(i) Initialization expression
(ii) Test expression
(iii) Update expression
(iv) The body of the loop

(i) for statement
(ii) while statement
(iii) do-while statement

Iteration statements is used to reduce the length of code, to reduce time, to execute program and takes less memory space.

An iteration (or looping) is a sequence of one or more statements that are repeatedly executed until a condition is satisfied. These statements are also called as control flow statements.

The if statement evaluates a condition, if the condition is true then a true-block (a statement or set of statements) is executed, otherwise the true-block is skipped.

if (expression)
true-block;
statement-x;

The set of statements that are executed again and again is called the body of the loop.

#include
using namespace std;
int main()
{
int i = 2:
while (i<=20)
cout << i <<"\t";
i+= 2;
}
return 0;
}
Output:
2 4 6 8 10 12 13 14 16 18 20

#include <iostream>
using namespace std;
int main()
{
for (int -21; i <31; i++)
cout << "value of i:"<<<i<<< endl;
return 0;
}
Output
Value of i: 21
Value of i: 22
Value of i: 23
Value of i: 24
Value of i: 25
Value of i: 26
Value of i: 27
Value of i: 28
Value of i: 29
Value of i: 30

Output:
2 4 6 8 10.

If the input given is (i) 2000 (ii) 2003 (iii) 2010?
(i) Output
(Input = 2000)
Leap
(ii) Output
(Input-2003)
Not leap Year
(iii) Output
(Input = 2010)
Not leap Year

The selection statement means the statements are executed depends upon a condition. If a condition is true, a true block (a set of statements) is executed, otherwise a false block is executed. This statement is also called decision statement or selection statement because it helps in making decision about which set of statements are to be executed.
Types: (i) Two way branching, (ii) Multiway branching.

Null statement
The "null or empty statement" is a statement containing only a semi colon. It takes the following
form:
;// it is a null statement.
Null statement are commonly used as place holders in iteration statements or as statements on which to place labels at the end of compound statements or functions.
Compound statement
C++ allows a group of statements enclosed by pair of braces (). This group of statements is called as a compound statement or a block.
The general format of compound statement is:
{
statement 1;
statement 2;
statement 3;
}
for example:
{
int a,b;
a-8;
b=a+8;
}