chapter 4 controlling program flow. the if statement allow a program to make decisions. example: if...
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CHAPTER 4
CONTROLLING PROGRAM FLOW
The if Statement
Allow a program to make decisions. Example:
if (Temp < 5) cout << “Wear a coat
today.” << endl;
Syntaxif (condition) statement;
Boolean expression
An expression that evaluates to either true or false.
Use relational operatorsOperator Meaning == equal
< less than <= less than or equal to > greater than >= greater than equal to != not equal to
Examples
ch <= ‘Z’;Grade == 12;FruitName == “Banana”;Radius != 0;2*pi*Radius >= Area;
/*Temperature program */
#include <iostream.h>
int main( ){ double Temp; cout << “Enter today’s temperature(Celsius): “; cin >> Temp;
if (Temp < 5) cout << “Wear a coat today.” << endl;
cout << “Have a gret day!” << endl; return (0);}
4.2 if Pitfalls
Never make equality comparisons (== or !=) with values that may have fractional parts.
if (480 == 4.8 * 100) cout << “These are equal!”;
Compare values of only the same typeExample: ‘2’ > 3; (page 7.12)
= vs == Misplaced semicolon
4. 3 The if-else Statement
Can include an else clause when condition is false
if (condition) statement;else statement;
Indention is good programming style.
if (Temp < 5)
cout << “Wear a coat today.” << endl;
else
cout << “Don’t wear a coat today.” << endl;
Modify the Circle Area program to display an error message when the value entered for Radius is negative. If Radius is a positive value or zero, only the resulting area in a message should be displayed. The error message should look similar to:
Radius entered was -3.4Negative radii are illegal.
#include <iostream.h>int main( ){
double Radius;cout << “Enter a radius: “;cin >> Radius;
if (Radius < 0){
cout << “Radius entered was “ << Radius;cout << “Negative radii are illegal.” << endl;
}else{
cout << “Radius entered was “ << Radius;cout << “Area = “ << (3.14 * Radius * Radius) <<
endl;}return (0);
}
4.5 Nested if StatementsAn if statement that contains another if statement
if (Votes1 == Votes2)
cout << “It was a tie!” << endl;
else
if (Votes1 > Votes 2)
cout << Candidate1 << “ is the winner”;
else
cout << Candidate2 << “ is the winner.”;
4.6 The else-if Ladder
Used to decide among three, four, or more actions.if (temp <= 0) cout << “Freezing”;else if (temp < 13) cout << “Cool”;else if (temp < 41) cout << “Hot”;else
cout << “Very hot”;
Does not contain an if statement. It is executed of all other conditions are false. DEFAULT.
4.8 Logical Operators
“and” represented by &&
(true) && (true) evaluates to true(true) && (false) evaluates to false(false) && (true) evaluates to false(false) && (false) evaluates to false
if ((Temp < 5) && (Wind > 24))cout << “Wear a coat today!”;
4.8 Logical Operators
“or” represented by ||
(true) || (true) evaluates to true(true) || (false) evaluates to true(false) || (true) evaluates to true(false) || (false) evaluates to false
if ((Temp < 5) || (Wind > 24))cout << “Wear a coat today!”;
4.8 Logical Operators
“not” represented by !
!(true) evaluates to false!(false) evaluates to true
if (!(Temp < 5))cout << “DON’T wear a coat today!”;
4.9: Looping – The do-while Statement
iteration Repeat one or more statements during
program execution. Referred to as looping. Form: do
{ statement;
} while (condition);
One or more C++
statements form the body
of the loopA Boolean expression used to determine if
the loop is to be repeated.
4.9 Continued
The do-while loop is executed at least once because the condition is not evaluated until AFTER the first iteration of the loop.
If the condition is true, statement is executed again and then the condition reevaluated.
Looping process is repeated until the condition is evaluated and found to be false.
#include <iostream.>int main(){
char Answer;cout << “Circle Area Calculator” << endl;do {const double PI = 3.14159;double Radius;cout << “Enter the radius: “;cin >> Radius;cout << “Area is “ << (PI * Radius * Radius)<< endl;// Ask if another calculation is desiredcout << “Do another circle (Y/N)?”;cin >> Answer;
}while (Answer == ‘Y’);cout << “Thanks for using the Circle Calculator” << endl;Return (0);}
4.11 Looping: The while Statement
Evaluates its condition BEFORE each iteration
May execute zero or more times.Syntax (form) of a while loop body:
while (condition) { statement;}
#include <iostream.h>
int main()
{
int number;
cout << “Enter a positive number: “;
cin >> number;
while (number < 0) {
cout << “Number must be positive\n”;
cout << “Please re-enter: “;
cin >> number;
}
4.16 Looping: The for Statement
Used to execute a loop body a fixed number of times.
Form:for (initialization, condition,
increment)statement;
Performed only once
Evaluated before each iteration
Performed after each iteration - counter
4.16 Looping: The for Statement
Example:The following loop display the
numbers 1 through 10.
for (int i = 1; i <= 10; i++)cout << I << endl;
4.17 – and -=
Decrement (decrease) Total--;
Special form of assignment operator indicates the value after the operator is to be subtracted from the value of the variable on the left of the operator. Total = Total – Value;
Total -=;
PROGRAMS
Review 21 and 22Exercises
14, p. 4-32 15, p. 4-32 16a, p. 4-33 18a, b, c, p. 4-33/34 21
Quiz on Friday (for statements)