Control Flow: Loops and Conditionals

# Control Flow: Loops and Conditionals
## Introduction to Python
### A part of the series at <a href="http://pythonslides.review">pythonslides.review</a>
### Last Updated: 2018-06-14

---

---

```python
def function_1(...,...):
...
def function_2(...,...):
...
.
.
.
def function_k(...,...):
...

#Comments and so on.

statement_1
statement_2
.
.
.
statement_n
```
]
.right-column-40[

## How Python Sees This

- The Python interpreter executes statements from top to bottom.

]

---

```python
def function_1(...,...):
...
def function_2(...,...):
...
.
.
.
def function_k(...,...):
...

#Comments and so on.

statement_1
statement_2
.
.
.
statement_n
```
]
.right-column-40[

## How Python Sees This

- The Python interpreter executes statements from top to bottom.

- The function definitions and comments (lines starting with the # character) are parsed but not executed.

]

---

```python
def function_1(...,...):
...
def function_2(...,...):
...
.
.
.
def function_k(...,...):
...

#Comments and so on.

statement_1
statement_2
.
.
.
statement_n
```
]
.right-column-40[

## How Python Sees This

- The Python interpreter executes statements from top to bottom.

- The function definitions and comments (lines starting with the # character) are parsed but not executed.

- Actual computation starts from .red[statement_1]
]

---

# Control Flow

- Need to vary computation sequence as values evolve.

- .red[Control flow] determines the order in which statements are executed.

- Conditional Execution

- Do something if something happens.
--

- Repeated Execution or _Loops_

--
     - Do something again and again as long as something is true.

--
  - Function definitions

--
     - These blocks don't even execute until you call them.

- Keeps your codebase organized 😎

---

#Control Flow
## Conditionals
 
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# If-Then-Else

---

#If-Then-Else

From the website .red[[IFTTT](http://www.ifttt.com)]:

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#If-Then-Else

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#If-Then-Else

We use conditional executions all the time.

--
```python
if (sleepy == True):
  snooze_alarm()
else:
  wakeup()
```

- The statements .blue[snooze_alarm()] and .blue[wakeup()] are executed based on whether .red[sleepy] is .green[True] or .green[False].

- The if/else blocks are indented by spaces/tabs. It's important that you are consistent with your usage of spaces and tabs!

- The else block is not mandatory.

- You can handle multiple scenario cases by using if-elif-else statements.

```python
if (sleepiness == 10):
  snooze()
elif (sleepiness == 5):
  coffee()
else:
  wakeup()
```

---

#If-Then-Else

.footnote[.pink[*]From .blue[[Programiz](https://www.programiz.com/python-programming/if-elif-else)].]

---

#If-Then-Else

.footnote[.pink[*]From .blue[[Programiz](https://www.programiz.com/python-programming/if-elif-else)].]

---
#If-Then-Else

.footnote[.pink[*]From .blue[[Programiz](https://www.programiz.com/python-programming/if-elif-else)].]

---
#If-Then-Else

#Control Flow
## Repetition
 
---

# Loops

<img src="images/loops.png" style="width:300px;" alt="Loops">
.footnote[This is [xkcd](https://xkcd.com/1411/) again.]
---

# Loops - Fixed Iterations

## .blue[Repeating something n times]

- Use the .blue[for] statement in such scenarios.

```python
n = 10
for i in range(n):
  print(i, end=" ")
print() # Prints a newline
```

```python
0 1 2 3 4 5 6 7 8 9
```
--

- The code below will print numbers from 5 (.green[inclusive]) to 10 (.red[exclusive]).

```python
x = 5
y = 10
for i in range(x,y):
  print(i, end=" ")
print() #Prints a newline
```
--

```python
5 6 7 8 9
```

---

# Loops - Example

## .blue[Factors of a number n]

List all the factors of a number n.

```python
n = 50
for i in range(1,n+1):
  if n%i == 0:
    print(i)
```

Cutting down the number of iterations:

```python
n = 50
from math import sqrt
for i in range(1,int(sqrt(n))+1):
  if n%i == 0:
    print(i)
    print(int(n/i))
```

---

# Loops - Iterations based on Conditions

Often we don't know the number of repetitions in advance.

```python
sleepy = 10
while(sleepy):
  snooze()
  sleepy = sleepy - 1
```

Executes the body if condition evaluates to .red[True].

After each iteration, check if condition is True again.

☠️ Note that this execution will not terminate if the condition never evaluates to .red[False]!

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#While Control Flow

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#🎉