1. CMP 131 Introduction to Computer Programming
Violetta Cavalli-Sforza
Week 7, Lecture (Wednesday)

2. TODAY IF-THEN and IF-THEN-ELSE statements
Using compound statements with them
IF-THEN-ELSE for multiple choice
CASE statement for multiple choice

3. Yes/No Choice
The logic: IF some condition or test is TRUE THEN perform some action [ ELSE do nothing ]
Examples:
If the car is running out of gas, put in gasoline.
If you are failing your class and you cannot drop it, study more.
If someone says hello and you don’t want to seem impolite, say hello back.

4. Yes/No Choice: The IF-THEN Statement
Syntax: IF <condition> THEN <statement>
Syntax graph:
Flowchart: IF
condition
THEN
statement
statement 1
yes
condition
statement 2 no
statement 3

5. {File: ABSVAL1.PAS}
PROGRAM AbsoluteValue;
VAR Number : real;
BEGIN
writeln('Type a number, then press enter: ');
readln(Number);
write('The absolute value of ', Number, ' is ');
IF Number < 0 THEN Number := -Number;
writeln(Number);
readln;
END.

6. Either/Or Choice
The logic: IF some condition or test is TRUE THEN perform some action ELSE perform a different action
Examples:
If the weather is hot wear light clothes, else wear warm clothes.
If a number is odd increment OddCount else increment EvenCount
If a noun starts with a capital letter categorize it as a proper noun (name) else as a common noun.

7. Either/Or Choice : The IF-THEN-ELSE Statement
Syntax: IF <condition> THEN <statement> ELSE <statement>
Syntax graph:
Flowchart: IF
condition
THEN
statement
ELSE
statement 1
condition
yes
statement 4
statement 2 no
statement 3

8. {File: ABSVAL2.PAS}
PROGRAM AbsoluteValue;
VAR Number : real;
BEGIN
writeln('Type a number, then press enter: ');
readln(Number);
write('The absolute value of ', Number, ' is ');
IF Number < 0
THEN writeln(-Number)
ELSE writeln(Number);
readln;
END.

9. IF-THEN(-ELSE) and Compound Statements
Conditional statements:
IF <condition> THEN <statement>
IF <condition> THEN <statement> ELSE <statement>
What is <statement>?
A single statement:
An input or output statement
An assignment
Another IF … THEN … ELSE statement (nested conditional statements)
A looping statement (FOR, WHILE, REPEAT) …
A compound statement:
BEGIN <statement> ; <statement> ; …. <statement>
END

10. {File: ORDER1.PAS}
PROGRAM Order2Numbers;
VAR Number1, Number2 : real;
BEGIN
writeln('Enter 2 numbers, then press ENTER: ');
readln(Number1, Number2);
IF Number1 < Number2
THEN writeln('The ordered numbers are:', Number1, Number2)
ELSE writeln('The ordered numbers are:', Number2, Number1);
readln
END.

11. {File: ORDER2.PAS}
PROGRAM Order2Numbers;
VAR Number1, Number2, Temp : real;
BEGIN
writeln('Enter 2 numbers, then press ENTER: ');
readln(Number1, Number2);
IF Number2 < Number1 THEN
Temp := Number2;
Number2 := Number1;
Number1 := Temp;
END;
writeln('The ordered numbers are:', Number1, Number2);
readln
END.

12. Ordering 3 numbers… What can happen?
Foo =< Bar =< Nem
Nem =< Foo =< Bar
Foo =< Nem =< Bar
Bar =< Foo =< Nem
Nem =< Bar =< Foo
Bar =< Nem =< Foo
So you have to test all the possible input combinations!!!

13. {File: ORDER3.PAS}
PROGRAM Order3Numbers;
VAR Foo, Bar, Nem, Temp : real;
BEGIN
writeln('Enter 3 numbers, then press ENTER: ');
readln(Foo, Bar, Nem);
IF Foo < Bar THEN
IF Bar < Nem THEN
writeln('The ordered numbers are:', Foo, Bar, Nem)
ELSE
IF Nem < Foo THEN
writeln('The ordered numbers are:', Nem, Foo, Bar)
writeln('The ordered numbers are:', Foo, Nem, Bar)
ELSE { Foo >= Bar }
IF Foo < Nem THEN
writeln('The ordered numbers are:', Bar, Foo, Nem)
IF Nem < Bar THEN
writeln('The ordered numbers are:', Nem, Bar, Foo)
writeln('The ordered numbers are:', Bar, Nem, Foo);
readln
END.

14. Test Set Foo Bar Nem Foo < Bar < Nem 1 2 3 Nem < Foo < Bar
Foo = Nem = Bar

15. If you want to add a statement
{File: ORDER3.PAS}
PROGRAM Order3Numbers;
VAR Foo, Bar, Nem, Temp : real;
BEGIN
writeln('Enter 3 numbers, then press ENTER: ');
readln(Foo, Bar, Nem);
IF Foo < Bar THEN
writeln(‘Foo < Bar’);
IF Bar < Nem THEN
writeln('The ordered numbers are:', Foo, Bar, Nem)
ELSE
IF Nem < Foo THEN
writeln('The ordered numbers are:', Nem, Foo, Bar)
writeln('The ordered numbers are:', Foo, Nem, Bar)
END
ELSE { Foo >= Bar }
writeln(‘Foo >= Bar’);
IF Foo < Nem THEN
writeln('The ordered numbers are:', Bar, Foo, Nem)
IF Nem < Bar THEN
writeln('The ordered numbers are:', Nem, Bar, Foo)
writeln('The ordered numbers are:', Bar, Nem, Foo);
readln
END.
compound statement
compound statement