1

Simple Flow of Control

CSIS1117 Computer Programming

c1117 lecture 3 2

Contents Selection control If-statement Data type -- bool If/else statement Dangling else Logical operators Short-circuit evaluation

c1117 lecture 3 3

Statements So far, we have learned to write simple programs,

which can interact with users. However, in many cases, we want some

statements to be executed only if certain criteria are satisfied. e.g. we need to check if y is zero to prevent the run time

error.

int x, y;cin >> x >> y; cout << x/y << endl;

We will have an runtime error if y=0

c1117 lecture 3 4

Selection control The problem can be solved by attaching a test

expression (condition). If the condition is true, the statement is executed. Otherwise, the statement is skipped.

int x, y;cin >> x >> y; if( y != 0) cout << x/y << endl;if( y == 0) cout << "The dividend can’ t be zero" << endl;

Test expression

Only execute if the test expression is true

not equal equal

c1117 lecture 3 5

if - statement

We can use an if-statement to determine whether certain code is required to execute.

An if-statement consists of a test expression that evaluates to true or false and the body.

The body (consequence) will be executed only if the test expression is true.

See cal-div.cc as an example

if(test expression) body;

c1117 lecture 3 6

Data type -- bool We’ ve discussed the data type int, double, and string, they use to represent different values. int: 7, -3, 1000, etc … double: 1.2, -3.45, 0.012, etc … string: "Peter", "Hello World!", etc …

bool – only for storing true or false value. Test expression has the value of type bool.

bool isZero = (y == 0);

c1117 lecture 3 7

A test expression is usually formed by comparing two numbers with Relational Operator. e.g. a>b, a>=b, a<=b, a<b, a==b, a!=b

bool isZero = (y == 0);if(isZero) cout << "The dividend can’ t be zero" << endl;

!= means NOT equal

== means equal

Equal to:if(isZero == true)

c1117 lecture 3 8

Block statement

Suppose now we want to print two lines if y is zero.

// This is wrong !! if(y == 0) cout << "The dividend can’ t be zero" << endl; cout << " ^^^^^^^^ " << endl;

This statement is executed independent of the result of the test expression

Note that the body of an if-statement is a single statement.

c1117 lecture 3 9

We can group one or more statements to form a block statement delimited by a pair of braces. A block statement is a single statement. We can even declare variables inside a block No need to put a semi-colon after a block.

// This is correct!if(y == 0){ cout << "The dividend can’ t be zero" << endl; cout << " ^^^^^^^^ " << endl;}

Specifying a block statement

c1117 lecture 3 10

if/else statement

Sometimes we want the program to choose only one of two alternatives.

int x, y;cin >> x >> y; if( y != 0) cout << x/y << endl;if( y == 0){ cout << "The dividend can’ t be zero" << endl; cout << " ^^^^^^^^ " << endl;} No matter what value of y, only one

of the bodies is executed.

c1117 lecture 3 11

The two alternatives are mutually exclusive, the input number y is either equal to zero (y==0) or not equal to zero (y!=0), but not both.

Using if/else-statement can represent this idea naturally and more clearly.

c1117 lecture 3 12

int x, y;cin >> x >> y; if( y == 0){ cout << "The dividend can’ t be zero" << endl; cout << " ^^^^^^^^ " << endl;}else cout << x/y << endl;

if/else statement

See cal-div-else.cc as an example

Execute if the test expression is false

Execute if the test expression (y==0) is true

c1117 lecture 3 13

More on test expression A test expression can be formed by comparing two

values.

// Examples on test expressions// x, y and z are numbers (x >= y);(x + z > y);(x + z > y * z);

Arithmetic expression also has a value, it will be evaluated before the comparison.

c1117 lecture 3 14

Dangling “ else” problem

if (x > 0) if (x % 2 == 0)

cout << "It is a positive even number"; else

cout << "It is a positive odd number";

Are they the same? Which one will give the wrong answer?

Consider the following two programs:

if (x > 0) if (x % 2 == 0)

cout << "It is a positive even number"; else cout << "It is a non-positive number";

c1117 lecture 3 15

Dangling “ else” problem

if (x > 0) if (x % 2 == 0)

cout << "It is a positive even number"; else

cout << "It is a positive odd number";

The second program give wrong answer, why?

if (x > 0) if (x % 2 == 0)

cout << "It is a positive even number"; else cout << "It is a non-positive number";

// if x = 10 It is a positive even number // if x = 7It is a positive odd number

// if x = 10 It is a positive even number // if x = 7It is a non-positive number

c1117 lecture 3 16

An else is always matched with the nearest preceding unmatched if.

We need to use parentheses to indicate the else for the outer if. So the second program should be modified as follow:

if (x > 0){ if (x % 2 == 0)

cout << "It is a positive even number"; }else cout << "It is a non-positive number";

See checking.cc

c1117 lecture 3 17

Example – leap year Write a program to determine whether a year is a

leap year. How to check if a year is a leap year?

1997 1996 1994 A leap year is divisible by 4

cin >> year;if (year % 4 == 0) // may be a leap year else cout << "It is not a leap year“ << endl;

c1117 lecture 3 18

Example – leap year How about … 1900

But it is divisible by 4 !! A leap year is divisible by 4, but not divisible by

100cin >> year;if (year % 4 == 0){ if (year % 100 == 0)

// may not be a leap year else

cout << "It is a leap year" << endl; }else cout << "It is a not a leap year" << endl;

c1117 lecture 3 19

Example – leap year How about … 2000

But it is divisible by 4 and 100 !!

A leap year is divisible by 4, but not divisible by 100. However, if it is divisible by 400, it is still a leap year.

c1117 lecture 3 20

Example – leap year

See leap-year.cc and leap-year-bool.cc

cin >> year;if (year % 4 == 0){ if (year % 100 == 0){

if (year % 400 == 0) cout << "It is a leap year" << endl;

else cout << "It is not a leap year" << endl;

}else cout << "It is a leap year" << endl;

}else cout << "It is not a leap year" << endl;

c1117 lecture 3 21

Logical operators

A test expression sometimes consists of other “ sub-expressions” , e.g. If your exam score is at least 70 and less than 75, you get a

B+ If you fail in the quiz or exam, you will receive a warning letter

Test expressions can be combined using logical operators, e.g. && (AND), || (OR) and ! (NOT). A logical operator applies on bool values and the result is

also of type bool.

c1117 lecture 3 22

We can simplify the program code by using Logical operator. cin >> mark;if (mark >= 70) if (mark < 75)

cout << “ You get a B+“ << endl;

cin >> mark;if (mark >= 70 && mark < 75)

cout << “ You get a B+“ << endl;

Simplified to

c1117 lecture 3 23

Note that a range condition, such as a<= x < b, has to be written in forms like (a<=x)&&(x<b) See range.cc and range-logical.cc

Common mistakes: Mixing “ =” with “ ==” , “ |” with “ ||” and “ &” with “ &&” These logical errors are difficult to detect as they cannot

be reported by the compiler. See misuse.cc

c1117 lecture 3 24

Operator Precedence

highest

lowest

prio

ri ty

! Logical NOT

*, /, %+, − Binary operators

<, <=, >, >===, !=

Relational operators

&& Logical AND || Logical OR

You can use parentheses to force the order of evaluation.

c1117 lecture 3 25

Add parentheses to the following expressions to show the precedence of the computation.

x + 3 >= 9 || z != x * 2 && !(y / 2 < 3)

x + 3 >= 9 || z != x * 2 && ! y / 2 < 3

( ) ( )( )

( ) ( )( )

()

()

c1117 lecture 3 26

Short-circuit evaluation are the following programs the same?

if(scores / count < 60 && count != 0) cout << "low average warning" << endl;

if(count != 0 && scores / count < 60) cout << "low average warning" << endl;

No!! They are not the same, the second one will give error if count is zero.

Why?

c1117 lecture 3 27

Sub-expressions in test expression are not evaluated if the value of the entire expression is already known. E.g. A test expression (expr1) && (expr2) is false if at

least one of expr1 and expr2 is false. Therefore, if the expr1 is known to be false, the expr2 will not be evaluated as the result of the entire expression is known after evaluating expr1.

The short-circuit feature is useful in simplifying some program code. However, you need to consider the order of the sub-expressions

Short-circuit evaluation

c1117 lecture 3 28

Short-circuit evaluation

if(scores / count < 60 && count != 0) cout << "low average warning" << endl;

if(count != 0 && scores / count < 60) cout << "low average warning" << endl;

It is evaluated first, if it is false, the second sub-expression (scores/count<60) is not evaluated.

If count is zero, this sub-expression will cause run time error.

c1117 lecture 3 29

Conditional operator

If expr0 is evaluated to true, Evaluate expr1 to be the value of the whole expression Otherwise, evaluate expr2 to be the value of the whole

expression. Since it is an expression, it can appear inside a

statement. See conditional-op.cc

Syntax expr0 ? expr1 : expr2

This is the only ternary operator in C++.