glenn stevenson csis 113a msjc csis 113a lecture 2

Glenn Stevenson CSIS 113A MSJC

CSIS 113A

Lecture 2


The bool type

• Can have only hold two separate values– true, false

bool empty = true;bool full;

full = false;

• Watch your case!– False and false are different


Relational Operators

• Used to create Boolean expressions– A statement that evaluates to true or false


Relational Operators II< Less than

> Greater than

<= Greater than or equals

>= Less than or equals

== Equals

!= Not Equals

int x = 3, y = 4;bool z = x > y;z = x < y;z = x == y;z = x !=y;

• What is the value of z on each line?


Primitive Relations• Each relational operator require 2 primitive

operands– The result is a Boolean value

• Only works with comparable primitive types– Most types are comparable

» Normally don’t need to worry about mixed type comparisons

Variables Comparisonsint b = 2; int sh = 3300; double d = 2.34; float f = 2.34F; char c = 'A';

b > sh is false c < b is false f == d is false f < d is true c >= d is true


Floating Point Relations

• General Rule– Never compare floating point operands using

== or != (.1 * 10.0) == 1.0; // C++ considers this to be true

• What about this:(.1+.1+.1+.1+.1+.1+.1+.1+.1+.1) == 1.0


Introduction to selection

• The relational operators, and the Boolean values that they produce, are important– They allow us to implement selection .

• Acts like a “highway divider” in your code.


If Statement

• If is considered a block of code.– So why doesn’t it have braces?

• If you only want to execute one statement as a result of the if you don’t need braces

• Condition should be derived from relational operators


An Example#include <iostream>using namespace std;int main(){int number; cout << "Enter a number and I will square it for you " << endl; cin >> number; if(number == 50) cout << "50 is a big number to square! " << endl;

cout << number << " squared is " << number * number << endl;

return 0; }


If / else

• If has an optional else– It cannot stand alone

• Must be preceded by an if statement


Multiple statements• When you want to execute multiple statements as

a result of the if condition– Must surround code to execute by braces


Indentation Styles I

• 3 acceptable styles– 1. opening brace on same line as if

• Can be difficult to spot missing braces with this style

if (amountSold <= 35000) { bonusPct = .035; bonusAmt = amountSold * bonusPct; } else { bonusPct = .075; bonusAmt = amountSold * bonusPct + 100; }


Indentation Style II

• Style I prefer– Braces are lined up on top of each other with code

indented• Make seeing missing braces easy



Indentation Styles 3

• Variation of number 2– Again, braces don’t stand so finding a missing one

could again be a problem



Why use braces?• Required if multiple lines of code are used

within and if or an if / else– What is wrong with the following code?

bonusAmt = 0; if (amountSold <= 35000) bonusPct = .035; else bonusPct = .075; bonusAmt+= 100;bonusAmt += amountSold * bonusPct;


Rule of thumb

• Beginning programmers should always use braces– Even if there is only one statement to execute

• It is ok to omit them if you are putting everything on a single line:

if (amt < 100) cost = .23;


Nested ifs

• What is a nested if?– One if (or if-else) appears as the "body"

of anotherif ( x == 3 ) if ( z == 4 ) y = 3; else y = 4;else if ( z == 4 ) y = 5; else y = 6;


Nested Ifs II


Selecting one of several I


Selecting one of several II

• Same problem using if-else-if– if (x == 1)

{ // action for 1} else if (x == 2) { // action for 2} else if (x == 3) { …}

– Same code, just reformatted


Use of Boolean Expression


Short Circuit Evaluation

• Precedence– Logical AND (&&) is higher than OR (||)

– What is ( 10 < 15 || 5 > 8 && 3 > 5 ) ?

if ( (a != 0) && ( b / a > 12 )) if ( (a > 10) || (b++ > 7))


Ladder-Style if...else

• Called "ladders" [if-else-if]– More easily understood than traditional nesting

• Note if-else-if statements are interdependentMust often be careful to place in correct order

if (age <= 7) fee = 8.00;else if (age <= 12) fee = 10.50;else fee = 21.75;


Phantom Semicolon• Common error,

– Remember, decisions are blocks• They are terminated by an ending brace

– That is unless you have one statement to execute

if (employeesInBuilding == 0); { demolishBuilding(); }

UhOh, this is a big problem!!


Two Logical Problems

glenn stevenson csis 113a msjc csis 113a lecture 2

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