slides-05-03.qmd

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

# CSc 110 - while loop aggregation


## `while` loops

-   Using an `index` variable:
    -   Defined before the loop
    -   Used in the **condition** of the loop
    -   Changed within the loop
-   Using a `temporary variable` for aggregation:
    -   Defined before the loop
    -   Changed within the loop
    -   **Returned** outside the loop

## `while` loops with aggregation

Adding numbers from 1 to 5:
```{python}
#| eval: true
#| echo: true
total = 0
index = 1
while index <= 5:
    print('adding ' + str(index))
    total = total + index
    index = index + 1
    
print(total)
```

## `while` loops table

Start with `total = 0`, `index = 1`:

| `index <= 5`   | total =  total + index | index = index + 1 |
| :-------: |:-------: |:-------: |
| True | total = 0 + 1 | index = 1 + 1 |
| True | total = 1 + 2 | index = 2 + 1 |
| True | total = 3 + 3 | index = 3 + 1 |
| True | total = 6 + 4 | index = 4 + 1 |
| True | total = 10 + 5 | index = 5 + 1 |
| False | - | - |

## Write a function

Its name is `sum_all` and takes two numeric arguments: `low` and `high`.
It runs a loop that iterates through the values `low` and `high` summing all values.
It returns the sum of all values between `low` and `high`.

HINT: Create a variable that will aggregate the sum. Use `while` (set the index as the `low` before the loop, use index in the `while` condition, update the index inside the loop). 

```{python}
#| eval: false
#| echo: true
assert sum_all(1, 3) == 6
assert sum_all(0, 5) == 15
```

## Write a function -- solution

```{python}
#| eval: true
#| echo: true
def sum_all(low, high):
  total = 0
  index = low
  while index <= high:
    total += index
    index += 1
  return total
    
def main():
  assert sum_all(1, 3) == 6
  assert sum_all(0, 5) == 15
  
main()
  
```

## Write a function

Write a function called `count_vowels` that takes a string as argument, and returns
an integer with the number of vowels in the string.

HINT: use `in` to determine if a character is a vowel. The vowels are `"a"`,`"e"`,`"i"`,`"o"`, and `"u"`.

```{python}
#| eval: false
#| echo: true
assert count_vowels("banana") == 3
assert count_vowels("fly") == 0
```

## Write a function -- solution

```{python}
#| eval: true
#| echo: true
def count_vowels(string):
  index = 0
  count = 0
  while index < len(string):
    if string[index] in "aeiou":
      count += 1
    index += 1
  return count

def main():
  assert count_vowels("banana") == 3
  assert count_vowels("fly") == 0
  print("End of tests.")
  
main()

```


## Write a function

Its name is `factorial` and takes a numeric argument `number`.
It returns the factorial of `number`. 
The factorial of a number is the product of that number and every number that comes before it, down to one. 

For example, factorial of 4: `1 * 2 * 3 * 4 = 24`.

Name your file `factorial.py` and submit to Gradescope.

```{python}
#| eval: false
#| echo: true
assert factorial(4) == 24
assert factorial(5) == 120
assert factorial(0) == 1
```


## Write a function -- solution

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

def main():
  assert factorial(4) == 24
  assert factorial(5) == 120
  assert factorial(0) == 1
  
main()
```


## Write a function

1.  Its name is `power`
2.  It takes two numeric arguments: `base` and `exp`
3.  It returns the `base` to the power of `exp`
4.  Don't use the `**` operator, use a `while` loop (define an `index` before the loop, use `index` in the `while` condition, change `index` inside the loop)

```{python}
#| eval: false
#| echo: true
assert power(2, 4) == 16
assert power(3, 2) == 9
```


## Write a function -- solution

Write more test cases for this function.

```{python}
#| eval: true
#| echo: true
def power(base, exp):
  result = 1
  index = 1
  while index <= exp:
    result *= base
    index += 1
  return result

def main():
  assert power(2, 4) == 16
  assert power(3, 2) == 9
    
main()
```


## Write a function

1.  Its name is `vowels_only`
2.  It takes a `string` argument
3.  It builds a new string containing only the vowels in the `string` argument
4.  It returns new string with vowels only (define an `index` before the loop, use index in the `while` condition, change `index` inside the loop)

```{python}
#| eval: false
#| echo: true
print( vowels_only("banana") ) # "aaa"
print( vowels_only("fly") ) # ""
```


## Write a function -- solution


```{python}
#| eval: true
#| echo: true
def vowels_only(string):
  new_string = ""
  index = 0
  while index < len(string):
    if string[index] in "aeiou":
      new_string += string[index]
    index += 1
  return new_string

def main():
  print( vowels_only("banana") ) # "aaa"
  
main()
```