Chapter 7 Selection Dept of Computer Engineering Khon Kaen University

Chapter 7 Selection

Dept of Computer EngineeringKhon Kaen University

178110: Computer Programming (II/2546) 2

Major Concepts The if statement The if...else… statement Statement blocks Compound conditions Short-circuiting Boolean expressions Nested selection statements The switch statement


Introduction The programs in the previous

chapters have sequential execution Each statement in the program

executes once, and they are executed in the same order that they are listed

What if we want some statements to be executed only if some condition is true?


The if Statement The if statement allows conditional

execution Its syntax is

if (condition)a block of statements

condition is an integral expression a block of statements consists of one or

more executable statements The statement will be executed only if the

value of the integral expression is nonzero


Flow Chart of “if” Statement

condition

statement

true

false


An “if” Exampleint main(){

int n, d;cout << “Enter two positive integers: “;cin >> n >> d;if (n%d)

cout << n << “ is not divisible by “ << d << endl;

return 0;}


An “if” Example (Cont.) if (n%d)

cout << n << “ is not divisible by “ … In which condition, the above cout

statement is executed? n = 66, d = 7 n = 6, d = 3

In C++, whenever an integral expression is used as a condition, The value 0 means “false” All other values mean “true”


The if…else… Statement What if we want to have the cout

statement when n is divisible by n? We need to have “if …else…”

statement The if…else… statement causes one

of two alternative statements to execute depending upon whether the condition is true


If…else… Statement (Cont.)

Its syntax isif (condition)

statement1;else

statement2; If the value of the condition is nonzero

then statement1 will execute; otherwise statement2 will execute


Flow Chart of “if…else…”

condition

statement 1

true

false

statement 2


An “if…else…” Exampleint main(){

int n, d;cout << “Enter two positive integers: “;cin >> n >> d;if (n%d)

cout << n << “ is not divisible by “ << d << endl;

elsecout << n “ << is divisible by “ << d <<

endl;return 0;

}


An “if…else” Example (Cont.)

if (n%d) cout << n << “ is not divisible by “

…else

cout << n << “ is divisible by “ … In these conditions, which cout statement

is executed? n = 66, d = 7 n = 6, d = 3

In “if…else…” clause, how many statement is there? One


Keywords in C++ A keyword is a word that is already

defined and is reserved for a unique purpose in programs written in that language

Standard C++ now has 74 keywords Examples of C++ keywords

if else int bool float double


Keywords in C++ There are two kinds of keywords

Reserved words Standard identifiers

A reserved word is a keyword that serves as a structure marker, used to define the syntax of the language Examples: if else

A standard identifier is a keyword that names a specific element of the language Examples: int bool


Comparison Operators The six comparison operators are

x < y // x is less than yx > y // x is greater than yx <= y // x is less than or equal to yx >= y // x is greater than or equal to yx == y // x is equal to yx != y // x is not equal to y

These can be used to compare the values of expressions of any ordinal type


Comparison Example1 int main(){

int m, n;cout << “Enter two integers: “;cin >> m >> n;if (m < n)

cout << m << “ is the minimum “ << endl;else

cout << n << “ is the minimum “ << endl;

}


Comparison Example2int a = 4;float b = 3.4;char c = 'c';if (a < b)

cout << "a is less than b" << endl;else

cout << "a is greater than or equal to b" << endl;

if (a < c) cout << "a is less than c" << endl;else

cout << "a is greater than or equal to b" << endl;


What is Wrong?int main(){

int n;cout << “Enter an integer:”;cin >> n;if (n = 2)

cout << n << “ = 2 “ << endl;else

cout << n << “!= 2 “ << endl;} (n = 2) assigns the value 2 to n (n == 2) compares the values of n and 2


Different Kinds of Errors Compile-time error: syntax error, e.g.,

forgetting a semicolon at the end of the statement Caught by a compiler

Run-time error: error occurs when running the program, e.g., diving a number by zero Caught by an operating system

Logical error: error in which the program does not work as the programmer wishes, e.g., typing ‘=‘ instead of ‘==‘ when comparing two numbers Cannot be caught by anything


The Minimum of Three Integers

#include <iostream>Using namespace std;int main(){ // finds the minimum of three input integers:

int n1, n2, n3;cout << “Enter three integers:”;cin >> n1 >> n2 >> n3;int min=n1; // now min = n1if (n2 < min)

min = n2; // now min <= n1 and min = n2if (n3 < min)

min = n3; // now min <= n1, min <= n2, and min = n3cout << “Their minimum is “ << min << endl;return 0;

}


The Minimum of Three Integers (Cont.)

int min=n1; // now min = n1if (n2 < min)

min = n2; // now min <= n1 and min = n2if (n3 < min)

min = n3; // now min <= n1, min <= n2, and // min = n3

After the first if statement executes, min is the minimum of the set {n1, n2}

After the last if statement executes, min is the minimum of the set {n1, n2, n3}


Statement Blocks A statement block is a sequence of

statements enclosed by { } A statement block can be used anywhere

that a single statement can be used Example:{

int temp = x;x = y;y = temp;

}


Example: Statement Blocks

#include <iostream>using namespace std;int main(){

int x, y;cout << "Enter two integers: ";cin >> x >> y;if (x > y){

int temp=x; x = y;y = temp;

} // swap x and ycout << x << " <= " << y << endl;return 0;

}


Example: Statement Blocks (Cont.)

if (x > y) { int temp=x; x = y; y = temp; } // swap x and

y The three statements are within the same

statement block Temp is declared inside the block. That

makes it local to the block, i.e. it only exists during the execution of the block

If the condition is false, then does temp exist?

No


Using Blocks to Limit Scope

1. int n = 44;2. cout << “n = “ << n << endl;3. {4. int n = 2; cout << “n = “ << n << endl;5. }6. {7. cout << “ n = “ << n << endl;8. }9. {10. int n; cout << “n = “ << n << endl;11. }12. cout << “n = “ << n << endl; What are the values of n at line 2, line 4, line 7, line

10, and line 12?


Compound Conditions Conditions such as (n % d) and (x >=

y) can be combined to form compound conditions

This is done using the logical operators && (and) || (or) ! (not)


Compound Conditions (Cont.)

p && q evaluates to true If and only if both p and q are evaluated

to true p || q evaluates to true

If and only if either p or q is true !p evaluates to true

If and only if p evaluates to false (2 < 0) || (3 > 2) evaluates to?

true


Three Logic Operators

p q p && q

p || q

F F F F

F T F T

T F F T

T T T T

Let T stands for True and F stands for False

p !p

F T

T F


Using Compound Conditions

int main(){

int n1 = 1, n2 = 2, n3 = 3;if (n1 <= n2 && n1 <= n3)

cout << “n1 is less than n2 and n3”;if (n1 <= n2 || n2 <= n3)

cout << “n1 is not the greatest number”;if (!(n1 – 1))

cout << “n1 is ” << n1;


Short-Circuiting Compound conditions that use && and ||

will not even evaluate the second operand of the condition unless necessary.

Short-circuiting: can evaluate the condition without evaluating every terms

For p && q, what is the value of p that makes us not need to evaluate q? p is false


Short-Circuiting (Cont.) For p || q, what is the value of p that

makes us not need to evaluate q? p is true

How do we use the short-circuiting to solve the problem of dividing a number by zero? How do we prevent the program not to

evaluate n%d when d = 0?


Short-Circuit (Cont.)int main(){

int n, d;cout << “Enter two positive integers: “;cin >> n >> d;if ((d != 0) && (n%d == 0))

cout << “n is divisible by d” << endl;else

cout << “n is not divisible by d” << endl;return 0;

} When d is zero the expression, n%d will not be

evaluated


Boolean Expressions A boolean expression is a condition

that is either true or false Boolean expressions evaluate to

integer values The value 0 means “false” and every

nonzero value means “true” Since all nonzero integer values are

interpreted as meaning “true”, boolean expressions are often disguised


Boolean Expressions (Cont.)

For example, the statementif (n)

cout << “n is not zero”;will print “n is not zero” precisely when

n is not zero because this is when the boolean expression (n) is interpreted as “true”

Since boolean expressions have integer values, it can lead to some surprising anomalies in C++


Another Logical Errorint main(){

int n1, n2, n3;cout < “Enter three integers: “;cin >> n1 >> n2 >> n3;if (n1 >= n2 >= n3)

cout << “max = ” << n1;} What is the output when n1 = 0, n2 = 0, n3

=1? max = 0


The Source of the Logical Error

1. n1 = 0, n2 = 0, n3 = 12. if (n1 >= n2 >= n3)3. cout << “max = “ << n1; The source of this error is the fact that

boolean expressions have numeric values The expression (n1 >= n2 >= n3) is

evaluated from left to right The first part n1 >= n2 evaluates to true since

0 >= 0


Source of Logical Error (Cont.)

The expression (n1 >= n2 >= n3) is evaluated from left to right (n1 >= n2) “true” “true” is stored as the numeric value 1 This value is then compared to the value

of n3 which is also 1 (1 >= n3) (1 >= 1) true

The complete expression evaluates to “true” even though it is really false!


Nested Selection Statements

Selection statements can be used wherever any other statement can be used

Thus, a selection statement can be used within another selection statement

This is called nesting statements


Nesting Selection (Cont.)if (d != 0)

if (n%d == 0)cout << n << “ is divisible by “

<< d;else

cout << n << “ is not divisible by “ << d;

elsecout << “d is zero”;


Coding Style Code fragment 1

if (a > 0) if (b > 0) ++a; else c--; Code fragment 2

if (a > 0){

if (b > 0)++a;

elsec--;

} Which one is better? Which one is more

readable?


Using Nesting Selection How do we find the minimum of (n1, n2,

n3) by using “if … else” statements?if (n1 < n2)

if (n1 < n3)cout << “Their minimum is “ << n1;

elsecout << “Their minimum is “ << n3;

elseif (n2 < n3)

cout << “Their minimum is “ << n2;else

cout << “Their minimum is “ << n3;


Multiple Conditionsif (score >= 90)

cout << “Your grade is A” << endl;else

if (score >= 80)cout << “Your grade is B” << endl;

else if (score >= 70)

cout << “Your grade is C” << endl;

elsecout << “Your grade is D” <<

endl;


The else if Construct It is better to use “else if” to test a

sequence of parallel alternativesif (score >= 90)

cout << “Your grade is A” << endl;else if (score >= 80)

cout << “Your grade is B” << endl;else if (score >= 70)

cout << “Your grade is C” << endl;else

cout << “Your grade is D” << endl;


The switch Statement The switch statement can be used instead of the

else if construct to implement a sequence of parallel alternatives

Its syntax isswitch (expression){

case constant1:statement1;

case constant2:statement2;

…default: statement0;

}


The Switch Statement (Cont.)

This evaluates the expression Then looks for its value among the case

constants If the value is found among the constants

listed Then the statements in the corresponding

statementList are executed If there is a default (which is optional),

then the program branches to its statementList


Example1: Switch Statement

switch (score){

case 5: cout << “your score is 5” << endl; break;


case 3:case 2:case 1:

cout << “you need to improve!” << endl; break;default:

cout << “Error: score is out of range” << endl;}


Example 2: Switch Statement

switch (score){




cout << “you need to improve!” << endl; break;

} What is an output when score = 0?


Example 3: Switch Statement

switch (score){

case 5: cout << “your score is 5” << endl;

case 4: cout << “your score is 4” << endl;


cout << “you need to improve!” << endl; } What happen when score = 5, score = 4, score =

1?


Conditional Expression Operator

C++ provides a special operator that often can be used in place of the “if…else” statement

It is called the conditional expression operator

It uses the ? and the : symbols in this syntax:condition ? expression1: expression2


Conditional Expression Operator (Cont.)

It is a ternary operator; i.e., it combines three operands to produce a value

That resulting value is either the value of expression1 or the value of expression2, depending upon the boolean value of the condition

For example, the assignment min = (x < y ? x : y);assign the minimum of x and y to min


Conditional Expression Operator (Cont.)

If the condition x<y is true The expression (x<y? x: y) evaluates

to x Otherwise, it evaluates to y

Conditional expression statements should be used sparingly Only when the condition and both

expressions are very simple


Example: Finding the Minimum

int main(){

int m, n;cout << “Enter two integers:”;cin >> m >> n;cout << (m<n? m: n)

<< “ is the minimum “ << endl;} (m<n ? m : n) evaluates to m if

m < n

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Chapter 7 Selection Dept of Computer Engineering Khon Kaen University