Week 3: Conditional Statements Introduction to Programming Lecturer: Qun Liu

CA146/CA247A
Introduction to Programming
Week 3: Conditional Statements
Lecturer: Qun Liu
Lab Tutor: Dasha Bogdanova
2nd Semester, 2014-2015 Academic Year
http://computing.dcu.ie/~qliu/CA146-CA247A
Content
Decisions
Combining logical tests
Negation
if…else
switch
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if statement
We can use an if statement to test whether something is true.
The thing we are testing is called an expression and is surrounded
by round brackets: ( ).
If the expression
evaluates to true
then the instructions
between the following
set of curly brackets { }
are executed.
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if statement
This program prompts
the user to enter a
number and
• if that number is
greater than zero the
program prints out a
message saying so.
• If the number
entered fails the test
(i.e. the expression
evaluates to false)
then the program
just exits.
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logical operator
The symbol > is a logical operator.
Here are some others:
>= greater than or equal to
< less than
<= less than or equal to
== equal to
! = not equal to
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logical operator
Note we use double equals == to test for equality (remember
we use a single equals = to assign a value).
For example this statement tests whether num
(i.e. the value currently assigned to num) is equal to zero:
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Content
Decisions
Combining logical tests
Negation
if…else
switch
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Combining logical tests
• This program waits for the user to input a number and then
tests whether it lies between 0 and 10.
• •An expression which
uses the logical
operator && will only
be true if both parts of
the expression are
true.
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Combining logical tests
• An expression which uses the logical operator | | will be true
if either part of the expression is true.
• For example this statement returns true if 𝑛𝑢𝑚 is less than
zero or greater than 10:
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Content
Decisions
Combining logical tests
Negation
if…else
switch
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Negation
• The logical operator ! negates the expression following it.
• For example:
is true if 𝑛𝑢𝑚 is not greater than zero.
It should be read “if it is false that 𝑛𝑢𝑚 is greater than zero”.
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Integer variables
as logical variables
• Integer variables can be regarded as logical variables:
is true if 𝑛𝑢𝑚 is not zero and:
is true if 𝑛𝑢𝑚 is equal to zero.
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Content
Decisions
Combining logical tests
Negation
if…else
switch
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if …else
This program prompts
the user to enter a
number and then
performs one of two
different actions
depending on the
value of the number.
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if …else
We can chain together
multiple if … else statements
to test against a succession of
possible conditions as follows:
As soon as one condition
evaluates to true the
corresponding actions are
executed and the program
exits the if … else. If none of
the conditions evaluate to true
then the statements in the
final else block are executed.
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Content
Decisions
Combining logical tests
Negation
if…else
switch
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switch
Sometimes it is handier to
use a switch statement
rather than a succession
of if … else statements:
This program prompts
the user to enter an
integer between 1 and 3
and stores it in a variable
called 𝑐ℎ𝑜𝑖𝑐𝑒.
It then carries out a
different action for each
possible value of 𝑐ℎ𝑜𝑖𝑐𝑒.
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switch
The different possible
values of 𝑐ℎ𝑜𝑖𝑐𝑒 appear
after the word case.
Each action set is
terminated by the word
break which causes the
program to exit the
switch statement.
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switch
A default action is
usually supplied which is
carried out if none of the
previous case conditions
are applicable.
The switch statement is
often used in simple
menu driven programs.
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Exercises
•
•
•
•
•
•
•
•
•
•
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Ordering numbers
Minimum of three numbers
Water
Digits
Days in a month
The elevator
Grades
Even and odd numbers
Triangles
Leap years
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Ordering numbers
• Write a program that reads in two numbers.
The program should:
• Print “bigger” if the second number is bigger
than the first
• Print “smaller” if the second number is
smaller than the first
• Print “equal” if the numbers are equal
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Minimum of three numbers
• Write a program that reads in three floating
point numbers and prints out the minimum.
• If there is no unique minimum value the
program should print nothing.
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Water
• Write a program that reads in a temperature
(in Celsius).
• The program should print out the state water
occupies at that temperature i.e. solid, liquid
or gas.
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Digits
• Write a program that reads in a positive
number and prints out the number of digits in
the number.
• If the number is greater than 9999 print a
message to say that numbers greater than
9999 are not handled.
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Days in a month
• Write a program that reads in a month
number (1 for January, 2 for February etc.)
and prints out the number of days in that
month.
• Do not use a separate if statement for each
month but combine all months that have the
same number of days into a single check.
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The elevator
• A building has 19 floors numbered 1 through 20
(13 is excluded because it is too unlucky).
• Write a program that reads in a floor number and
checks its validity. The program should print out an
error message (and exit) if any of the following are
true:
– 13 is entered
– Zero or a negative number is entered
– A number greater than 20 is entered
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Grades
• Write a program that reads in an exam mark
and prints out the corresponding grade. The
rules are as follows:
–
–
–
–
–
0-39 is an F
40-54 is a D
55-69 is a C
70-84 is a B
85-100 is an A
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Even and odd numbers
• Write a program that reads in a number. The
program should:
– Print “even” if the number is even
– Print “odd” if the number is odd
• Hint: Make use of the modulus operator % in
your solution.
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Triangles
• Write a program that reads in the lengths of each of
the three sides of a triangle. The program should:
– Check if any of the numbers are less than or equal to zero
and exit if so
– Confirm that the length of each side is less than the sum of
the other two. If this is not the case a triangle cannot be
formed and the program should exit.
– Check whether the lengths form an equilateral triangle
(all sides equal).
– Check whether the lengths form an isosceles triangle (two
sides equal).
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Leap years
• Write a program that checks whether it is a
leap year. It is a leap year if:
– The year is divisible by 4 (e.g. 1980), but,
– it is not a leap year if divisible by 100 (e.g. 1900),
unless,
– the year is divisible by 400 (e.g. 2000)
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Discussion
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Acknowledgement
• Most of the content of this lecture is based on
previous lecture given by Gerard Marks.
– http://student.computing.dcu.ie/~ca146lab/html/index.html
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