slides-04-02.qmd

---
title: "if statements (slides)"
format: revealjs
slide-number: true
---

# CSc 110 If statements

## Expression vs Statement

-   *Expression* evaluates to value
-   *Statement* does something

```{python}
#| echo: true
#| eval: false
3 + 4 # expression
print(3 + 4) # statement

a = 8 # statement
a != 9 # expression
print(a) # statement
```

## The if statement

-   If statements can be used to **run code conditionally**

    -   **Before if-statements:** Code has pretty much just run in a straight line

    -   **With ifs:** Can run code optionally, depending on the value of a condition

This means our code can **branch** in different directions

## The if statement

```{=html}
<pre>
if condition:
    statement 1
    statement 2
    . . .
    statement N
</pre>
```
The condition is an expression that is evaluated to a bool.

Example:

```{python}
#| eval: false
#| echo: true
def greeting(name):
  if name == "Bond":
    return "Welcome on board 007."
  else:
    return "Hello, " + name

def main():
  user_name = input("Enter your name:\n")
  print( greeting(user_name) )

main()
```

## Improve the function

Instead of checking if the name entered is "Bond", also check whether the name is "James Bond".

## Conditional execution

The computer program branches out, or makes decisions

```{python}
#| echo: true
#| eval: true
def greater_than_zero(n):
  if n > 0:
    return "Greater than zero"
  else:
    return "Not greater than zero"
  
def main():
  result = greater_than_zero(4)
  print(result)
  
  result = greater_than_zero(0)
  print(result)
  
  result = greater_than_zero(-3)
  print(result)
  
main()
```

## Write a function

Write a Python function called `absolute` that takes one numeric argument (integer or float) `n`
and returns the absolute value of `n`: if `n` is positive, it results `n`, if `n` is negative, it returns `n * -1`

Test cases:

```{python}
#| echo: true
#| eval: false
print( absolute(4) ) # 4
print( absolute(-4) ) # 4
print( absolute(0) ) # 0
```


## Write a function -- solution 1

```{python}
#| echo: true
#| eval: true
def absolute(n):
  if n > 0:
    return n
  else:
    return -n
  
def main():
  print( absolute(4) )
  print( absolute(-4) )
  print( absolute(0) )
  
main()
```

## Write a function -- solution 2

```{python}
#| echo: true
#| eval: true
def absolute(n):
  if n > 0:
    return n
  return -n
  
def main():
  print( absolute(4) )
  print( absolute(-4) )
  
main()
```

Which lines run in the function `absolute`?

-   When expression `n > 0` is `True`, line `3` runs (`return` is used to exit a function and returns a value)
-   When expression `n > 0` is `False`, line `4` runs (skip line `3`)


## Write a function

1.  Function `age_milestones` takes one integer `age`
2.  It returns:
    -   'You may apply to join the military.' if `age` is greater or equal to `18`
    -   'You may drink.' if `age` greater or equal to `21`
    -   'You may run for president.' if `age` greater or equal to `35`
3.  The latter conditions include former so concatenate them
    
```{python}
#| echo: true
#| eval: false
print( age_milestones(18) ) # You may apply to join the military.
print( age_milestones(30) ) # You may apply to join the military. You may drink.
print( age_milestones(0) ) # 
```

## Write a function -- solution

```{python}
#| eval: true
#| echo: true
def age_milestones(age):
  '''
  This function prints an informative message based on,
  a person's age.
  Args:
    age: integer representing a person's age
  Returns:
    A string with a message to the user
  '''
  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 main():
  print( age_milestones(18) ) # You may apply to join the military.
  print( age_milestones(30) ) # You may apply to join the military. You may drink.
  print( age_milestones(0) ) # 
 
main()
```

## Quiz 04

<center>
<div class="cleanslate w24tz-current-time w24tz-large" style="display: inline-block !important; visibility: hidden !important; min-width:300px !important; min-height:145px !important;"><a href="//24timezones.com/Tucson/time" style="text-decoration: none" class="clock24" id="tz24-1695057604-c1393-eyJob3VydHlwZSI6IjEyIiwic2hvd2RhdGUiOiIwIiwic2hvd3NlY29uZHMiOiIwIiwiY29udGFpbmVyX2lkIjoiY2xvY2tfYmxvY2tfY2I2NTA4ODZjNDg0OWVlIiwidHlwZSI6ImRiIiwibGFuZyI6ImVuIn0=" title="World Time :: Tucson" target="_blank" rel="nofollow"></a>current time<div id="clock_block_cb650886c4849ee"></div></div>
<script type="text/javascript" src="//w.24timezones.com/l.js" async></script>
</center>

You have 10 minutes to complete the quiz.

No need to write `main()` function.

# Input validation

## String methods

In addition to having built-in **functions** (`len()`, `print()`, `int()`, `float()`, etc.), Python also has a number of **methods** we will be using in this class.

Check the [documentation](https://docs.python.org/3/library/stdtypes.html#str.isnumeric) for string methods and read what `.isnumeric()` does.

## Validating numbers

-   The `input()` function always returns a `string`
-   We can use the string built-in method `.isnumeric()` to determine if a string represents an integer
-   `.isnumeric()` returns True if all the characters are numeric (0-9)

Try these out:

```{python}
#| eval: true
#| echo: true
name = "Jimmy42"
name.isnumeric()
```


```{python}
#| eval: false
#| echo: true
age = 37
age.isnumeric() # this throws an error
```

## Write a validation function

Write a Python function `validate_age` that takes a string argument `age`.
It returns `True` if `age` contains only 0-9 digit characters, and `False` otherwise.

In your previous file for age milestone, in the `main` function:

-   call `input()` to type an age 
-   call `validate_age` to validate your input 
-   if it is `True`, call `age_milestones` to print messages
-   if it is `False`, print 'Invalid age entered'

## Age milestones

```{python}
#| eval: false
#| echo: true
def age_milestones(age):
  '''
  This function prints an informative message based on,
  a person's age.
  Args:
    age: integer representing a person's age
  Returns:
    A string with a message to the user
  '''
  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():
  '''
  This functions takes input from the user and calls the
  check_age() functiont to print a message
  '''
  age = input('How old are you?\n')
  if validate_age(age):
    print(age_milestones(int(age)))
  else:
    print("Invalid age entered")
 
main()
```