# Iteration - while loops
[Class
recording](https://www.scaler.com/meetings/i/beginner-iteration-while-loops-6/archive)
[lecture
notes](https://drive.google.com/file/d/1aNoe7EzTVerq4CZsmtaHgT5UhG-avXMW/view?usp=share_link)
## **Content**
1. Need for loops
2. While loop - structure
3. Example questions
## Meanwhile when people are joining (5-7 mins)
- Instructor's welcome.
- Breaking ice with students.
- casual conversation to bond.
---
title: Motivation behind while loops
description:
duration: 200
card_type: cue_card
---
## Motivation behind while loops
### **Number Lock**
- In a number lock with 3 digit key, one would have to iterate digit
by digit repetitively until the lock opens, loops can be used here
with appropriate condition.
- On using only conditional statements we would have to check 1000
conditions(from 000 -999)
```python=
While lock is not open:
keep trying
```
- Start with 000, if it doesnt work, increase by 1, try again, until
lock opens.
---
title: Explaining While loop
description:
duration: 200
card_type: cue_card
---
### **Initialization**
- where the loop has to start from.
- if we know the password is \>500 we wuld start trying from 501.
``` python=
combnation=0
```
### **Condition**
- check the condition where the loop has to stop
``` python=
while(lock still closed): #combination!= actual password
try this combination
```
### **Updation** update the iterator variable.
``` python=
combination+=1
```
---
title: Using While loop
description:
duration: 600
card_type: cue_card
---
## **Use while loop to print numbers from 1 to 5**
Code:
``` python=
num=1
while(num<=5):
print(num)
num+=1
```
>Output
```bash=
1
2
3
4
5
```
## **Question**: Take an input from user and print all the alternate
Code
``` python=
number = int(input("Please input a number - "))
i = 1
while i <= number:
print(i)
i += 2
```
>Output
```bash=
Please input a number - 10
1
3
5
7
9
```
## **Question**: Take an input from the user for start and end values and print every third number
Code:
``` python=
start = int(input())
end = int(input())
while start <= end:
print(start)
start += 3
```
>Output
```bash=
10
20
10
13
16
19
```
## **Question**: Take an input from the user for start and end values. Print all even numbers between them.
- Start at the number and print if the number is divisible by 2,
increment number by 1
Code:
``` python=
start = int(input())
end = int(input())
loop_count = 0
while start <= end:
loop_count += 1
if start % 2 == 0:
print(start)
start += 1
print("Loop was run", loop_count,"times!")
```
>Output
```bash=
10
20
10
12
14
16
18
20
Loop was run 11 times!
```
- If number is even, start from number, if odd, start from number+1,
print number, increment by 2
Code
``` python=
start = int(input())
end = int(input())
if start % 2 == 0:
i = start
else:
i = start +1
loop_count = 0
while i <= end:
loop_count += 1
print(i)
i += 2
print("Loop was run", loop_count,"times!")
```
>Output
```bash=
10
20
10
12
14
16
18
20
Loop was run 6 times!
```
- Second answer is more optimized, since the loop runs less number of
times.
## **Question**: You are a bowler and you have to bowl 1 over. (6 balls) Print "Bowling ball number - ball_number" where ball_number is the number of current ball
Code
``` python=
ball_number = 1
while ball_number <= 6:
print("Bowling ball number -", ball_number)
ball_number += 1
```
>Output
```bash=
Bowling ball number - 1
Bowling ball number - 2
Bowling ball number - 3
Bowling ball number - 4
Bowling ball number - 5
Bowling ball number - 6
```
## **Question**: Print all numbers from 1 to N (input from user) in a single line.
- Use `end =" "` to print space seperated values.
Code
``` python=
number = int(input())
i = 1
while i <= number:
print(i, end=" ")
i += 1
```
>Output
```bash=
7
1 2 3 4 5 6 7
```
---
title: Explaining end in print statement
description:
duration: 300
card_type: cue_card
---
## `end` in print
- `end` parameter can take values such as `" "`, `\t`, `\n` etc.
Code
``` python=
print("random", end="ANYTHING_AT_ALL")
print("print", end="ANYTHING_AT_ALL")
print("statements!",end="ANYTHING_AT_ALL")
```
>Output
```bash
randomANYTHING_AT_ALLprintANYTHING_AT_ALLstatements!ANYTHING_AT_ALL
```
---
title: Explaining sep in print statement
description:
duration: 300
card_type: cue_card
---
## `sep` in print
- Seperator works like end but for values in the same print statement.
Code
``` python=
print("these", "are", "multiple", "strings", 45, 67.8, sep="!WHATEVER!")
```
>Output
```bash these!WHATEVER!are!WHATEVER!multiple!WHATEVER!strings!WHATEVER!45!WHATEVER!67.8
```
---
title: Revising previous concepts
description:
duration: 300
card_type: cue_card
---
## **Question**: Given a number, print it's multiplication table!
Code
``` python=
number = int(input())
i = 1
while i <= 10:
print(number*i, end=" ")
i += 1
```
>Output
```bash=
10
10 20 30 40 50 60 70 80 90 100
```
## **Question**:
1. Take number of test_cases (count) from a user. (Count of numbers)
2. Take int(count) inputs from the user
3. Print multiplication table for all these inputs!
``` python=
# 3
# 4
# multiplication table of 4
# 5
# multiplication table of 5
# 6
# multiplication table of 6
```
Code
``` python=
test_cases = int(input())
t = 1
while t <= test_cases:
number = int(input("Please enter the number -"))
i = 1
print("Printing multiplication table of -", number)
while i <= 10:
print(number*i, end=" ")
i += 1
print("\n")
t += 1
```
>Output
```bash=
2
Please enter the number -12
Printing multiplication table of - 12
12 24 36 48 60 72 84 96 108 120
Please enter the number -4
Printing multiplication table of - 4
4 8 12 16 20 24 28 32 36 40
```
---
title: Quiz-1
description: Quiz-1
duration: 60
card_type: quiz_card
---
# Question
**Which of the following statements is true about while loops in
Python?**
# Choices
- [ ] A while loop executes a specific number of times
- [x] A while loop executes until a specific condition is met
- [ ] While loops are not used in Python
---
title: Quiz-2
description: Quiz-2
duration: 60
card_type: quiz_card
---
# Question
**What will be the output of the following code?**
```python=
x = 1
while x <= 100:
if x % 3 == 0 and x % 5 == 0:
print("FizzBuzz")
elif x % 3 == 0:
print("Fizz")
elif x % 5 == 0:
print("Buzz")
else:
print(x)
x += 1
```
# Choices
- [ ] FizzBuzz
- [ ] Fizz
- [ ] Buzz
- [x] A combination of Fizz, Buzz, and/or integers
---
title: Quiz-3
description: Quiz-3
duration: 60
card_type: quiz_card
---
# Question
**What is the output of the following code?**
```python=
count = 0
while True:
if count == 5:
break
print(count)
count += 1
```
# Choices
A:
```bash=
0
1
2
3
4
```
B: 5
C: an infinite loop
- [x] A
- [ ] B
- [ ] C
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