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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

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slide1

Chapter 4

October 22, 2013

the if statement
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 statement1
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 statement2
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
slide5

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
If-Else Statement
  • Contains an else clause
    • Excuted when if is false
  • If(condition){

statement(s);

}

else{

statement(s);

}

compound statements
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
slide8

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;

}

}

slide9

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;

else if ladder
Else-if Ladder
  • 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

else if ladder1
Else-if Ladder

if(condition){

statement(s);

}

else if(condition){

statement(s);

}

else{

}

logical operators
Logical Operators
  • Used to form Boolean expressions
  • && represents and
  • | | represents or
  • Evaluated based on the following rules
logical operators1
Logical Operators

&&- AND

True/False Operator T/F Expressions

T && T T

T && F F

F && T F

F && F F

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

statement(s);

}

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

cout<<“It’s really cold!!”<<endl;

}

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

statement(s);

}

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

cout<<“It’s too cold to be outside”;

cout<< endl;

}

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

statement(s);

}while(condition);

  • Condition
    • Boolean expression
    • Determines if loop continues
do while1
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
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
Infinite loops
  • Continue FOREVER
  • Causes
    • misplaced semicolons
    • missing curly brackets
    • logic errors
algorithms
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 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 summing1
Counting and Summing
  • Counting
    • counters
      • variables that count
      • need to be initialized
      • gives it a starting value; usually 0
      • int i = 0;
counting and summing2
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 summing3
Counting and Summing
  • Sentinel
    • constant
    • value the loop should end on
    • Easily changed
increment and decrement
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 decrement1
Increment and Decrement
  • +=
    • adds a value to a sum
    • sumOfValues +=Value;
      • adds value to sumOfValues
      • stores new value in sumOfValues.
increment and decrement2
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
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
The for Statement
  • Condition
    • Boolean Expression
    • Evaluated before each loop
  • increment
    • performed after each loop
    • Advances the counter
the for statement1
The for Statement
  • Initialization can be performed outside the loop
    • ex: int x = 0;

for(; condition; increment){

statement(s);

}

slide32

#include <iostream>

using namespace std;

int main()

{

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

cout<< x <<endl;

}

}

the bool library
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