"Ticket issued" print/]
    B -- No --> D[/"18+ only" print/]
    C --> E([End])
    D --> E

Recall the even/odd example from the pseudocode post; that too was a double-branch conditional:

IF n MOD 2 = 0 THEN
    PRINT "Even"
ELSE
    PRINT "Odd"
ENDIF

MOD gives the remainder of a division; if a number’s remainder when divided by 2 is 0, it’s even. With a single conditional we split every number into two.

Multi-branch conditional: “one of several”

Sometimes two options aren’t enough either. Say we want to turn a score into a letter grade: 90 and up is A, the 80s are B, the 70s are C, below that is Fail. That’s four possible outcomes. For this we turn the conditions into a chain: ELSE IF.

IF score ≥ 90 THEN
    PRINT "A"
ELSE IF score ≥ 80 THEN
    PRINT "B"
ELSE IF score ≥ 70 THEN
    PRINT "C"
ELSE
    PRINT "Fail"
ENDIF

We drew this as a chain of decisions in the flowcharts post; now we have the same chain in written form. But there is a critical, easily missed rule here:

ELSE IF isn’t really a new thing; it’s shorthand for “if the previous condition was false, then ask this one.” The plain ELSE at the very end means “if none of the above matched” — a kind of safety net. In a multi-branch conditional it’s usually good practice to include that final catch-all.

Combining conditions: AND, OR, NOT

So far each conditional asked a single question. But real life often demands several conditions at once: “if it’s not raining and it’s warm, go for a picnic.” Those words “and,” “or,” “not” exist in the world of conditionals too, combining small questions into a single true/false answer. They are called logical operators.

AND — all must be true

AND is true only if all the conditions it joins are true; if even one is false, the result is false. It’s a strict, exacting gatekeeper; everyone’s ticket must be in order.

IF isStudent AND isWeekday THEN
    PRINT "Discounted ticket"
ELSE
    PRINT "Full-price ticket"
ENDIF

The discount applies only if you’re both a student and it’s a weekday. A student on the weekend gets no discount; a non-student on a weekday gets none either. AND means “both.”

OR — at least one will do

OR is true if at least one of the conditions it joins is true; it’s false only when all are false. It’s a lenient gatekeeper; a single true answer opens the gate.

IF day = "saturday" OR day = "sunday" THEN
    PRINT "It's a day off"
ELSE
    PRINT "It's a working day"
ENDIF

The day being either of the two counts as a day off. Note that everyday “or” sometimes means “one or the other, but not both” — in programming OR is not like that: if at least one is true, the result is true, even if both are.

NOT — flip the answer

NOT inverts a condition’s result: it turns true into false and false into true. It’s often used to phrase a “negative” situation more readably.

IF NOT loggedIn THEN
    PRINT "Please log in first"
ENDIF

NOT loggedIn reads as “if not logged in.” Writing IF loggedIn = false amounts to the same thing, but NOT often reads more smoothly.

Here are all three together, with their everyday meanings:

Nested conditionals: a decision inside a decision

Sometimes we only want to ask a question if another question came back “yes.” We place a second conditional inside the block of the first; this is a nested conditional.

Let’s first check whether a user is logged in, and then — only if they are — whether they’re an admin:

IF loggedIn THEN
    IF isAdmin THEN
        PRINT "Welcome to the admin panel"
    ELSE
        PRINT "Welcome"
    ENDIF
ELSE
    PRINT "Please log in"
ENDIF

The outer condition opens the door; only once you’re inside (logged in) is the inner question asked. If not logged in, the program never even asks “is admin?” — control never reaches it. Notice the inner block is indented one level deeper; here too, indentation shows what belongs to what.

Testing a condition on paper

Remember the trace table habit from the variables post. We do something similar with conditions: run the same condition with different inputs on paper and mark which branch we land in each time. The most valuable inputs are the boundary values — the exact thresholds.

Let’s try the ticket example (age ≥ 18) with a few values:

Pay special attention to the 18 row: because ≥ is “greater or equal,” a person exactly 18 gets the ticket. Had we written age > 18, an 18-year-old would be turned away — see how one symbol makes a huge difference at the boundary? Always test conditions at these threshold values; most bugs hide right there.

Common mistakes

Try it yourself

Pen and paper are all you need — no other tools. First write the condition in pseudocode, then run it on paper with a few different inputs and check that it lands in the right branch.

Exercise 1 — Pass or fail? (easy)

A student has an exam score. If the score is 50 or above, print “Pass”; below that, print “Fail”.

Exercise 2 — Free shipping (medium)

If a shopping cart’s total is 200 or more or the customer is a member, shipping is free; if neither is true, add a shipping fee of 30.

Exercise 3 — What to wear? (mini project)

Give advice based on temperature: 30 and above “t-shirt,” 15–29 “jacket,” below 15 “coat.” And if it’s raining — whatever the temperature — add “take an umbrella too” to the advice.

Summary