Python File I/O, Strings, Control Flow and Data Structures

Posted on Feb 20, 2026 in Computers

Python File I/O, Strings, Control Flow and Data Structures

File Input Examples

infile.read(1)
'T'
>>>>
infile.read(5)
'he 3 '
>>>>
infile.readline()
'lines in this file end with the new line
character.\n'
>>>>
infile.read()
'\nThere is a blank line above this line.\n'
>>>>
infile.close()

File Output Examples

outfile = open('test.txt', 'w')
>>>>
outfile.write('T')
1
>>>>
outfile.write('his is the first line.')
22
>>>>
outfile.write(' Still the first line...\n')
25
>>>>
outfile.write('Now we are in the second line.\n')
31
>>>>
outfile.write('Non string value like '+str(5)+' must be converted first.\n')
49
>>>>
outfile.write('Non string value like {} must be converted first.\n'.format(5))
49
>>>> outfile.close()

Conditional Statements (If / Else)

if temp > 86:
    print('It is hot!')
    print('Be sure to drink liquids.')
else:
    print('It is not hot.')
    print('Bring a jacket.')
print('Goodbye.')

BMI Function Example

def BMI(weight, height):
    'prints BMI report'
    bmi = weight * 703 / height**2
    if bmi < 18.5:
        print('Underweight')
    elif bmi < 25:
        print('Normal')
    else:  # bmi >= 25
        print('Overweight')

For Loops and Ranges

String Slicing and Methods

Link = '<a href="http://www.main.com/smith/index.html">http://www.main.com/smith/index.html</a>'
>>>> Link[:4]
'http'
>>>> Link[:4].upper()
'HTTP'
>>>> Link.find('smith')
20
>>>> Link[20:25]
'smith'
>>>> Link[20:25].capitalize()
'Smith'
>>>> Link.replace('smith','ferreira')
'<a href="http://www.main.com/ferreira/index.html">http://www.main.com/ferreira/index.html</a>'
>>>> Link
'<a href="http://www.main.com/smith/index.html">http://www.main.com/smith/index.html</a>'
>>>> New = Link.replace('smith','ferreira')
>>>> New
'<a href="http://www.main.com/ferreira/index.html">http://www.main.com/ferreira/index.html</a>'
>>>> Link.count('/')
4
>>>> Link.split('/')
['http:', '', 'www.main.com', 'smith', 'index.html']

Table = Str.maketrans(':,-' 3*' ')
Event.transl8(table)
'tuesdayFeb292012335Pm'
Event.transl8(table).split()
['tuesday', 'feb', '29', '2012', '3', '35', 'pm']

Formatting and Print Examples

Print(prodCostWght2talSep=';')
Morels1390.569.5
Print(prodCostWght2talSep=':::')
Morels:::139:::0.5:::69.5

Print('{},{} {},{} @ {}:{}:{}'.4mat(weekdayMonth,
DayYearHourMinute2nd))
WednesdayMarch102012 @11:45:33

List Iteration and String Formatting

Iterating Over Collections

for animal in pets:
    print(animal, end=' ')
cat dog fish bird

Sorted Check Function (Pseudo / Corrected)

def checkSorted(lst):
    'return True if sequence lst is increasing, False otherwise'
    # iterate through indices and compare adjacent items
    for i in range(len(lst) - 1):
        # compare lst[i] with lst[i+1]
        if lst[i] > lst[i+1]:
            # incorrectly ordered, return False
            return False
    # all adjacent pairs are in order
    return True

Arithmetic Sequence Check

def arithmetic(lst):
    if len(lst) < 2:
        # a sequence of length < 2 is arithmetic by definition
        return True
    # check that the difference between successive numbers is
    # equal to the difference between the first two numbers
    diff = lst[1] - lst[0]
    for i in range(1, len(lst) - 1):
        if lst[i+1] - lst[i] != diff:
            return False
    return True

Sum and Product Examples

Acronym and Divisors Functions

def acronym(phrase):
    'return the acronym of the input string phrase'
    # split phrase into a list of words
    words = phrase.split()
    # accumulate first character, as an uppercase, of every word
    res = ''
    for w in words:
        res = res + w[0].upper()
    return res


def divisors(n):
    'return the list of divisors of n'
    res = []  # accumulator initialized to an empty list
    for i in range(1, n + 1):
        if n % i == 0:  # if i is a divisor of n
            res.append(i)  # accumulate i
    return res

Nested Loops and 2D Printing

Finding First Negative and Row Processing

def greater(lst):
    # iterate through list lst and
    # compare each number with 0
    # Using counter loop pattern
    for i in range(len(lst)):
        if lst[i] < 0:
            # number at index i is first
            # negative number, so return i
            return i
    # if no negative number is found, return None
    return None


def before0(table):
    for row in table:
        for num in row:
            if num == 0:
                break
            print(num, end=' ')
        print()

>>>> before0(table)
2 3
4 5 6

def ignore0(table):
    for row in table:
        for num in row:
            if num == 0:
                continue
            print(num, end=' ')
        print()

>>>> ignore0(table)
2 3 6
3 4 5
4 5 6

User-Defined Indexes and Dictionaries

User-defined indexes and dictionaries

Lookup and Frequency Examples

def complete(abbreviation):
    'returns day of the week corresponding to abbreviation'
    days = {
        'Mo': 'Monday', 'Tu': 'Tuesday', 'We': 'Wednesday',
        'Th': 'Thursday', 'Fr': 'Friday', 'Sa': 'Saturday',
        'Su': 'Sunday'
    }
    return days[abbreviation]


def frequency(itemList):
    'returns frequency of items in itemList'
    counters = {}
    for item in itemList:
        if item in counters:
            # increment item counter
            counters[item] += 1
        else:
            # create item counter
            counters[item] = 1
    return counters

All examples and outputs preserved. Spelling and grammar corrected in explanatory text and comments. Code and outputs are shown as recorded interactive examples.