# C# Loops

When to use loops?
- Repeat
- Pattern

Types
- for
- while
- do while
- foreach

for loop:
Patter:

for (initialization; condition; re-initialization/changes in the condition)
{
statements;
}

example 1:
q:

a: 
for (i = 1; i <= n; i++)
{
   print i;
}
output:
n = 5;
1
2
3
4
5

for decreasing order
for (i = 5; i > 0; i--)
{
    print i;
}
5
4
3
2
1

example 2:
Find the sum all even numbers upto 100;

var sum = 0;
for (i = 1 ; i <= 100; i++)
{
    if (i % 2 == 0)
    {
        sum += i;
    }
}

print i;

example 3:
find n factorial (n!);

var factorial = 1;
for (i = 1 ; i <= n; i++)
{
    factorial *= i;
}

print factorial;

example 4:
q: find the sum of first n prime numbers;

var sum = 0;
for (i = 1; i <= n; i++)
{
    if (IsPrime(i))
    {
        sum += i;
    }
}

print i;

**Nested loops:**
for ()
{
for()
{

}
}

example:
Print table of n numbers upto 10 terms;

for (i = i; i <= n; i++)
{
    for (j = 1; j <= 10; j++)
    {
        print j * i;
    }
}

Pattern printing:
q:

*
**
***
****
*****
******

for (i = 1; i <= n; i++)
{
    for (for j = 1; j <= i; j++)
    {
        print *;
    }
}

q:

*****
****
***
**
*

for (i = n; i > 0; i--)
{
    for (for j = 1; j <= i; j++)
    {
        print *;
    }
}

q:

1000000
0100000
0010000
0001000
0000100
0000010
0000001

for (i = 0; i < n; i++)
{
    for (j = 0; j < n; j++)
    {
        if (i == j)
        {
            print 1;
        }
        else
        {
            print 0;
        }
    }
}

q:

1000001
0100010
0010100
0001000
0010100
0100010
1000001

for (i = 0; i < n; i++)
{
    for (j = 0; j < n; j++)
    {
        if (i == j || i + j == n)
        {
            print 1;
        }
        else
        {
            print 0;
        }
    }
}

q:

   1
  121
 12321
1234321
 12321
  121
   1

for (i = 1; i <=4; i++)
{
    for (j = 1; j <= i; j++)
    {
       print j;
    }
    
    for (j = i - 1; j > 0; j--)
    {
        print j;
    }
}

for (i = 3; i > 0; i--)
{
    for (j = 1; j <= i; j++)
    {
       print j;
    }
    
    for (j = i - 1; j > 0; j--)
    {
       print j;
    }
}

q:

*
**
***
**
*

for (int i = 1; i <= n; i++)
{
    for (int j = 1; j <= i; j++)
    {
        print *;
    }
}

for (int i = n - 1; i > 0; i--)
{
    for (int j = i; j > 0; j--)
    {
        print *;
    }
}

q:

     *
    **
   ***
  ****
 *****
******

for (var i = 1; i <= n; i++)
{
    for (var j = 1; j <= n - i; j++)
    {
        print " ";
    }
    
    for (var j = 1; j <= i; j++)
    {
        print *;
    }
}