Lec 11

Lec 11, Mon 02/10

Lecture 11: File I/O + Binary Numbers

Feedback from this week’s lab:

Confused with pytest, please provide additional examples in the lab.
- Look back at previous labs!
- Look at the homework for using pytest with a list
- There will be pytest examples in this week’s lab
Liked a more practical lab! (:

Announcements

HW07

Due Sat 2/15 at 11 am
https://ucsb-cs8.github.io/w20/work_list
HW 1-4 replacement assignment
Will be released a little later

No Common Final

Will be defaulting to times scheduled on Gold

Review

range()
while loops
- a condition-controlled loop that repeats while condition is true
- syntax:
```
  while CONDITION:
  # loop body
```

for loops

a count-controlled loop that repeats a specified number of times

syntax:

  for VAR in COLLECTION:
  # loop body
  One or more statements

For both loops, anything on the same indentation as the for and while statement will be executed when the loop ends
continue will get you out of the loop that you are in and will recheck the condition
break takes us out of the loop that it is in
- example:
```
  For loop
  While loop
  # Statments
  Break
  # Statements
```
  - Break will get us out of the while loop, not the for loop!

What is the output of this code?

for x in range(1, 4, 2):
    print (2**x, end = " "

Output: 2 8

Answer explained:

range(starting point, ending point, step-size)
range(1, 4, 2) evaluates to 1, and 3
2**x raises 2 to the power of x
print(2**x, end = " " prints 2 to the power of 1 and 2 to the power of 3
- Thus, the ouput is 2 8

What does `end = " "` do?

python puts a new line at the end of a printed value as a defaults
We are overwriting the default, and setting the end as a space
Similarly, python puts a space between printed value as a default. So, sep = overwrites the default separator

Starting iteration using `range()`

start = 2
end = 9
step = 3
for i in range(start, end, step):
    print i

i defaults as the start value, then increases by the step value, and the for loop will continue to execute while i < end

String iteration

s = "Hi there!"
for c in s:
    print(c) # prints the characters in s
    
for i in range(len(s)):
    print (i) # prints the indicies of the letters in s

Indexing a string

for i in range(len(s)):
    print(s[i]) # prints each letter in s

File I/O

Opening and reading

We will be opening and reading text files.

fobj = open("info.txt") # Open "info.txt"
content = fobj.readlines() 
fobj.close() # Close "info.txt"

Get in the habit of closing files!
Note: python has a way to open a file without having to close it, but we will not be using that method in this class!
.readlines() reads a bunch of lines that python gets from this file and stores it as a list
- The output is a list with all the lines of text in the file
open() opens the filename passed inside
- Returns a file object that is connected to the file, we get to name the object
Does this only work for text files?
- No! Works with any file
- Will be able to open it but may not succeed to get the contents inside the file

Opening and writing

Write contents produced back into a different file.

fobj = open("info.txt", "w") # "w" is a write flag that says we will overwrite any contents in file 
content = fobj.write()
fobj.close()

.write() tells file object what we want to write
- Arguement must be a string

Demo

fobj = open("sample_file.txt", "w")

for x in range(1, 4, 2):
    fobj.write(str(2**x) + " ")

fobj.close

Output: contents in “sample_file.txt” are changed. Now the file contains: 2 8

fobj = open("sample_file.txt", "a") # "a" is an append flag that says we will add to any contents in file 

for x in range(1, 4, 2):
    fobj.write(str(2**x) + " ")

fobj.close

Output: 2 8 is added to contents in “sample_file.txt”

Can be tedious to open file every time, so we can store the contents in the file in a variable.

fobj = open("sample_file.txt", "r") # "r" is an read flag that reads the contents in file 

contents = fobj.readlines() # Store file object in a list variable 

fobj.close

print(contents)

for line in contents:
    print(line) # Print each line separately instead of as a list

Another example:

file_obj = open("another_sample.txt")

message = file_obj.readlines() # Each line in another_sample.txt is an element in the list, message

for line in message:
    print(line, end = "") # Prints each element in message
    
# Could also print each element in message using indexing explicitly:
for i in range(len(message)):
    print(message[i], end = "")
    
print("Contents of message:", message) # Prints message as a list

Looking forward to next topic:

This is how we count:

In Binary, we only have two digits: 0 and 1

00 –> 00
01 –> 01
10 –> 02
11 –> 03
100 –> 04

We will be writing a function where given a string, we will confirm if this is a binary string or not (contains only binary characters)

def only_binary(input_str):
    """
    If input_str has only 1s and 0s 
        return True
    Otherwise,
        return False
    """

    for char in input_str:
        if char != '1' or char != '0':
            return False
    return "stub"

only_binary('1')

False

only_binary('0')

False

Hmmm, not what we expected. Debugging skill: any time your function is not acting as expected, use the simplest possible input to see what is happening.

def only_binary(input_str):
    """
    If input_str has only 1s and 0s 
        return True
    Otherwise,
        return False
    """

    for char in input_str:
        if char != '1' and char != '0': # Notice we changed OR to an AND
            return False
    return True

only_binary('1')

True

only_binary('0')

True

Now that our function is working properly, we can give it more complicated inputs.

only_binary('1011')

True

only_binary('10171')

False