slides-08-02.qmd

---
title: "for loops -- iterating over items (slides)"
format: revealjs
slide-number: true
df-print: kable
---

# CSc 110 - the other type of `for` loop

## Announcements

* Midterm 2 will be on next Wednesday, Oct 23.
* Review session next Tuesday, Oct 22 5-7pm
  * Location: Gittings 129b 
* Study guide: [xinchenyu.github.io/csc110-fall2024/pdfs/study-guide-2.pdf](https://xinchenyu.github.io/csc110-fall2024/pdfs/study-guide-2.pdf) 
  

## Review of `for in range():`

```{python}
#| echo: true
#| eval: true
for n in range(5):
  print(n)
```

```{python}
#| echo: true
#| eval: true
numbers = [2, 1, 4, 6, 23, 2]
for i in range(len(numbers)):
  print(numbers[i])
```

## Introducing `for x in string:`

```{python}
#| echo: true
#| eval: true
string = "hello"
for n in string:
  print(n)
```

## Introducing `for x in list:`

```{python}
#| echo: true
#| eval: true
numbers = [2, 1, 4, 6, 23, 2]
for n in numbers:
  print(n)
```

## Write a function

Modify your `count_vowels` and `count_chars` functions.
Use `for x in string` instead of `for x in range(len(string))`.

```{python}
#| eval: false
#| echo: true
assert count_vowels("") == {"a": 0, "e": 0, "i": 0, "o": 0, "u": 0}
assert count_vowels("banana") == {"a": 3, "e": 0, "i": 0, "o": 0, "u": 0}
assert count_chars("") == {}
assert count_chars("banana") == {"b": 1, "a": 3, "n": 2}
```

## Write a function -- solution

```{python}
#| eval: true
#| echo: true
def count_vowels(string):
  counts = {"a": 0, "e": 0, "i": 0, "o": 0, "u": 0}
  for char in string:
    if char in counts:
      counts[char] += 1
  return counts

def main():
  assert count_vowels("") == {"a": 0, "e": 0, "i": 0, "o": 0, "u": 0}
  assert count_vowels("banana") == {"a": 3, "e": 0, "i": 0, "o": 0, "u": 0}
  print("Passed all tests.")
  
main()
    
```


## Write a function -- solution

```{python}
#| eval: true
#| echo: true
def count_chars(string):
  counts = {}
  for char in string:
    if char in counts:
      counts[char] += 1
    else:
      counts[char] = 1
      
  return counts

def main():
  assert count_chars("") == {}
  assert count_chars("banana") == {"b": 1, "a": 3, "n": 2}
  print("Passed all tests.")

main()
```


## Quiz 07

<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.

## Write a function

1. Its name is `max_list` that takes a list of numbers as argument.
2. It mutates the list, multiply each number by 2
3. It returns the highest value in the mutated numbers

Test cases:

```{python}
#| eval: false
#| echo: true
assert max_list([]) == None
assert max_list([2, 2, 1]) == 4
assert max_list([1, -3]) == 2 
```

## Write a function -- solution

```{python}
#| eval: true
#| echo: true
def max_list(numbers):
    max = None
    for i in range(len(numbers)):
        numbers[i] *= 2
    for i in range(len(numbers)):
        if max == None or numbers[i] > max:
            max = numbers[i]
    return max

def main():
  assert max_list([]) == None
  assert max_list([2, 2, 1]) == 4
  assert max_list([1, -3]) == 2 
  print("pass")
  
main()
```

## Write a function

1. Its name is `tally_negatives`
1. It takes a list of `numbers` as argument
1. It returns a dictionary that maps each negative value in `numbers` to its frequency in `numbers`

Name your file as `tally_negatives.py` and submit to gradescope. Test cases:

```{python}
#| eval: false
#| echo: true
assert  tally_negatives([1, -2, 0, -4, -2]) == {-2: 2, -4: 1}
assert tally_negatives([]) == {}
```

## Write a function -- solution

```{python}
#| eval: true
#| echo: true
def tally_negatives(numbers):
  tally = {}
  for n in numbers:
    if n < 0:
      if n not in tally:
        tally[n] = 0
      tally[n] += 1
  return tally

def main():
  assert  tally_negatives([1, -2, 0, -4, -2]) == {-2: 2, -4: 1}
  assert tally_negatives([]) == {}
  print("Passed all tests.")
  
main()
```


## What happens when we do `for k in dictionary:`

```{python}
#| eval: true
#| echo: true
scores = {'A': 10, 'B': 25, 'C': 27, 'D': 10, 'E': 5}
for key in scores:
  print(key)
```

## Methods for dictionaries

We can use `.values()` to get only the values in a dictionary:

```{python}
#| eval: true
#| echo: true
scores = {'A': 10, 'B': 25, 'C': 27, 'D': 10, 'E': 5}
scores.values()
```

We can use `.keys()` to get only the keys in a dictionary:

```{python}
#| eval: true
#| echo: true
scores = {'A': 10, 'B': 25, 'C': 27, 'D': 10, 'E': 5}
scores.keys()
```

We can use `.items()` to get tuples for keys and values:

```{python}
#| eval: true
#| echo: true
scores = {'A': 10, 'B': 25, 'C': 27, 'D': 10, 'E': 5}
scores.items()
```

## `for x in list`

```{python}
#| eval: true
#| echo: true
scores = {'A': 10, 'B': 25, 'C': 27, 'D': 10, 'E': 5}
for value in scores.values():
  print(value)
```


```{python}
#| eval: true
#| echo: true
scores = {'A': 10, 'B': 25, 'C': 27, 'D': 10, 'E': 5}
for key in scores.keys():
  print(key)
```

## `for key, value in dictionary.items()`

```{python}
#| eval: true
#| echo: true
scores = {'A': 10, 'B': 25, 'C': 27, 'D': 10, 'E': 5}
for key, value in scores.items():
  print(key, value)
```


## Write a function

1. Its name is `keys_and_values`
1. It takes a `dictionary` as argument
1. It returns a list with all the keys and values in the `dictionary`

Test cases:

```{python}
#| eval: false
#| echo: true
assert keys_and_values({'A': 10, 'B': 25, 'C': 27, 'D': 10, 'E': 5}) == \
    ['A', 10, 'B', 25, 'C', 27, 'D', 10, 'E', 5]
```

## Write a function -- solution

```{python}
#| eval: true
#| echo: true
def keys_and_values(dictionary):
  new_list = []
  for key, value in dictionary.items():
    new_list.append(key)
    new_list.append(value)
  return new_list

def main():
  assert keys_and_values({'A': 10, 'B': 25, 'C': 27, 'D': 10, 'E': 5}) == ['A', 10, 'B', 25, 'C', 27, 'D', 10, 'E', 5]
  
main()
```

## Write a function

1. Its name is `merge_dictionaries`
1. It takes two arguments: `dict_1` and `dict_2`
1. It mutates `dict_1`, by adding to it all key-values pairs in `dict_2`
1. If a key is in both dictionaries, the values are added
1. Don't use `.update()`

Test cases:

```{python}
#| echo: true
#| eval: false
dict_1 = {"a": 20, "e": 5}
dict_2 = {"e": 10, "i": 2}
assert merge_dictionaries(dict_1, dict_2) == {"a": 20, "e": 15, "i": 2}
```

## Write a function -- solution

```{python}
#| echo: true
#| eval: true
def merge_dictionaries(dict_1, dict_2):
  for key, value in dict_2.items():
    if key in dict_1:
      dict_1[key] += value
    else:
      dict_1[key] = value
  return dict_1
      
def main():
  dict_1 = {"a": 20, "e": 5}
  dict_2 = {"e": 10, "i": 2}
  assert merge_dictionaries(dict_1, dict_2) == {"a": 20, "e": 15, "i": 2}
  
main()
```