slides-05-01.qmd

---
title: "while loop intro (slides)"
format: revealjs
slide-number: true
---

# CSc 110 - Intro to While Loops

## Announcements

* Midterm 1 Febuary 19 -- BRING PHOTO ID
* Modules 1 to 5 -- study guide under [practice problems](practice-01.html) page on website

## While loops

-   A **while** loop allows a programmer to repeat code
-   You can think of it as an if-statement with the potential to repeat

```{python}
#| eval: false
#| echo: true
statements . . .

while conditionA:
    statementA
    statementB
    . . .
    statementN

statements . . .
```

## What will happen?

```{python}
#| eval: false
#| echo: true
number = 15
while number < 50:
    print('number is less than 50')
```

## While loops

-   What if the condition never evaluates to `False`?
    -   Infinite loop!
-   There are two ways around this:
    -   Break (do not use in this class!)
    -   Designing the code such that the condition will eventually become `False`

## What will happen?

To ensure our condition (`number < 50`) will eventually be evaluated as `False`, we need to updated `number` inside our loop:

```{python}
#| eval: false
#| echo: true
number = 15
while number < 50:
    print('number is less than 50')
    number += 5
print('End')
```

## While loops -- visualization

Go to [Python Tutor](https://pythontutor.com/visualize.html#mode=edit) to visualize how the `while` loop runs.

## While loop -- example

Factorial: 5! = 1 * 2 * 3 * 4 * 5 = 120

```{python}
#| eval: true
#| echo: true
def factorial(n):
  result = 1
  current = 1
  while current <= n:
    result = result * current
    current += 1
  return result

def main():
  print( factorial(5) )
  print( factorial(6) )
  
main()
```

## `while` loops table

Consider `n = 5`, start with `result = 1`, `current = 1`:

| `current <= 5`   | result = result * current | current += 1 |
| :-------: |:-------: |:-------: |
| True | result = 1 * 1 | current = 1 + 1 |
| True | result = 1 * 2 | current = 2 + 1 |
| True | result = 2 * 3 | current = 3 + 1 |
| True | result = 6 * 4 | current = 4 + 1 |
| True | result = 24 * 5 | current = 5 + 1 |
| False | - | - |

## Write a function

Write a function called `add_up_to` that takes an numeric argument `n`.
The function should add all numbers from 1 to `n` in a while loop, and then (outside the loop) return the sum

```{python}
#| eval: false
#| echo: true
print( add_up_to(5) ) # 15
print( add_up_to(10) ) # 55
```

Name your file `sum_up.py` and submit it to attendance on gradescope

## Write a function -- solution

```{python}
#| eval: true
#| echo: true
def add_up_to(n):
  sum = 0
  current = 0
  while current <= n:
    sum += current
    current += 1
  return sum

def main():
  print( add_up_to(5) )
  print( add_up_to(10) )
  
main()
```

## Age milestones

Modify `main` function: use a `while` loop to request a valid input from the user.

```{python}
#| eval: false
#| echo: true
def age_milestones(age):
  message = ""
  if age >= 18:
      message += 'You may apply to join the military.'
     
  if age >= 21:
      message += ' You may drink.'
     
  if age > 35:
      message += ' You may run for president.'
      
  return message

def validate_age(age):
  return age.isnumeric()
     
def main():
  str_age = input('How old are you?\n')
  if validate_age(str_age):
    age = int(str_age)
    print(age_milestones(age))
  else:
    print("Invalid age entered.")
 
main()
```

## Age milestones continued

Program behavior in your standard output:
```{=html}
<pre style="font-size: 30px">
How old are you?
abc
How old are you?
10abc
How old are you?
55
You may apply to join the military. You may drink. You may 
run for president.
</pre>
```

## Age milestones -- solution

Modify `main` function: use a `while` loop to request a valid input from the user.

```{python}
#| eval: false
#| echo: true
def age_milestones(age):
  message = ""
  if age >= 18:
      message += 'You may apply to join the military.'
     
  if age >= 21:
      message += ' You may drink.'
     
  if age > 35:
      message += ' You may run for president.'
      
  return message

def validate_age(age):
  return age.isnumeric()
     
def main():
  str_age = input('How old are you?\n')
  while validate_age(str_age) == False:
    str_age = input('How old are you?\n')
  age = int(str_age)
  print(age_milestones(age))
  
main()
```

## Write functions

1. Write a function `to_do` that takes a numeric argument `temp`.
2. It returns
    -   'go for a swim' if is greater equal to `86` and less than `104`
    -   'go for a hike' if is greater equal to `68` and less than `86`
    -   'stay at home' for all other cases
3. Write a `main` function to request input from user until it is valid (only digits 0-9), print `"Let us"` and the message

## Write functions continued

Program behavior in your standard output:
```{=html}
<pre style="font-size: 30px">
Enter a temperature:
abc
Enter a temperature:
100abc
Enter a temperature:
89
Let us go for a swim
</pre>
```

## Write functions -- solution

```{python}
#| eval: false
#| echo: true
def to_do(temp):
    if temp >= 86 and temp < 104:
        return "go for a swim"
    elif temp >= 68 and temp < 86:
        return "go for a hike"
    else:
        return "stay at home"
    

def main():
    str_temp = input("Enter a temperature:\n")
    while str_temp.isnumeric() == False:
        str_temp =  input("Enter a temperature:\n")
    message = to_do(int(str_temp))
    print("Let us", message)

main()
```