Decisions with if and else

A condition is a boolean

The condition after if is any expression of type boolean: a comparison, a variable, or comparisons joined with and, or and not. Comparisons use =, <>, <, >, <= and >=.

Two of those are worth stopping on. = compares and := assigns; confusing them is the first mistake everyone makes, and the compiler catches it because the types do not match. <> means not equal, since Pascal has no bang or double equals.

Text compares too, letter by letter, so 'Ada' < 'Bob' is true. That makes sorting names a plain comparison rather than a call to a library.

The type matters because the compiler will not accept a number where a boolean belongs. if count then is a type error even though other languages treat zero as false; write if count > 0 then and say what you mean. The same strictness catches a comparison written with a single = in the wrong place: the compiler sees a boolean where a statement belongs and stops.

if, then, else

The shape is if condition then statement else statement. Both branches take exactly one statement, and only one of them runs.

There is no semicolon before else. A semicolon ends a statement, and the if is not finished until its else has been read, so putting one there ends the if early and leaves the else with nothing to belong to.

Chains work by nesting: if ... then ... else if ... then ... else .... Pascal reads them exactly as written, and the indentation in the examples is a reading aid rather than something the compiler needs.

An if is a statement, not an expression: it does something rather than producing a value, so it cannot sit on the right of an assignment. When a value has to be chosen between two options, write the assignment inside each branch, or move the choice into a function that returns the answer.

Two statements need a block

When a branch should do more than one thing, wrap the statements in begin … end. The block counts as one statement, so it fits where a single statement goes, and the semicolons inside it separate its own statements.

This is the only place a beginner needs begin and end in the middle of a program: for a group of statements where one is expected. The same block used after then needs no semicolon after its final end when an else follows.

A chain and a block

program Main;
var
  score: integer;
begin
  score := 72;
  if score >= 90 then
    writeln('A')
  else if score >= 70 then
  begin
    writeln('B');
    writeln('Solid pass');
  end
  else
    writeln('C');
end.

Which if owns the else

An else belongs to the nearest if above it that does not already have one. Indentation is invisible to the compiler, so a nested if can steal the else you meant for the outer branch even when the layout says otherwise. A begin … end around the inner branch settles the question.

and and or have an order too: not binds tightest, then and, then or. Comparisons bind tighter than all three, so score > 0 and score < 100 groups as two comparisons joined by and. When a condition mixes and with or, put the grouping you mean in brackets; the compiler will not infer it from the line breaks.

Boundaries are where branches go wrong

A chain of comparisons is only correct if the cases meet without gaps or overlaps. With score >= 90, score >= 70 and a final else, a score of 70 takes the second branch and a score of 69 takes the third; nothing falls between them. Reverse the order and the first test swallows everything above 70, so the later branches can never run.

Pick the boundaries first and write them down: 90 and above is an A, 70 to 89 is a B, below 70 is a C. Then write the tests from the most specific to the most general, and check the exact boundary values — 69, 70, 89, 90 — rather than a comfortable middle like 75. Bugs live at the edges, and a middle value exercises none of them.

Try these next

• Rewrite the grade chain so a score of exactly 70 prints the B branch, and check the boundary.
• Add an inner if without begin and end, predict which branch takes the else, then fix it with a block.
• Join two comparisons with or and add brackets so the meaning is obvious to a reader.