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@section('title')
    Lab 4, Part 1: Predicates and Functions
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# Lab 4, Part 2: Working with Strings
## How to take apart and put together strings ![](cs111beads.gif)

It's useful to know how to extract parts of strings. First, let's set
up some string variables (you can type these directly into your
Canopy interactive window -- it's better to type than to copy and
paste because the indentation will generate errors). Or you can copy
them with your mouse and paste them into a file, and then run the
file.

```py
   theTruth = 'CS111 rocks my world!'
   name = 'Tina Turner'
   line1 = 'What\'s love got to do with it'  # note the backslash 
```

<span class="note">Note: does it matter if you use single quotes (') or double quotes (") in pyt hon around strings? Short answer: no. [Click here for a long explanation.](https://docs.python.org/2/tutorial/introduction.html#strings)</span>  
Now, try out these python commands and see what you get:

```py
   theTruth[0]
   name[0]
   name[1:5]
   name[8:10]
```


Let's get a little bit tricker with our string examples:

```py
   name[-1]
   name[-3]
   line1[::-1]
   line1[len(line1)]  # oops. Why error message?
```
<span class="note">Note that we use the built-in python function `len` in the examples above. Here is a list of all the [built-in python functions](https://docs.python.org/2/library/functions.html).</span>  

Here are some special python string operators (`in` and `not in`):
```py
  'leia' in str2 ==> False  
  'our' in str2 ==> True
  'yoda' not in str2 ==> True
  
```

---
## Table of some string operations

<table class="ops" style="border:1">

<tbody>

<tr>
   <th>symbol</th>
   <th>operation</th>
   <th>example</th>
</tr>

<tr>
   <td class="up" align="center" style="font-size: 200%;">+</td>
   <td class="up">concatenation</td>
   <td class="up">
      str1 = 'I am '  <br>
      str2 = 'your father '  <br>
      darth = str1 + str2  <br>
      yoda = str2 + str1 <br>
</tr>
<tr>
     <td class="up" align="center" style="font-size: 200%;">*</td>
     <td class="up">repetition</td>
     <td class="up">'hee'*3 ==> 'heeheehee'</td>
</tr>

<tr>
     <td class="up" align="center" style="font-size: 200%;">[]</td>
     <td class="up">slice</td>
     <td class="up">str2[5]==> 'f'</td>
</tr>
<tr>
	<td class="up" align="center" style="font-size: 200%;">[:]</td>
	<td class="up">range</td>
	<td class="up">str2[2:5]==> 'ur '</td>
</tr>
<tr>
	<td class="up" align="center" style="font-size: 200%;">in</td>
	<td class="up">in</td>
	<td class="up">'dad' in str2 ==> False <br>
	     'at' in str2 ==> True</td>
</tr>

<tr>
	<td class="up" align="center" style="font-size: 200%;">not in</td>
	<td class="up">not in</td>
	<td class="up">str1 not in str2 ==> True  <br>
	str1 not in darth ==> False</td>
</tr>
</tbody>
</table>

## Write some simple string predicates
## Task 2A.
 `endsWithA` returns True if the string ends with the letter a; False otherwise. *Hint:* `word[-1]` *gives the last letter of a word, e.g.* `rain[-1]` returns `n`.

Examples:
```py
endsWithA('Lyn') ==> False
endsWithA('Justin') ==> False
endsWithA('Justina') ==> True
endsWithA('Mia') ==> True
endsWithA('ANNA') ==> True
```

## Task 2B.
 `hasPop` returns True if the string contains the substring 'pop'
; False otherwise.

Examples:
```py
hasPop('popcorn') ==> True
hasPop('lollipop') ==> True
hasPop('hippopotamus') ==> True
hasPop('poopy') ==> False
```

## Task 2C. Writing simple string functions

Add the functions below to your `lab04.py` file. You may, if you like,
use the built-in Python function `len(s)`, which returns the length of
a string `s`. Copy and paste the function definitions template below
into your `lab04.py` file. You'll need to fill in the missing code to
make sure that your code works correctly.  <span class="note">Note
that each function should be just one line of code.</span>

```py
def first(s):
    """returns the first letter of the given string
    e.g. first("tahoe") => t"""
    # ____________________
    # replace the blank line above with your code

def last(s):
    """returns the last letter of the given string
    e.g. last("tahoe") => e"""
    # ____________________
    # replace the blank line above with your code

def sameLength (s1, s2):
    """determines whether strings s1 and s2 have the same length
     e.g. sameLength("tahoe","snow") => False
     e.g. sameLength("cold","snow") => True"""
    # ___________________
    # replace the blank line above with your code

def average(a,b):
    """returns the average of two numbers
    e.g. average(9,12) => 10.5"""
    # ____________________
    # replace the blank line above with your code
```

## Task 2D. Writing more complex predicates

In this task, you'll write the following three string predicates (include them in your `lab04.py` file):

1.  **`isBookend(s)`**: A string is a bookend if it begins and ends with the same letter. For example, "neon" is a bookend and "cake" is not a bookend.
2. ** `siblings(s1,s2)`**: Two strings are siblings if
    *   they have the same length and
    *   they either begin or end with the same letter. For example, "Emily" and "Elise" and siblings; "Tucker" and "Oliver" are siblings, but "Sam" and "Scott" are not and "hi" and "hawaii" are not.
3.  **`stringAverage(s1,s2,s3)`**: Checks to see if the middle string has a length that i s the average of the other two strings. For example, "bowling" has a length that is the average of the length of the strings "football" and "soccer".

Write the predicates one at a time, testing each one to convince yourself that it works before moving on to the next one.


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