loops + strings (class slides)

CSc 110 - looping through strings

Announcements

• Midterm 1 October 2 – BRING PHOTO ID
• Modules 1 to 5 – study guide under practice problems page on website
• Review session Tuesday, October 1, 5pm to 7pm
  • Gittings 129b

String Manipulation

String indexing

• Strings are sequences in Python
• We can retrieve values in a sequence using [ ]

name = "Adriana"
name[0]

'A'

Note that the first index in a sequence is always zero.

String indexing

Since the first index of a sequence is zero, the last index is going to be the length of the string minus 1

name = "Adriana"
len(name)

7

name = "Adriana"
name[6]

'a'

name = "Adriana"
name[7] # this will throw an error

Evaluate expressions

my_str = "Clint Eastwood"
my_str[11] + my_str[1] + my_str[13]

my_str = "cider"
my_str[0] + my_str[4] + my_str[1] + my_str[3] + my_str[2]

my_str = "dusty"
my_str[2] + my_str[3] + my_str[1] + my_str[0] + my_str[4]

Evaluate expressions

my_str = "Clint Eastwood"
my_str[11] + my_str[1] + my_str[13]

'old'

my_str = "cider"
my_str[0] + my_str[4] + my_str[1] + my_str[3] + my_str[2]

'cried'

my_str = "dusty"
my_str[2] + my_str[3] + my_str[1] + my_str[0] + my_str[4]

'study'

Write a function

1. Its name is four_letter_anagram
2. Its parameters are a string, and four integers a, b, c and d
3. It returns the anagram of the string according to the indices a, b, c and d – concatenate the individual characters
4. Test case: four_letter_anagram("balm", 2, 1, 3, 0) should return lamb

Write a function - solution + add to it

Add the following test cases to the code below:

• loin returns lion
• lugs returns slug
• reap returns pear

def four_letter_anagram(word, a, b, c, d):
  return word[a] + word[b] + word[c] + word[d]

def main():
  print( four_letter_anagram("balm", 2, 1, 3, 0) )

main()

lamb

Submit code for attendance

Submit your four_letter_anagram function to Gradescope for attendance.

Name your file anagram.py

in operator

The in operator determines whether a given value is a constituent element of a sequence (such as a string)

"a" in "Adriana"

True

"ana" in "Adriana"

True

"i" in "aeiou"

True

Evaluate the expressions

"a" in "aeiou"
"b" in "aeiou"
not "b" in "aeiou"
"0" in "0987654321."
"10.0" in  "0987654321."

Evaluate the expressions

"a" in "aeiou"

True

"b" in "aeiou"

False

not "b" in "aeiou"

True

"0" in "0987654321."

True

"10.0" in  "0987654321."

False

Before we proceed, a refresher …

Remember the string method.isnumeric()?

user_input = "paul"
user_input.isnumeric()

False

user_input = "12"
user_input.isnumeric()

True

user_input = "12.5"
user_input.isnumeric()

False

Write your own is numeric for one character

Write a Python function that does the following:

1. Its name is is_numeric_one_char
2. It takes one string argument of length of 1 (check if argument is valid with len())
3. It returns True if the argument is a digit (0-9), False otherwise (use the in operator)

print( is_numeric_one_char("0") ) # True
print( is_numeric_one_char("000") ) # False
print( is_numeric_one_char("9") ) # True
print( is_numeric_one_char("a") ) # False

Write a function – solution

def is_numeric_one_char(char):
  if len(char) > 1:
    return False
  
  return char in "0123456789"

def main():
  print( is_numeric_one_char("0") ) # True
  print( is_numeric_one_char("000") ) # False
  print( is_numeric_one_char("9") ) # True
  print( is_numeric_one_char("a") ) # False
  
main()

True
False
True
False

Write a is_numeric() for any string length

1. Its name is is_numeric
2. It takes one string argument of any length
3. It returns True if the first character of the argument is a digit (0-9), False otherwise (remember to use [ ] to index the string and the in operator)

Test cases:

print( is_numeric("0") ) # True
print( is_numeric("000") ) # True
print( is_numeric("9") ) # True
print( is_numeric("a") ) # False

Write a function – solution

def is_numeric(char):
  return char[0] in "0123456789"

def main():
  print( is_numeric("0") ) # True
  print( is_numeric("000") ) # True
  print( is_numeric("9") ) # True
  print( is_numeric("a") ) # False
  
main()

True
True
True
False

Improving on is_numeric()

What if we want to check every character in a string of any length?

What if the string is of length 2 or 5 or 45?

While loops – using an index

• One technique that can be used to control the number of loop iterations is using an index variable
• For while loops, an index variable is:
  • Defined before the loop
  • Used in the condition of the loop
  • Incremented within the loop

While loops – using an index

index = 0
while index < 5:
    print('Print ' + str(index))
    # Can add other lines here too
    index = index + 1

Print 0
Print 1
Print 2
Print 3
Print 4

index = 5
while index > 0:
    print('Print ' + str(index))
    # Can add other lines here too
    index = index - 1

Print 5
Print 4
Print 3
Print 2
Print 1

Improve on your is_numeric()

1. Its name is is_numeric
2. It takes one string argument of any length
3. It returns True if every character in the argument is a digit (0-9), False otherwise
4. Remember to use [ ] to index the string, use a while loop with a len() condition, and an index – something like string[index] with index being updated inside a while loop

print( is_numeric("0") ) # True
print( is_numeric("1090") ) # True
print( is_numeric("95") ) # True
print( is_numeric("10a") ) # False

is_numeric() solution

def is_numeric(my_string):
  index = 0
  while index < len(my_string):
    if my_string[index] not in "0123456789":
      return False
    index += 1
  return True

def main():
  print( is_numeric("234") ) # True
  print( is_numeric("abc") ) # False
  print( is_numeric("12c") ) # False
  print( is_numeric("12.3") ) # False
  
main()

True
False
False
False

is_numeric() solution

How do we change our function so that the last test case (“12.3”) returns True instead?

def is_numeric(my_string):
  index = 0
  while index < len(my_string):
    if my_string[index] not in "0123456789":
      return False
    index += 1
  return True

def main():
  print( is_numeric("234") ) # True
  print( is_numeric("abc") ) # False
  print( is_numeric("12c") ) # False
  print( is_numeric("12.3") ) # False
  
main()

True
False
False
False

is_numeric() solution

How do we change our function so that the last test case (“12.3”) returns True instead?

def is_numeric(my_string):
  index = 0
  while index < len(my_string):
    if my_string[index] not in "0123456789.":
      return False
    index += 1
  return True

def main():
  print( is_numeric("234") ) # True
  print( is_numeric("abc") ) # False
  print( is_numeric("12c") ) # False
  print( is_numeric("12.3") ) # True
  print( is_numeric("1.2.3") ) # True
  
main()

True
False
False
True
True

is_numeric() solution

def is_numeric(my_string):
  # create control variable for one decimal point
  decimal_point = False
  # create index variable
  index = 0
  while index < len(my_string):
    # first check, can character be found in a number?
    if my_string[index] not in "0123456789.":
      return False
    # second check, if a period, has a period been found before?
    if my_string[index] == ".":
      if decimal_point: # a previous period was detected
        return False
      else: # first period detected
        decimal_point = True
    # increment index  
    index += 1
  # while loop executed without returning False
  # that means every character is valid, so return True
  return True

def main():
  print( is_numeric("234") ) # True
  print( is_numeric("abc") ) # False
  print( is_numeric("12c") ) # False
  print( is_numeric("12.3") ) # True
  print( is_numeric("1.2.3") ) # False
  
main()

True
False
False
True
False

Quiz 04

current time

You have 10 minutes to complete the quiz

• No need for comments, no need for a main(), no need to print test cases
• Just write your function and what’s inside the function
• DO NOT USE NESTED CONDITIONALS

Built-in functions you can use: round(), input(), float(), str(), int() — you don’t have to use all of these