Chapter 3: Control Structures

Control structures decide the flow of a program: which statements run, and how many times. Pascal has several kinds, including conditional statements and loops.

Conditional statements

If-then statement

The if-then statement runs a block of code only when a condition is true.

if condition then
  statement;

Example

program IfThenExample;

var
  num: integer;

begin
  num := 10;
  
  if num > 5 then
    writeln('num is greater than 5');
end.

If-then-else statement

With if-then-else, a condition decides which of two blocks of code runs.

if condition then
  statement1
else
  statement2;

Example

program IfThenElseExample;

var
  num: integer;

begin
  num := 10;
  
  if num > 15 then
    writeln('num is greater than 15')
  else
    writeln('num is not greater than 15');
end.

Case statement

The case statement picks one block of code out of many, based on the value of a variable.

case variable of
  value1: statement1;
  value2: statement2;
  ...
  else statementN;
end;

Example

program CaseExample;

var
  grade: char;

begin
  grade := 'B';
  
  case grade of
    'A': writeln('Excellent');
    'B': writeln('Good');
    'C': writeln('Fair');
    'D': writeln('Poor');
    'F': writeln('Fail');
  else
    writeln('Invalid grade');
  end;
end.

Loops

A loop runs a block of code repeatedly.

For loop

The for loop runs a block of code a fixed number of times.

for variable := start_value to end_value do
  statement;

Example

program ForLoopExample;

var
  i: integer;

begin
  for i := 1 to 10 do
    writeln(i);
end.

While loop

A while loop keeps repeating its block of code as long as a condition is true.

while condition do
  statement;

Example

program WhileLoopExample;

var
  i: integer;

begin
  i := 1;
  
  while i <= 10 do
  begin
    writeln(i);
    i := i + 1;
  end;
end.

Repeat-until loop

The repeat-until loop runs its block of code until a condition becomes true.

repeat
  statement;
until condition;

Example

program RepeatUntilExample;

var
  i: integer;

begin
  i := 1;
  
  repeat
    writeln(i);
    i := i + 1;
  until i > 10;
end.

Nested loops

You can also nest loops by placing one loop inside another.

Example

program NestedLoopExample;

var
  i, j: integer;

begin
  for i := 1 to 3 do
  begin
    for j := 1 to 3 do
    begin
      writeln('i = ', i, ', j = ', j);
    end;
  end;
end.

Breaking out of loops

The break statement exits a loop early. The continue statement skips the rest of the current iteration and moves on to the next one.

Example: using break

program BreakExample;

var
  i: integer;

begin
  for i := 1 to 10 do
  begin
    if i = 5 then
      break;
    writeln(i);
  end;
end.

Example: using continue

program ContinueExample;

var
  i: integer;

begin
  for i := 1 to 10 do
  begin
    if i = 5 then
      continue;
    writeln(i);
  end;
end.