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Updated 9-25-2020

Python Tutorial 2:

Strings, Lists, Loops, and Conditionals

More fun with strings

Strings can be:

• Indexed (using [ ]) : retrieve one character from a particular location in a string

  sample_string = "sample"
  i1 = sample_string[0] #note, index counts start at 0 NOT 1
  print(i1)
  > s
  

  i2 = sample_string[1] #second letter
  print(i2)
  > a
  

  i3 = sample_string[-1] #last letter
  print(i3)
  > e
  

• Sliced (using [ : ]) : retrieve multiple contiguous characters from a particular location in a string

  #slices
  s1 = sample_string[0:1] #start at position 0, grab characters until you get to position 1
  s2 = sample_string[0:2] #start at position 0, grab characters until you get to position 2
  s3 = sample_string[1:-1] #start at position 1, grab characters until you get to last position
  s4 = sample_string[1:] #start at position 1, grab all following characters
  s5 = sample_string[:2] #start at beginning, grab characters until you get to position 2
  print(s3)
  > ampl
  

• Concatenated (using “+”) : strings can be added together to make longer strings

  #concatenation
  sample1 = "awesome"
  sample2 = "super"
  sample3 = sample2 + sample1
  print(sample3)
  > superawesome
  

  sample4 = sample2 + " " + sample1 #here we add a space
  print(sample4)
  > super awesome
  

• Searched (using in) : we can check if certain characters (or character combinations) are in a string

  #searching in strings
  if "a" in sample1:
    print("yay!")
  else:
    print("nope")
  > yay!
  

  #searching in strings
  if "a" in sample2:
    print("yay!")
  else:
    print("nope")
  

  > nope
  

Lists

Lists are awesome.

In Python:

• lists are indicated by square brackets [ ].
• items within a list of separated by commas.
• Almost any Python object can be a list item.

Lists can be:

• Indexed (using [ ]) : retrieve one list item from a particular location in a list

  #indexing
  l1 = ["Windows","Mac","Linux"]
  print(l1[1]) #just like strings, we can get particular list items
  

  > Mac
  

• Sliced (using [ : ]) : retrieve multiple contiguous list items from a particular location in a list

  #slicing
  print(l1[1:]) #just like strings, we can get multiple list items
  

  > ['Mac', 'Linux']
  

• Concatenated (using +) : combined two lists to make a longer list

  l2 = [1,2,3]
  l3 = l1+l2
  print(l3)
  

  ["Windows","Mac","Linux",1,2,3]
  

• Appended (using .append()): add particular objects to a list

  #append
  l1.append("new string")
  print(l1)
  

  > ['Windows', 'Mac', 'Linux', 'new string']
  

• Changed (using list_name[position] = x ) : We can replace items in certain positions in lists

  #Change items
  l1[0] = "rubbish"
  print(l1)
  

  > ['rubbish', 'Mac', 'Linux', 'new string']
  

• Searched (using loops and conditionals, examples in next section) : we can check if certain objects are in a list

Conditional statements

Conditional statements are very powerful. The most commonly used conditional statement is the ‘if’ statement and its children, the ‘elif’ and ‘else’ statements. Conditional statements use operators (see table below).

Python Operators:

Operator	Description
x == y	x is equal to y
x != y	x is not equal to y
x > y	x is greater than y
x < y	x is less than y
x >= y	x is greater than or equal to y
x <= y	x is less than or equal to y
x in y	x is a part of y

The basic format for conditional statments is if conditional_stmnt: do something. Some examples:

• Working with numbers

  int1 = 2
  if int1 < 2:
    #note that the "do something" statement is indented (by a tab or 4 spaces)
    print("The number is smaller than 2")
  elif int1 == 2:
    print("The number is 2!")
  else:
    print("The number is greater than two")
  

  > The number is 2!
  

• Working with strings

  #we can check to see if two string are the same
  str1 = "sample string"
  if str1 == "sample string":
    print("match!")
  

  > match!
  

  #we can also check to see if a character is in a string
  if "a" in str1:
    print("yay!")
  

  > yay!
  

• Working with lists

  #we can check to see if two lists are the same
  list1 = ["this", "is", "a", "list", "of", "strings"]
  if list1 == ["this", "is", "a", "list", "of", "strings"]:
    print("match!")
  

  > match!
  

  #we can also check to see if an item (int, float, string, etc.) is in a list
  if "list" in list1:
    print("yay!")
  

  > yay!
  

Loops

Loops are awesome. With conditional statements, they are the backbone of programming

Things to remember when working with loops:

• You can loop over any iterable item (e.g., strings, lists, tuples, and dictionaries)
• Loops use local variables, but can also use global variables. Don’t confuse the two!
• You can have conditional statements inside of loops
• You can have loops inside of loops
• You must be careful with loop levels

The basic syntax for a loop is for local_variable in iterable: do something

Examples:

#Creating a loop:
sl = ["a", "wonderful", "list", "this", "is"]

for x in sl: #note that 'x' is a local variable and is completely arbitrary
	print(x) #we can call the local variable
#note that the "do something" statement (i.e., print(x)) is indented by a tab (or 4 spaces)

> a
> wonderful
> list
> this
> is

#if statements in loops
for x in sl:
	if "t" in x:
			print(x)
		else:
			continue #continue tells the loop to go to the next item

> list
> this

Exercises

1. Assign the string “This is an awesome sample sentence” to the variable a

2. Split the string into a list of words and assign it to variable b

3. Write a loop that prints each item in b

4. Define a new empty list and assign it to variable c

5. Write a loop that adds each item in b to c if the last letter in the item is “e”