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<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8">
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  <title>INST126 — 7: Strings</title>
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<body>
<div class="reveal">
<div class="slides">

<!-- ===== TITLE ===== -->
<section>
  <h1>7: Strings</h1>
  <p class="subtitle">INST126 — Introduction to Programming</p>
</section>

<!-- ===== PART 1: WHY STRINGS ===== -->
<section>
  <h2>Why should we care about strings?</h2>
</section>

<section>
  <h2>Strings are everywhere</h2>
  <ul>
    <li>Email addresses</li>
    <li>Webpage URLs</li>
    <li>Names</li>
    <li>Documents, words</li>
    <li>Sales records</li>
  </ul>
  <p class="fragment">Strings are the ultimate "lingua franca" between systems<br>
  <small>(e.g., JSON: JavaScript <strong>Object</strong> Notation)</small></p>
</section>

<section>
  <h2><code>input()</code> returns a string!</h2>
  <pre><code class="language-python">age = input('What is your age?')
# user types 37
age</code></pre>
  <pre class="fragment"><code class="language-python">'37'   # &lt;-- a string, not the integer 37!</code></pre>
  <p class="fragment">What data type is in <code>age</code>?</p>
</section>

<!-- ===== PART 2: STRINGS AS SEQUENCES ===== -->
<section>
  <h2>Strings are sequences of characters</h2>
  <pre><code class="language-python">s = "banana"
for char in s:
    print(char)</code></pre>
  <pre class="fragment"><code>b
a
n
a
n
a</code></pre>
  <p class="fragment">Just like iterating through a list!</p>
</section>

<section>
  <h2>Characters ≠ just letters</h2>
  <pre><code class="language-python">a = ""   # empty string
b = " "  # string with one space character
print(len(a), len(b))  # 0  1
print(a == b)           # False</code></pre>
  <div class="key-point fragment">
    A blank space <em>is</em> a character! An empty string and a space are <strong>not</strong> the same.
  </div>
</section>

<section>
  <h2>Whitespace matters!</h2>
  <pre><code class="language-python">a = "James"
b = " James"
c = "James "
print(a == b)   # False
print(a == c)   # False</code></pre>
  <p class="fragment">Leading/trailing whitespace creates a <em>different</em> string.</p>
</section>

<section>
  <h2>Special whitespace characters</h2>
  <ul>
    <li><code>\t</code> — tab</li>
    <li><code>\n</code> — newline</li>
  </ul>
  <pre class="fragment"><code class="language-python">s = "a\ttab\nand a newline"
print(s)</code></pre>
  <pre class="fragment"><code>a	tab
and a newline</code></pre>
</section>

<section>
  <h2>Whitespace → structured data!</h2>
  <pre><code class="language-python">records = "NAME\tSCORE\tGRADE\nJoel\t81\tB-\nRony\t98\tA+\nSravya\t99\tA+"
print(records)</code></pre>
  <pre class="fragment"><code>NAME    SCORE   GRADE
Joel    81      B-
Rony    98      A+
Sravya  99      A+</code></pre>
</section>

<!-- ===== PART 3: PROPERTIES ===== -->
<section>
  <h2>Properties of strings</h2>
  <div class="key-point">
    Like lists, strings are <strong>sequences</strong>: ordered collections that support indexing, slicing, <code>len()</code>, and <code>in</code>.<br><br>
    But strings are <em>not</em> lists — and strings are <strong>immutable</strong>.
  </div>
</section>

<section>
  <h2>Strings are ordered</h2>
  <pre><code class="language-python">a_string = "hello world Hi"

# can be indexed
a_string[0]       # 'h'

# can be sliced
a_string[:5]      # 'hello'

# can be sorted
sorted(a_string)  # [' ', ' ', 'H', 'd', 'e', 'h', 'i', 'l', 'l', 'l', 'o', 'o', 'r', 'w']</code></pre>
</section>

<section>
  <h2>Strings have length</h2>
  <pre><code class="language-python">a_string = "hello world Hi"
print(len(a_string))   # 14</code></pre>
  <p class="fragment">Remember: spaces count as characters!</p>
</section>

<section>
  <h2>Strings are IMMUTABLE</h2>
  <pre><code class="language-python">a_string = "hello"
a_string.upper()     # this does NOT change a_string
print(a_string)      # still "hello"

# to preserve the change, you must reassign
a_string = a_string.upper()
print(a_string)      # now "HELLO"</code></pre>
  <div class="key-point fragment">
    Anytime you modify a string, you <strong>must</strong> assign the result to a variable to preserve the change.
  </div>
</section>

<!-- ===== ENCODING ASIDE ===== -->
<section>
  <h2>Aside: string encoding</h2>
  <p>Each character has a unique numeric code.<br>
  Use <code>ord()</code> to see it:</p>
  <pre><code class="language-python">print("A:", ord("A"))   # 65
print("a:", ord("a"))   # 97
print("0:", ord("0"))   # 48
print(" :", ord(" "))   # 32</code></pre>
  <p class="fragment"><code>'A'</code> is 65, <code>'a'</code> is 97 — completely different!</p>
</section>

<section>
  <h2>Why encoding matters</h2>
  <pre><code class="language-python">s1 = "James"
s2 = "james"
s3 = "James "
print(s1 == s2)   # False — different case
print(s1 == s3)   # False — trailing space</code></pre>
  <div class="key-point fragment">
    Always <strong>normalize</strong> your strings before comparing them!
  </div>
</section>

<!-- ===== PART 4: BASICS ===== -->
<section>
  <h2>Working with strings: basics</h2>
  <p class="subtitle">Indexing · Slicing · Concatenation · <code>in</code></p>
</section>

<section>
  <h2>Indexing</h2>
  <pre><code class="language-python">s = "my name is inigo montoya..."

s[0]    # 'm'
s[-1]   # '!'
s[11]   # 'i'</code></pre>
  <p class="fragment">Works exactly like list indexing. Negative indices count from the end.</p>
</section>

<section>
  <span class="practice-label">PRACTICE</span>
  <h3>Get the first number of the level</h3>
  <pre><code class="language-python">code = "INST201"
# How would you get the '2'?</code></pre>
</section>

<section>
  <h2>Slicing</h2>
  <pre><code class="language-python">code = "INST201"

area = code[:4]     # "INST"
number = code[4:]   # "201"</code></pre>
  <div class="key-point">
    <code>[start:stop]</code> — start is included, stop is <em>not</em>.<br>
    Omit start → from beginning. Omit stop → to the end.
  </div>
</section>

<section>
  <span class="practice-label">PRACTICE</span>
  <h3>How would you get the first 2 letters?</h3>
  <pre><code class="language-python">name = "Michelle"
# your code here</code></pre>
</section>

<section>
  <h2>Slicing + filtering pattern</h2>
  <pre><code class="language-python">names = ["Eliana", "John", "Elias", "Esther",
         "Joseph", "Ebenezer", "Eric", "Josiah",
         "Joe", "Eliza", "Frank", "Ellie"]

count = 0
for name in names:
    if name[:3] == "Eli":
        count += 1

count   # 4</code></pre>
</section>

<section>
  <span class="practice-label">PRACTICE</span>
  <h3>Grab the names that begin with <code>Jo</code></h3>
  <pre><code class="language-python">names = ["Eliana", "John", "Elias", "Esther",
         "Joseph", "Ebenezer", "Eric", "Josiah",
         "Joe", "Eliza", "Frank", "Ellie"]

target_names = []
# your code here</code></pre>
</section>

<section>
  <h2>Concatenation</h2>
  <div class="two-col">
    <div>
      <h3>Strings</h3>
      <pre><code class="language-python">s1 = "Hello"
s2 = " World!"
print(s1 + s2)
# Hello World!</code></pre>
    </div>
    <div>
      <h3>Lists</h3>
      <pre><code class="language-python">l1 = [1, 2, 3]
l2 = [4, 5, 6]
print(l1 + l2)
# [1, 2, 3, 4, 5, 6]</code></pre>
    </div>
  </div>
  <p class="fragment">Same <code>+</code> operator, same "join" idea!</p>
</section>

<section>
  <h2>The <code>in</code> operator</h2>
  <pre><code class="language-python">message = "hello, my name is inigo montoya"
keyword = "ingo"
print(keyword in message)   # True</code></pre>
  <p>Checks if a <em>substring</em> exists inside a string.</p>
</section>

<section>
  <span class="practice-label">PRACTICE</span>
  <h3>Only grab the <code>.edu</code> emails</h3>
  <pre><code class="language-python">emails = ["oasislab@gmail.com",
          "joelchan@terpmail.umd.edu",
          "rony@terpmail.com",
          "joelchan@umd.edu",
          "joelchan@gmail.com",
          "sarahp@umd.edu",
          "sarah@umd.org"]
target_emails = []
# your code here</code></pre>
</section>

<!-- ===== PART 5: STRING METHODS ===== -->
<section>
  <h2>Working with strings: advanced</h2>
  <p class="subtitle">String methods</p>
  <p style="font-size:0.8em;">Full list: <a href="https://docs.python.org/3/library/stdtypes.html#string-methods">docs.python.org</a></p>
  <p class="fragment" style="font-size:0.85em;">No need to memorize — just know they exist and how to look them up!</p>
</section>

<section>
  <h2>Checking a string</h2>
  <ul>
    <li><code>.isnumeric()</code> — all numeric?</li>
    <li><code>.isalnum()</code> — all letters and numbers?</li>
    <li><code>.isalpha()</code> — all letters?</li>
    <li><code>.startswith()</code> — starts with a substring?</li>
    <li><code>.endswith()</code> — ends with a substring?</li>
  </ul>
</section>

<section>
  <h2><code>.isnumeric()</code></h2>
  <pre><code class="language-python">a = " 123"
a.isnumeric()   # False! (leading space)

b = "567"
b.isnumeric()   # True</code></pre>
  <pre class="fragment"><code class="language-python"># practical: validate before converting
a = "x123"
b = "567"
if a.isnumeric() and b.isnumeric():
    print(int(a) * int(b))
else:
    print("One of the input strings contains non-digits!")</code></pre>
</section>

<section>
  <h2>Cleaning a sales record</h2>
  <pre><code class="language-python">sales_record = "$1,000,000"

cleaned = ""
for char in sales_record:
    if char.isnumeric():
        cleaned += char

print(cleaned)   # "1000000"</code></pre>
</section>

<section>
  <h2><code>.startswith()</code> / <code>.endswith()</code></h2>
  <pre><code class="language-python">l = ["INST201", "INST126", "INFM322", "CMSC126",
     "joelchan@umd.edu", "joelchan", ".edu", "sarah@umd.edu"]

for item in l:
    if item.startswith("INST"):
        print(item)</code></pre>
  <pre class="fragment"><code>INST201
INST126</code></pre>
</section>

<section>
  <span class="practice-label">PRACTICE</span>
  <h3>Use <code>.endswith()</code> to grab <code>.edu</code> emails</h3>
  <pre><code class="language-python">emails = ["oasislab@gmail.com",
          "joelchan@terpmail.umd.edu",
          "rony@terpmail.com",
          "joelchan@umd.edu",
          "joelchan@gmail.com",
          "sarahp@umd.edu",
          "sarah@umd.org"]
target_emails = []
# your code</code></pre>
</section>

<!-- ===== CLEANING / NORMALIZING ===== -->
<section>
  <h2>Cleaning / normalizing strings</h2>
  <ul>
    <li><code>.lower()</code> / <code>.upper()</code> — normalize case</li>
    <li><code>.replace()</code> — replace parts of a string</li>
    <li><code>.strip()</code> — remove leading/trailing whitespace</li>
  </ul>
</section>

<section>
  <h2>Chaining methods</h2>
  <pre><code class="language-python">def normalize_sales_record(sale):
    return sale.replace("$", "").replace(",", "")

sales_record = "$1,000,000"
cleaned = normalize_sales_record(sales_record)
print(cleaned)   # "1000000"</code></pre>
  <div class="key-point fragment">
    Chaining works because each <code>.method()</code> returns a <em>new</em> string, which you can immediately call another method on.
  </div>
</section>

<section>
  <h2>Normalizing for comparison</h2>
  <pre><code class="language-python">def normalize_string(s):
    return s.upper().strip()

n = " Josh Lyman"
m = "JOSH LYMAN"
print(n == m)   # False

n_normal = normalize_string(n)
m_normal = normalize_string(m)
print(n_normal == m_normal)   # True!</code></pre>
</section>

<!-- ===== PARSING / SPLIT ===== -->
<section>
  <h2>Parsing strings with <code>.split()</code></h2>
  <p>Use a <em>separator</em> to divide a string into a <strong>list</strong> of parts.</p>
  <ul>
    <li>Parse an email</li>
    <li>Parse a URL</li>
    <li>Parse a sentence into words</li>
    <li>Parse a timestamp</li>
  </ul>
</section>

<section>
  <h2>Example: parse an email</h2>
  <pre><code class="language-python">email = "joelchan@umd.edu"

elements = email.split("@")
print(elements)   # ['joelchan', 'umd.edu']

username = elements[0]
print(username)   # 'joelchan'</code></pre>
</section>

<section>
  <h2>Multi-step splitting</h2>
  <pre><code class="language-python">email = "joelchan@umd.edu"

split1 = email.split("@")       # ['joelchan', 'umd.edu']
domainserver = split1[1]         # 'umd.edu'

split2 = domainserver.split(".") # ['umd', 'edu']
domain = split2[1]               # 'edu'
print(domain)</code></pre>
</section>

<section>
  <span class="practice-label">PRACTICE</span>
  <h3>Get the hour from a timestamp</h3>
  <pre><code class="language-python">timestamp = "13:30:31"
# your code here</code></pre>
</section>

<section>
  <span class="practice-label">PRACTICE</span>
  <h3>Get the words in a sentence</h3>
  <pre><code class="language-python">message = "She sells seashells by the sea shore"
# your code here</code></pre>
</section>

<section>
  <h2>Parsing records</h2>
  <pre><code class="language-python">records_string = "NAME\tSCORE\tGRADE\nJoel\t81\tB-\nRony\t98\tA+\nSravya\t99\tA+"

records = []
for row in records_string.split("\n"):
    row_data = row.split("\t")
    records.append(row_data)

records</code></pre>
  <pre class="fragment"><code>[['NAME', 'SCORE', 'GRADE'],
 ['Joel', '81', 'B-'],
 ['Rony', '98', 'A+'],
 ['Sravya', '99', 'A+']]</code></pre>
</section>

<!-- ===== IMMUTABILITY REDUX ===== -->
<section>
  <h2>REMEMBER:<br>Strings are immutable!</h2>
</section>

<section>
  <h2>The change is lost!</h2>
  <pre><code class="language-python">a = "Hello"
b = "WORLD"
a.lower()   # this returns "hello" but...
b.lower()   # this returns "world" but...
print(a, b) # still "Hello WORLD"!</code></pre>
  <div class="key-point fragment">
    <strong>Fix:</strong> reassign the result!
    <pre><code class="language-python">a = a.lower()   # now a is "hello"
b = b.lower()   # now b is "world"</code></pre>
  </div>
</section>

<!-- ===== F-STRINGS ===== -->
<section>
  <h2>String formatting with f-strings</h2>
  <p class="subtitle">Composing strings from variables</p>
</section>

<section>
  <h2>The basics</h2>
  <p>Prefix with <code>f</code>, then put variables in <code>{}</code> slots:</p>
  <pre><code class="language-python">msg = "hello"
friend = "rony"
name = "anna"
output = f"{msg} {friend}, my name is {name}!"
print(output)</code></pre>
  <pre class="fragment"><code>hello rony, my name is anna!</code></pre>
</section>

<section>
  <h2>Expressions inside <code>{}</code></h2>
  <pre><code class="language-python">weight_kgs = 120
print(f"{weight_kgs} is {weight_kgs * 2.2} lbs")
# 120 is 264.0 lbs</code></pre>
  <pre class="fragment"><code class="language-python">birth_year = 1956
this_year = 2023
name = "Joel"
msg = f"Happy birthday, {name}! You are {this_year - birth_year} this year!"
print(msg)
# Happy birthday, Joel! You are 67 this year!</code></pre>
</section>

<section>
  <h2>f-strings in loops</h2>
  <pre><code class="language-python">names = ["Joel", "Sarah", "Michael", "Kacie"]
for name in names:
    message = f"Welcome, {name}!"
    print(message)</code></pre>
  <pre class="fragment"><code>Welcome, Joel!
Welcome, Sarah!
Welcome, Michael!
Welcome, Kacie!</code></pre>
</section>

<section>
  <h2>f-strings for debugging</h2>
  <pre><code class="language-python">sales = ["$100", "$250", "$500"]

for i in range(len(sales)):
    sale = sales[i]
    print(f"Processing the item at index {i}: {sale}")</code></pre>
  <pre class="fragment"><code>Processing the item at index 0: $100
Processing the item at index 1: $250
Processing the item at index 2: $500</code></pre>
</section>

<section>
  <h2>Controlling decimal places</h2>
  <pre><code class="language-python">x = 2
y = 3
message = f"{x} divided by {y} is {x/y:.2f}"
print(message)</code></pre>
  <pre class="fragment"><code>2 divided by 3 is 0.67</code></pre>
  <div class="key-point fragment">
    <code>:.2f</code> → show 2 decimal places for a float
  </div>
</section>

<section>
  <span class="practice-label">PRACTICE</span>
  <h3>Complete the output message</h3>
  <pre><code class="language-python">tip = 0.18
check = 25.00
total_value = check + check * tip
# print: "Please charge my card for $29.50"
# msg = f"..."
# print(msg)</code></pre>
</section>

<!-- ===== RECAP ===== -->
<section>
  <h2>Recap</h2>
  <ul>
    <li>Strings are <strong>sequences</strong> of characters (like lists)</li>
    <li>Strings are <strong>immutable</strong> (unlike lists) — always reassign!</li>
    <li><strong>Indexing &amp; slicing</strong> work just like lists</li>
    <li><strong>String methods</strong>: <code>.isnumeric()</code>, <code>.startswith()</code>, <code>.endswith()</code>, <code>.lower()</code>, <code>.upper()</code>, <code>.replace()</code>, <code>.strip()</code>, <code>.split()</code></li>
    <li><strong>f-strings</strong>: <code>f"Hello, {name}!"</code></li>
    <li>Always <strong>normalize</strong> before comparing!</li>
  </ul>
</section>

<section>
  <h1>Questions?</h1>
</section>

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