Programming with Python

Using range, write a loop that uses range to print the first 3 natural numbers.

for i in range(1, 4):
   print(i)

1
2
3

Write a loop that calculates the same result as 5 ** 3 using multiplication (and without exponentiation).

result = 1
for i in range(0, 3):
   result = result * 5
print(result)

125

Write a loop that takes a string, and produces a new string with the characters in reverse order.

newstring = ''
oldstring = 'Newton'
length_old = len(oldstring)
for char_index in range(length_old):
   newstring = newstring + oldstring[length_old - char_index - 1]
print(newstring)

'notweN'

Use a for loop to convert the string "hello" into a list of letters:

my_list = []
for char in "hello":
    my_list.append(char)
print(my_list)

["h", "e", "l", "l", "o"]

Which of the following would be printed if you were to run this code? Why did you pick this answer?

if 4 > 5:
    print('A')
elif 4 == 5:
    print('B')
elif 4 < 5:
    print('C')

C gets printed, because the first two conditions, 4 > 5 and 4 == 5 are not true, but 4 < 5 is true.

After reading and running the code below, explain the rules for which values are considered true and which are considered false.

if '':
    print('empty string is true')
if 'word':
    print('word is true')
if []:
    print('empty list is true')
if [1, 2, 3]:
    print('non-empty list is true')
if 0:
    print('zero is true')
if 1:
    print('one is true')

First line prints nothing: an empty string is false Second line prints 'word is true': a non-empty string is true Third line prints nothing: an empty list is false Fourth line prints 'non-empty list is true': a non-empty list is true Fifth line prints nothing: 0 is false Sixth line prints 'one is true': 1 is true

Write some conditions that print True if the variable a is within 10% of the variable b and False otherwise.

a = 5
b = 5.1

if abs(a - b) < 0.1 * abs(b):
    print('True')
else:
    print('False')

Another possible solution:

print(abs(a - b) < 0.1 * abs(b))

This works because the boolean objects True and False have string representations which can be printed.

Write some code that sums the positive and negative numbers in a list separately, using in-place operators.

positive_sum = 0
negative_sum = 0
test_list = [3, 4, 6, 1, -1, -5, 0, 7, -8]
for num in test_list:
    if num > 0:
        positive_sum += num
    elif num == 0:
        pass
    else:
        negative_sum += num
print(positive_sum, negative_sum)

21 -14

Here pass means “don’t do anything”. In this particular case, it’s not actually needed, since if num == 0 neither sum needs to change, but it illustrates the use of elif.

Explain what the overall effect of this code is:

left = 'L'
right = 'R'

temp = left
left = right
right = temp

The code swaps the contents of the variables right and left.

Compare it to:

left, right = right, left

Do they always do the same thing? Which do you find easier to read?

Yes, although it’s possible the internal implementation is different. Answers will vary on which is easier to read.

Write a function called fence that takes two parameters called original and wrapper and returns a new string that has the wrapper character at the beginning and end of the original.

def fence(original, wrapper):
    return wrapper + original + wrapper

Write a function called outer that returns a string made up of just the first and last characters of its input.

def outer(input_string):
    return input_string[0] + input_string[-1]

Write a function rescale that takes an array as input and returns a corresponding array of values scaled to lie in the range 0.0 to 1.0. (Hint: If \(L\) and \(H\) are the lowest and highest values in the original array, then the replacement for a value \(v\) should be \((v-L) / (H-L)\).)

def rescale(input_array):
    L = input_array.min()
    H = input_array.max()
    output_array = (input_array - L) / (H - L)
    return output_array

Run the commands help(numpy.arange) and help(numpy.linspace) to see how to use these functions to generate regularly-spaced values, then use those values to test your rescale function. Once you’ve successfully tested your function, add a docstring that explains what it does.

'''Takes an array as input, and returns a corresponding array scaled so
that 0 corresponds to the minimum and 1 to the maximum value of the input array.

Examples:
>>> rescale(np.arange(10.0))
array([ 0.        ,  0.11111111,  0.22222222,  0.33333333,  0.44444444,
       0.55555556,  0.66666667,  0.77777778,  0.88888889,  1.        ])
>>> rescale(np.linspace(0, 100, 5))
array([ 0.  ,  0.25,  0.5 ,  0.75,  1.  ])
'''

Rewrite the rescale function so that it scales data to lie between 0.0 and 1.0 by default, but will allow the caller to specify lower and upper bounds if they want. Compare your implementation to your neighbor’s: do the two functions always behave the same way?

def rescale(input_array, low_val=0.0, high_val=1.0):
    '''rescales input array values to lie between low_val and high_val'''
    L = input_array.min()
    H = input_array.max()
    intermed_array = (input_array - L) / (H - L)
    output_array = intermed_array * (high_val - low_val) + low_val
    return output_array

What does the following piece of code display when run - and why?

f = 0
k = 0

def f2k(f):
    k = ((f-32)*(5.0/9.0)) + 273.15
    return k

print(f2k(8))
print(f2k(41))
print(f2k(32))

print(k)

259.81666666666666
287.15
273.15
0

k is 0 because the k inside the function f2k doesn’t know about the k defined outside the function.

Read the traceback below, and identify the following pieces of information about it:

1. How many levels does the traceback have?
2. What is the file name where the error occurred?
3. What is the function name where the error occurred?
4. On which line number in this function did the error occurr?
5. What is the type of error?
6. What is the error message?

import errors_02
errors_02.print_friday_message()

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-2-e4c4cbafeeb5> in <module>()
      1 import errors_02
----> 2 errors_02.print_friday_message()

/Users/jhamrick/project/swc/novice/python/errors_02.py in print_friday_message()
     13
     14 def print_friday_message():
---> 15     print_message("Friday")

/Users/jhamrick/project/swc/novice/python/errors_02.py in print_message(day)
      9         "sunday": "Aw, the weekend is almost over."
     10     }
---> 11     print(messages[day])
     12
     13

KeyError: 'Friday'

1. 3 levels
2. errors_02.py
3. print_message
4. 11
5. KeyError
6. There isn’t really a message; you’re supposed to infer that Friday is not a key in messages.

1. Read the code below, and (without running it) try to identify what the errors are.
2. Run the code, and read the error message. Is it a SyntaxError or an IndentationError?
3. Fix the error.
4. Repeat steps 2 and 3, until you have fixed all the errors.

def another_function
    print("Syntax errors are annoying.")
     print("But at least python tells us about them!")
    print("So they are usually not too hard to fix.")

SyntaxError for missing (): at end of first line, IndentationError for mismatch between second and third lines.

Fixed version:

def another_function():
    print("Syntax errors are annoying.")
    print("But at least python tells us about them!")
    print("So they are usually not too hard to fix.")

1. Read the code below, and (without running it) try to identify what the errors are.
2. Run the code, and read the error message. What type of NameError do you think this is? In other words, is it a string with no quotes, a misspelled variable, or a variable that should have been defined but was not?
3. Fix the error.
4. Repeat steps 2 and 3, until you have fixed all the errors.

for number in range(10):
    # use a if the number is a multiple of 3, otherwise use b
    if (Number % 3) == 0:
        message = message + a
    else:
        message = message + "b"
print(message)

3 NameErrors for number being misspelled, for message not defined, and for a not being in quotes.

Fixed version:

message = ""
for number in range(10):
    # use a if the number is a multiple of 3, otherwise use b
    if (number % 3) == 0:
        message = message + "a"
    else:
        message = message + "b"
print(message)

abbabbabba

1. Read the code below, and (without running it) try to identify what the errors are.
2. Run the code, and read the error message. What type of error is it?
3. Fix the error.

seasons = ['Spring', 'Summer', 'Fall', 'Winter']
print('My favorite season is ', seasons[4])

IndexError; the last entry is seasons[3], so seasons[4] doesn’t make sense.

Fixed version:

seasons = ['Spring', 'Summer', 'Fall', 'Winter']
print('My favorite season is ', seasons[-1])

Suppose you are writing a function called average that calculates the average of the numbers in a list. What pre-conditions and post-conditions would you write for it? Compare your answer to your neighbor’s: can you think of a function that will pass your tests but not hers or vice versa?

# a possible pre-condition:
assert len(input) > 0, 'List length must be non-zero'
# a possible post-condition:
assert input.min() < average < input.max(), 'Average should be between min and max of input values'

Given a sequence of values, the function running returns a list containing the running totals at each index. Explain in words what the assertions in this function check, and for each one, give an example of input that will make that assertion fail.

def running(values):
    assert len(values) > 0
    result = [values[0]]
    for v in values[1:]:
        assert result[-1] >= 0
        result.append(result[-1] + v)
        assert result[-1] >= result[0]
    return result

• The first assertion checks that the input sequence values is not empty. An empty sequence such as [] will make it fail.
• The second assertion checks that the first value in the list is positive. Input such as [-1,0,2,3] will make it fail.
• The third assertion checks that the running total always increases. Input such as [0,1,3,-5,4] will make it fail.

Fix range_overlap. Re-run test_range_overlap after each change you make.

import numpy

def range_overlap(ranges):
    '''Return common overlap among a set of [low, high] ranges.'''
    if len(ranges) == 1: # only one entry, so return it
        return ranges[0]
    lowest = -numpy.inf # lowest possible number
    highest = numpy.inf # highest possible number
    for (low, high) in ranges:
        lowest = max(lowest, low)
        highest = min(highest, high)
    if lowest >= highest: # no overlap
        return None
    else:
        return (lowest, highest)

Write a command-line program that does addition and subtraction:

$ python arith.py add 1 2

3

$ python arith.py subtract 3 4

-1

# this is code/arith.py
import sys

def main():
    assert len(sys.argv) == 4, 'Need exactly 3 arguments'

    operator = sys.argv[1]
    assert operator in ['add', 'subtract', 'multiply', 'divide'], \
        'Operator is not one of add, subtract, multiply, or divide: bailing out'
    try:
        operand1, operand2 = float(sys.argv[2]), float(sys.argv[3])
    except ValueError:
        print('cannot convert input to a number: bailing out')
        return

    do_arithmetic(operand1, operator, operand2)

def do_arithmetic(operand1, operator, operand2):

    if operator == 'add':
        value = operand1 + operand2
    elif operator == 'subtract':
        value = operand1 - operand2
    elif operator == 'multiply':
        value = operand1 * operand2
    elif operator == 'divide':
        value = operand1 / operand2
    print(value)

main()

Using the glob module introduced earlier, write a simple version of ls that shows files in the current directory with a particular suffix. A call to this script should look like this:

$ python my_ls.py py

left.py
right.py
zero.py

# this is code/my_ls.py
import sys
import glob

def main():
    '''prints names of all files with sys.argv as suffix'''
    assert len(sys.argv) >= 2, 'Argument list cannot be empty'
    suffix = sys.argv[1] # NB: behaviour is not as you'd expect if sys.argv[1] is *
    glob_input = '*.' + suffix # construct the input
    glob_output = sorted(glob.glob(glob_input)) # call the glob function
    for item in glob_output: # print the output
        print(item)
    return

main()

Rewrite readings.py so that it uses -n, -m, and -x instead of --min, --mean, and --max respectively. Is the code easier to read? Is the program easier to understand?

# this is code/readings-07.py
import sys
import numpy

def main():
    script = sys.argv[0]
    action = sys.argv[1]
    filenames = sys.argv[2:]
    assert action in ['-n', '-m', '-x'], \
           'Action is not one of -n, -m, or -x: ' + action
    if len(filenames) == 0:
        process(sys.stdin, action)
    else:
        for f in filenames:
            process(f, action)

def process(filename, action):
    data = numpy.loadtxt(filename, delimiter=',')

    if action == '-n':
        values = data.min(axis=1)
    elif action == '-m':
        values = data.mean(axis=1)
    elif action == '-x':
        values = data.max(axis=1)

    for m in values:
        print(m)

main()

Separately, modify readings.py so that if no parameters are given (i.e., no action is specified and no filenames are given), it prints a message explaining how it should be used.

# this is code/readings-08.py
import sys
import numpy

def main():
    script = sys.argv[0]
    if len(sys.argv) == 1: # no arguments, so print help message
        print("""Usage: python readings-08.py action filenames
              action must be one of --min --mean --max
              if filenames is blank, input is taken from stdin;
              otherwise, each filename in the list of arguments is processed in turn""")
        return

    action = sys.argv[1]
    filenames = sys.argv[2:]
    assert action in ['--min', '--mean', '--max'], \
           'Action is not one of --min, --mean, or --max: ' + action
    if len(filenames) == 0:
        process(sys.stdin, action)
    else:
        for f in filenames:
            process(f, action)

def process(filename, action):
    data = numpy.loadtxt(filename, delimiter=',')

    if action == '--min':
        values = data.min(axis=1)
    elif action == '--mean':
        values = data.mean(axis=1)
    elif action == '--max':
        values = data.max(axis=1)

    for m in values:
        print(m)

main()

Separately, modify readings.py so that if no action is given it displays the means of the data.

# this is code/readings-09.py
import sys
import numpy

def main():
    script = sys.argv[0]
    action = sys.argv[1]
    if action not in ['--min', '--mean', '--max']: # if no action given
        action = '--mean'    # set a default action, that being mean
        filenames = sys.argv[1:] # start the filenames one place earlier in the argv list
    else:
        filenames = sys.argv[2:]

    if len(filenames) == 0:
        process(sys.stdin, action)
    else:
        for f in filenames:
            process(f, action)

def process(filename, action):
    data = numpy.loadtxt(filename, delimiter=',')

    if action == '--min':
        values = data.min(axis=1)
    elif action == '--mean':
        values = data.mean(axis=1)
    elif action == '--max':
        values = data.max(axis=1)

    for m in values:
        print(m)

main()

Write a program called check.py that takes the names of one or more inflammation data files as arguments and checks that all the files have the same number of rows and columns. What is the best way to test your program?

# this is code/check.py
import sys
import numpy

def main():
    script = sys.argv[0]
    filenames = sys.argv[1:]
    if len(filenames) <=1: #nothing to check
        print('Only 1 file specified on input')
    else:
        nrow0, ncol0 = row_col_count(filenames[0])
        print('First file %s: %d rows and %d columns' % (filenames[0], nrow0, ncol0))
        for f in filenames[1:]:
            nrow, ncol = row_col_count(f)
            if nrow != nrow0 or ncol != ncol0:
                print('File %s does not check: %d rows and %d columns' % (f, nrow, ncol))
            else:
                print('File %s checks' % f)
        return

def row_col_count(filename):
    try:
        nrow, ncol = numpy.loadtxt(filename, delimiter=',').shape
    except ValueError: #get this if file doesn't have same number of rows and columns, or if it has non-numeric content
        nrow, ncol = (0, 0)
    return nrow, ncol

main()

Write a program called line-count.py that works like the Unix wc command:

• If no filenames are given, it reports the number of lines in standard input.
• If one or more filenames are given, it reports the number of lines in each, followed by the total number of lines.

# this is code/line-count.py
import sys

def main():
    '''print each input filename and the number of lines in it,
       and print the sum of the number of lines'''
    filenames = sys.argv[1:]
    sum_nlines = 0 #initialize counting variable

    if len(filenames) == 0: # no filenames, just stdin
        sum_nlines = count_file_like(sys.stdin)
        print('stdin: %d' % sum_nlines)
    else:
        for f in filenames:
            n = count_file(f)
            print('%s %d' % (f, n))
            sum_nlines += n
        print('total: %d' % sum_nlines)

def count_file(filename):
    '''count the number of lines in a file'''
    f = open(filename,'r')
    nlines = len(f.readlines())
    f.close()
    return(nlines)

def count_file_like(file_like):
    '''count the number of lines in a file-like object (eg stdin)'''
    n = 0
    for line in file_like:
        n = n+1
    return n

main()