slides-08-01.qmd

---
title: "intro to dictionaries (slides)"
format: revealjs
slide-number: true
df-print: kable
---

# CSc 110 - dictionaries

## Announcements

* Last lab for short project 06 is Today -- no lab for the rest of the week, we re-start lab sessions Tuesday October 22


# Dictionaries

## Data Structure

-   **A data-structure is a way of arranging and organizing data in a computer program**

-   Python has several useful data-structures built into the language

    -   One is a **list** (already covered)

    -   Another, **dictionary**


## Mapping

-   Many data structures allow data to be stored and retrieved using a concept called **mapping**

-   **Mapping** is the process of associating one value with another (a **key** with a **value**)

    -   Sometimes also referred to as Hashing or Associativity
    

## Mapping

-   Lists map **keys** to **values** too!

    -   Indices **of** the list are the **keys**

    -   Elements **in** the list are the **values**

-   Keys (indices) are used to acess or modify the elements in the list

![](images/list_key_value.png)

## Mapping and Lists

```{python}
#| eval: false
#| echo: true
numbers = [12, 49, -2, 26, 5, 17, -6]
```

-   What are the keys?
-   What are the values?
-   Which keys map to which values?

## Mapping and Lists

```{python}
#| eval: false
#| echo: true
numbers = [12, 49, -2, 26, 5, 17, -6]

# Using the key 3 to lookup the associated value of 26
# and then save the value into variable
new = numbers[3]

# Modifying the list so that the key 5 now maps to 77
# instead of 17
numbers[5] = 77
```

## Dictionary
- Like lists:
  
    -   Associates a set of keys to their corresponding values 
    -   Each key has exactly 1 associated value 
    
- Unlike lists:

    -   The keys can be types other than ints: strings 

## Dictionary

Example (mapping strings to integers)

```{python}
#| eval: true
#| echo: true
singers = { "Taylor Swift": 1989,
            "JVKE":  2001,
            "Bruno Mars":  1983}
        
# Using the key "JVKE" 
# to lookup the number 2001
singers["JVKE"]

# Modifying the value associated with "Bruno Mars" 
singers["Bruno Mars"] = 1985

# add a new key and value pair
singers["Doja Cat"] = 1995

print(singers)
```

## Evaluate the expressions

```{python}
#| eval: false
#| echo: true
word_count = {"and": 324, "why": 134, "cannot": 76, "sanded": 13}
word_count["cannot"] = 90
word_count["and"] = 110
word_count["foot"] = "feet"
word_count["and"] += 10

# what will these evaluate to?
word_count["and"] 
word_count["cannot"]
word_count["foot"]
```

## Evaluate the expressions

```{python}
#| eval: true
#| echo: true
word_count = {"and": 324, "why": 134, "cannot": 76, "sanded": 13}
word_count["cannot"] = 90
word_count["and"] = 110
word_count["foot"] = "feet"
word_count["and"] += 10

# what will these evaluate to?
word_count["and"]
```

```{python}
#| eval: true
#| echo: true
word_count["cannot"]
```

```{python}
#| eval: true
#| echo: true
word_count["foot"]
```

## Evaluate the expressions

```{python}
#| eval: false
#| echo: true
num_to_player = {}   # A valid, but empty dictionary
num_to_player[13] = "Paul George"
num_to_player[3]  = "Chris Paul"
num_to_player[23] = "Lebron James"
num_to_player[13] = "James Harden"

# what will these evaluate to?
num_to_player[23] 
num_to_player[3]
num_to_player[13]
num_to_player
```

## Evaluate the expressions

```{python}
#| eval: true
#| echo: true
num_to_player = {}   # A valid, but empty dictionary
num_to_player[13] = "Paul George"
num_to_player[3]  = "Chris Paul"
num_to_player[23] = "Lebron James"
num_to_player[13] = "James Harden"

# what will these evaluate to?
num_to_player[23]
```

```{python}
#| eval: true
#| echo: true
num_to_player[3]
```

```{python}
#| eval: true
#| echo: true
num_to_player[13]
```

```{python}
#| eval: true
#| echo: true
num_to_player
```

## Attendance
Attendance Evaluate the expression on Gradescope.


## Review: list methods

-   `.append(value)`
-   `.remove(value)`
-   `.pop(index)`

## Dictionary operations

```{python}
#| eval: false
#| echo: true
scores = {'A': 10, 'B': 25, 'C': 27, 'D': 10, 'E': 5}
 
scores['A+'] = 7     # Adds a key/value pair

scores['B'] = 20     # Changes value associated with a key

scores['C']    # Retrieves a value, given a key

scores.pop('E')      # Removes a key/value pair
```

## The `in` operator

With strings:

```{python}
#| eval: true
#| echo: true
"a" in "aeiou"
```

With lists:

```{python}
#| eval: true
#| echo: true
1 in [1, 2, 3, 4, 5]
```

And dictionary keys:

```{python}
#| eval: true
#| echo: true
word_count = {"and": 324, "why": 134, "cannot": 76, "Sanded": 13}
"why" in word_count
```


## Write a function

Its name is `count_vowels` that takes a `string` argument
It creates a `dictionary` and returns the `dictionary` with the count of every lowercase vowel in `string` (iterate over the string with a `for` loop)

Hint: initialize a dictionary with all the vowels as keys and the associated count as 0.

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

## 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 i in range(len(string)):
    char = string[i]
    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("pineapple") == {"a": 1, "e": 2, "i": 1, "o": 0, "u": 0}
  
main()
```


## Write a function

1.  Its name is `count_chars`
2.  It takes a `string` argument
3.  It creates a `dictionary`
4.  It returns the `dictionary` with the count of every characters in `string`

```{python}
#| eval: false
#| echo: true
print( count_chars("") ) # {}
print( count_chars("banana") ) # {"b": 1, "a": 3, "n": 2}
```


## Write a function -- solution

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

def main():
  print( count_chars("") ) # {}
  print( count_chars("banana") ) # {"b": 1, "a": 3, "n": 2}

main()
```