# Introduction to Python Programming/Python Programming - Control Structures

## 4. Control Structures

A set of actions define the flow of events as decided by the flow chart. Within Python programming, none “None” values return True while variables with “None” values return False.

### 4.1. If statements

The simplest control if statements return the value True when a condition is met. There is the simple one line if condition, followed by the if-else, and continued by the if-elif statements.

```   >>> var = 100
>>> if var==100:
```

print "yay! the variable is truly 100!!!"

```   yay! the variable is truly 100!!!
>>>
>>>
>>> if (var==100): print "value is 100, its looks great!"
```
```   value is 100, its looks great!
>>>
var = 100
if var == 100:
print "yay! the variable is truly 100"
else:
print "Nope! Better luck next time!!!!"
>>>var=10
>>> if var == 100:
print "yay! the variable is truly 100"
else:
print "Nope! Better luck next time!!!!"
Nope! Better luck next time!!!!
```

### 4.2. For Loops

The “for” loops in Python executes a certain block of code for a known number of iterations. The specialty of “for” loop in Python is that the block of code can be iterated for the number of items existing within a list, dictionary, string variable, over a particular range of numbers in counted number of steps, and for the number of items existing in a tuple.

```   >>> a=(10,20,30,40,50)
>>> for b in a:
print "square of " + str(b) + " is " +str(b*b)
square of 10 is 100
square of 20 is 400
square of 30 is 900
square of 40 is 1600
square of 50 is 2500
>>> c=["new", "string", "in", "python"]
>>> for d in c:
print "the iteration of string is %s" %d
the iteration of string is new
the iteration of string is string
the iteration of string is in
the iteration of string is python
>>>
>>> for i in range(10):
```

print "the value of "+str(i)+" is "+str(i)

```   the value of 0 is 0
the value of 1 is 1
the value of 2 is 2
the value of 3 is 3
the value of 4 is 4
the value of 5 is 5
the value of 6 is 6
the value of 7 is 7
the value of 8 is 8
the value of 9 is 9
>>> a={'a':10,'b':20,'c':30}
>>> for k in a:
print "the key is %s and the value is %d" %(k,a[k])
the key is a and the value is 10
the key is c and the value is 30
the key is b and the value is 20
```

### 4.3. While Loop

The while loop is executed till a conditional statement returns true. The conditional statement is evaluated every time before a block of code is executed, and the execution stops the moment the conditional statement returns false.

```   >>> count = 0
>>> while (count<9):
print "the count is at iteration %d" %count
count+=1
the count is at iteration 0
the count is at iteration 1
the count is at iteration 2
the count is at iteration 3
the count is at iteration 4
the count is at iteration 5
the count is at iteration 6
the count is at iteration 7
the count is at iteration 8
```