[C++] Chap 02 - Flow of Control

Branching Mechanisms

If-else statements

if (<boolean_expression>) 
    <yes_statement>
else
    <no_statement>

• Use brackets for compound statements
• Logical Operators
  • Logical AND ( && )
  • Logical OR (||)
• Boolean Expression
  • any nonzero number → true, 0 → false

bool result = (1<2);
cout<<result<<endl; // 1

• Short-circuit evaluation
  • Evaluation stops when the result is already decided

int x = 0;
int y = 0;
bool result = (x>=1) && (y++); // y++ is not executed
cout<<y;

• Do not use (x<y<z)!

int x(1), y(2), z(2);
cout<<(x<y<z) <<endl; // 1
cout<<(x<y && y<z) <<endl; // 0

• Consider

int x(3), y(4), z(5), r(0);
bool result = !x; //0
bool result = (!0*x); // 1
int result = (!0*x); // 3
bool result = (x<5) ||(x/r); //1, short circuit evaluation
bool result = (x&&y) + (!z); // 1. bool 1 + bool 1 = bool 1

• Precedence of Operators

  Postfix/Prefix → +,-,*,/,→ Comparison Operators → Equal, Not equal → AND OR → Assign

• x + 1 > 2 || x + 1 < -3
• Roughly, arithmetic done before comparison & logical
• + → > → ||
• evaluating assignment expression is the Rvalue.

if (x=12)  // (x=12) = 12 -> true
    Do_Something
    
if (12==x) // works same with x==12
if (12=x) //error

Switch statement

• Uses controlling expression which returns either (Bool, Enu, Integer, Character)

switch (Controlling_Expression) {
	case Constant_1:
		Statement_seq1
		break;
	case Constant_2:
		Statement_seq2
		break;
	case Constant_3:
	case Constant_4:
		Statement_seq3
		break;
	default:
		Default_Statement_sequence
}

• Omitting break will continue until it meets new ‘break’ or end of switch statement

Enumeration types

• List of constants of type int ← Only integer is valid
• When no values are assigned, consecutive values starting from 0 are assigned.

enum MyEnum { a=17, b, c, d=-3, e};
// is equivalent with
enum MyEnum2 { a=17, b=18, c=19, d=-3, e=-2};

Conditional Operator

• Also called ternary operator

max = (n1 > n2) ? n1 : n2;

• Shorthand version of if-else

Loop statements

• While / Do-while / for

Do-while

do {
	Statement
} while (Boolean_Expression); // Don't forget the semicolon!

• do loop is executed at least once
• while : checks before the body is executed / do-while : checks after the body is executed
• Generally diescouraged to use increment/decrement operators in boolean exp. in while

int n, cnt, one, t;
cin>>n; //enter number of integers to be inputted
t=0;
cnt=1;
while (cnt++<=n) { // Boolean expression executed before executing body
	cin>>one;
	t+=one;
}
cout<<t<<endl; // prints total number of inputted values
cout<<cnt<<endl; // if n==4, cnt is 6.
/*
cnt=4 <=4 passes, then cnt++, cnt=5
cnt=5 <=4 blocked, but cnt++, cnt=6
*/

For loop syntax

for (Initialization_Action; Boolean_Expression;Update_Action) // No semicolon
{
	Body
}

• The third Update Action is optional

sum=0;
int n;
for (n=1;n<=10;n++)
	sum = sum + n;

for (sum=0, n=1;n<=10;n++)
	sum = sum + n;
for (sum=0, n=1;n<=10;) // Update Action omitted 
{
	sum+=n;
	n++;
}

for (sum=0;n=1;n<=10;sum+=n, n++); //Semicolon needed.

• Two semicolons are required inside the Expressions.
• Initialization → {check condition → loop body → update action} → and so on …
• Common pitfalls : misplaced ;

for (int cnt = 1;cnt<=10;cnt++);
		cout<<"Hello"; // Nothing happens..

• Similar issue exists for the while loop

Break and continue statements

• break : Ends the entire loop
• continue : Ends the current iteration of the loop body

IOstream

Opening a text file

 #include <fstream>
 using namespace std;
 
 ifstream is;
 is.open("filename.txt"); // the file is the same directory as the code
 is>>var1 >> var2; // by spacing. if 1 2, var1=1, var2=2 
 is.close();

• Reading a string is done UP TO a whitespace character( " ", “\t”, “\n”)
• When done, close the file with .close();
• ifstream : input only, ofstream : output only, fstream both I/O

Using a loop to read the text file input

string text;
fstream iS;
iS.open("player.txt");
while (iS >> text)  // Performs two actions : Read, check boolean result 
{
	cout<<text<<endl; //not line by line.
}

• The boolean expression is evaluated to false when there is no data left to read from the file