## Loop Statement : While Loop

### Introduction 

- While loops repeat as long as a certain boolean condition is met.
- The while loop in Python is used to iterate over a block of code as long as the test expression (condition) is true.
- Mostly while loop is used when we want to iterate a dynamical number of times repetition of an object.
- The loop iterates while the condition is true.
- When the condition becomes false, program control passes to the line immediately following the loop.
- Iteration means executing the same block of code over and over, potentially many times.

Syntax:
```python
while [condition]:
           #run this code until while condtion is true
```
Example:
```python
a=10
while a!=0:
  print(a)
  a-=1
```
Output:
```
10
9
8
7
6
5
4
3
2
1
```

### Interruption of Loop  Iteration
  - In each example you have seen so far, the entire body of the while loop is executed on each iteration. 
  - Python provides two keywords that terminate a loop iteration prematurely:
```
1. break
2. continue
```

### 1.break
- Immediately terminates a loop entirely. 
- Program execution proceeds to the first statement following the loop body.

Example:
```python
n= 5
while n > 0:
   n -= 1
   if n == 2:
      break
   print(n)
print('Loop ended.')
```

Output:
```
4
3
Loop ended.
```

when n becomes 2 the break statement is executed.loop is terminated completely, 
and program execution jumps to the **print()** statement on line 7.

### 2. continue
- Immediately terminates the current loop iteration. 
- Execution jumps to the top of the loop, and the controlling expression is re-evaluated to determine whether the loop will execute again or terminate.

Example:
```python
n = 5
while n > 0:
 n -= 1
 if n == 2:
     continue
 print(n)
print('Loop ended.')
```

Output
```
4
3
1
0
Loop ended.
```
when n is 2, the continue statement causes termination of that iteration. 
Thus, 2 isn’t printed.
Execution returns to the top of the loop, the condition is re-evaluated, and it is still true.