CSCI 150, Spring 2003

Conditionals (if ... then ... else)

Programming Decision Trees
In Pascal, a decision is made using the if ... then ... else statement (also called a conditional statement).

Example:

	program Diagnose;
	var
		answer: string;
	begin
		writeln('Does your head ache?');
		readln(answer);
		if answer = 'yes' then
		begin
			writeln('Take two aspirin');
		end
		else
		begin
			writeln('Good!');
		end;
	end;

General form of if ... then ... else:

	if Condition then
	begin
		Pascal Code A
	end
	else
	begin
		Pascal Code B
	end;

Other types of conditions:
Conditions can be any expression that has a value of true or false.

Example:

	program Compare;
	var
		x, y: integer;
	begin
		writeln('Enter two integers');
		readln(x);
		readln(y);
		if x > y then
		begin
			writeln(x, ' is greater than ', y);
		end
		else
		begin
			writeln(y, ' is greater than ', x);
		end;
	end.

Programming Three way Decision Trees
We can program a three-way decision by adding an else if ... to the if ... then ... else statement.

Example:

	program medals;
	var
		answer: string;
	begin
		writeln('Did you get first, second or third?');
		readln(answer);
		if answer = 'first' then
		begin
			writeln('You get the gold medal!');
		end
		else if answer = 'second' then
		begin
			writeln('You get the silver medal.');
		end
		else
		begin
			writeln('You get the bronze medal.');
		end;
	end.

Note that the second condition is tested in the else if clause in the middle.

If statements within other if statements (Nested Conditionals):
When a decision tree has several decision nodes along a single branch, this requires another if statement within the original if statement. This is called a nested conditional.

Example:

	program skiSlopes;
	var
		skiAnswer, downHill : string;
	begin
		writeln('Do you like to ski?');
		readln(skiAnswer);
		if skiAnswer = 'yes' then
		begin
			writeln('Downhill or cross country?');
			readln(downHill);
			if downHill = 'downhill' then
			begin
				writeln('Try Wachusett Mountain');
			end
			else
			begin
				writeln('Try the local trails');
			end;
		end
		else
		begin
			writeln('That is too bad!');
		end;
	end.