Chapter 4 October 22, 2013

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# Chapter 4 October 22, 2013 - PowerPoint PPT Presentation

Chapter 4 October 22, 2013. The If Statement. Programs make decisions If(condition){ Statement(s); } Condition boolean expression Evaluates to either true or false formed using relational operators. The If Statement. Operator Meaning = = equal to < less than

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Chapter 4

October 22, 2013

The If Statement
• Programs make decisions
• If(condition){

Statement(s);

}

• Condition
• boolean expression
• Evaluates to either true or false
• formed using relational operators
The If Statement

Operator Meaning

• = = equal to
• < less than
• <= less than or equal to
• > greater than
• >= greater than or equal to
• != not equal to
The If Statement
• Statement
• single or complex
• Directions on what to do
• Pitfalls
• do not use = = or ! = with decimals
• roundoff errors
• occur because doubles cannot be exactly represented in binary

The If Statement

• Only compare values of the same type
• Do not confuse assignment (=) with equal to (==).
• Will compile but output will vary
• Misplaced semicolons
• if(score ==21);
If-Else Statement
• Contains an else clause
• Excuted when if is false
• If(condition){

statement(s);

}

else{

statement(s);

}

Compound Statements
• More than one statement enclosed in { }
• Ex: if(temp > 5){

cout<<“Wear a coat!”<<endl;

cout<<“It is cold outside”<<endl;

}

Nested if Statements

• Controls flow in 3 or more situations
• if statement within an if statement

Nested if Statements

• If statement within an if statement
• If(condition){

if(condition){

statement(s);

}

}

• Ex: if(temp < 5){

if(temp< 2){

cout<<“Wear a coat”<<endl;

}

}

Nested if Statements

• Dangling else
• A logic error associated with nested if statements
• reason we use brackets
• clarify and group

Ex: if(temp < 5)

if(temp< 2)

cout<<“Wear a coat”<<endl;

else

cout<<“It’s hot”<<endl;

• Decides between three or more actions
• order of if and else is very important
• Last statement is executed iff the statements before ALL fail.
• Ex: if(temp < 0)

else if (temp< 2)

else if (temp < 4)

else

if(condition){

statement(s);

}

else if(condition){

statement(s);

}

else{

}

Logical Operators
• Used to form Boolean expressions
• && represents and
• | | represents or
• Evaluated based on the following rules
Logical Operators

&&- AND

True/False Operator T/F Expressions

T && T T

T && F F

F && T F

F && F F

OR –If example
• If((condition) | | (condition)){

statement(s);

}

If((temp < 10) | | (wind> 20 )){

cout<<“It’s really cold!!”<<endl;

}

AND –If example
• If((condition) && (condition)){

statement(s);

}

If((temp < 10) && (wind> 20 )){

cout<<“It’s too cold to be outside”;

cout<< endl;

}

Looping
• Control program through iteration
• Repeat one or more statements
• Referred to as looping
Do-while
• do {

statement(s);

}while(condition);

• Condition
• Boolean expression
• Determines if loop continues
Do-while
• executed at least once
• condition executed after first loop
• if condition is true then statements are executed again and condition is reevaluated.
while Statements
• evaluates before each loop
• Can be executed 0 or more times
• while(condition){ statement(s);

}

• if the condition is true then the statements are executed and the condition is reevaluated until condition is false.
Infinite loops
• Continue FOREVER
• Causes
• misplaced semicolons
• missing curly brackets
• logic errors
Algorithms
• series of steps that tell how to solve a problem
• usually written out first
• helps with structure of code
• helps eliminate logic errors
• caused by statements that are syntactically correct but produce undesired results
Counting and Summing
• Counting
• counts the number of values entered by the user
• really counts number of loops
• numOfValues = numOfValues + 1;
• takes values currently in variable, adds one to it and re-stores that sum in the original variable.
Counting and Summing
• Counting
• counters
• variables that count
• need to be initialized
• gives it a starting value; usually 0
• int i = 0;
Counting and Summing
• Summing
• sums the values entered by a user
• sumOfValues = sumOfValues + value;
• takes value currently stored and adds the new value and re-stores it back in sumOfValues
• needs to be initialized
• usually zero.
Counting and Summing
• Sentinel
• constant
• value the loop should end on
• Easily changed
Increment and Decrement
• ++
• after a variable indicates addition of 1
• numOfValues++;
• adds one and re-stores new value in numOfValues
• increment operator
• Where C++ came from
Increment and Decrement
• +=
• adds a value to a sum
• sumOfValues +=Value;
• stores new value in sumOfValues.
Increment and Decrement
• --
• subtraction of 1 from variable
• total --;
• subtracts 1 from total
• stores back in total
• - =
• subtracts a value from a total
• total -=value;
• subtracts value from total
• Stores new value in total
The for Statment
• executes a loop for a fixed number of times
• for(initialization; condition; increment){

statement(s);

}

• initialization
• performed once
• type counter = startingPoint;
• int x = 0;
The for Statement
• Condition
• Boolean Expression
• Evaluated before each loop
• increment
• performed after each loop
The for Statement
• Initialization can be performed outside the loop
• ex: int x = 0;

for(; condition; increment){

statement(s);

}

#include <iostream>

using namespace std;

int main()

{

for ( int x = 0; x < 10;x++) {

cout<< x <<endl;

}

}

The bool Library
• Boolean variables only hold true or false
• library used to implement bool type
• #include<bool>

using namespace std;

• bool Tuesday = false;
• no quotations marks