1. Chapter 4: For Loops, Strings, and Tuples
C10061 – Introduction to Computer Programming
Kent State University
Chapter 4: For Loops, Strings, and Tuples

2. Chapter Topics Covered
A new look at constants
Sequences
For loops and their uses
String functions
Sequence operators
Indexing sequences
String immutability
Slicing sequences
Tuples
Chapter 4: For Loops, Strings, and Tuples

3. A New Look at Constants
Recall that constants are variable values that are unchangeable.
Valuable in two ways:
Make programs clearer and easy to interpret
Prevents retyping or mistyping of the same value
New twist to our current naming convention—constants use all capital letters
Example: string = “Hello, World!”
STRING = “Hello, World!”
Chapter 4: For Loops, Strings, and Tuples

4. Sequences A sequence is an ordered list of elements.
The elements are arranged in order from the first element in the sequence to the last element.
Example: Given five elements in a sequence, the
elements are arranged as:
Computer’s
reference
Element 1
Element 2
Element 3
Element 4
Element 5
Chapter 4: For Loops, Strings, and Tuples

5. Sequences One type of sequence is a string.
Every character of the string is an element.
Example: Given the string “index”, the sequence
elements would be as follows:
Computer’s
reference i n d e x
Chapter 4: For Loops, Strings, and Tuples

6. For Loops Repetitious and execution is based on a sequence.
Unlike while loops that execute based on a condition.
Provides for sequential access of data.
From element 1 to element x.
Each element is processed in the loop body.
The last element processed ends the loop.
Chapter 4: For Loops, Strings, and Tuples

7. For Loops Loop variables do not need to be part of the loop body.
Actions inside the loop body may not use the variable.
Less restrictive in Python than in other programming languages.
Other languages require a counter variable to control the for loop (includes generics like i, j, k).
Python allows for direct access through the sequence—no counter variable is used.
Chapter 4: For Loops, Strings, and Tuples

8. For Loops
Syntax
for variable-name-for-elements in sequence-to-process:
loop body
Required colon
Sequence operator
Reserved word in Python
Chapter 4: For Loops, Strings, and Tuples

9. For Loop Uses Counting with sequences: range( ) function
One argument—positive number
Sequence range generated is 0 < positive number
Example: range(7)
will generate the sequence of
[0, 1, 2, 3, 4, 5, 6]
Chapter 4: For Loops, Strings, and Tuples

10. For Loop Uses Counting with sequences: range( ) function
Three arguments—starting point, ending point, number to count by
Sequence range generated is beginning point < ending point with intervals determined by number to count by
Example: range(5,-5,-1)
will generate the sequence of
[5, 4, 3, 2, 1, 0, -1, -2, -3, -4]
Chapter 4: For Loops, Strings, and Tuples

11. String Functions Counting sequence lengths: len( ) function
One argument needed
Argument may be any sequence
A specific string
Result of an I/O read
The length of the sequence is returned
The number of elements in the sequence
Includes all characters, spaces and symbols
Chapter 4: For Loops, Strings, and Tuples

12. Sequence Operators Checking existence or conditions: in operator
Use anywhere in a Python program
To check if an element is a member of a sequence (single member only or an error).
To create a condition to check.
Syntax: for element-to-check-for in sequence-to-check:
Syntax: if element-to-check-for in sequence-to-check:
Chapter 4: For Loops, Strings, and Tuples

13. Indexing Sequences
Recall that all sequences are stored sequentially, one element after another.
Recall that all sequences are stored sequentially, one element after another.
What if you want to access the fourth element only?
Use indexing for random access.
Computer’s
reference
Element 1
Element 2
Element 3
Element 4
Element 5
Chapter 4: For Loops, Strings, and Tuples

14. Indexing Sequences Steps needed: Assign a sequence to a variable.
Specify the index location as part of the variable.
Positive index is for sequential or positive random
Negative index is for negative random
Example: variable-name[x]
where x is the specific element to access.
Chapter 4: For Loops, Strings, and Tuples

15. Indexing Sequences Note: Computers begin counting at zero.
The range of sequence elements is 0 to sequence length – 1.
Attempting to access beyond the bounds of a sequence will result in an IndexError.
Positive index
Element 1
Element 2
Element 3
Element 4
Element 5
Negative index
Chapter 4: For Loops, Strings, and Tuples

16. String Immutability
Sequences may be changeable (mutable) or unchangeable (immutable).
Strings fall into the immutable category.
If we attempt to change the element in a string through an assignment statement, an error is generated (TypeError).
Example: string = “name”
string[0] = “s” will generate an error
Chapter 4: For Loops, Strings, and Tuples

17. String Immutability To “change” a string, create a new string.
Each character of a string is an element.
Elements may be concatenated, thus creating a new string.
May be accomplished through a for loop and by using the augmented assignment (+=) operator.
Example: user_input = raw_input(“Enter some text: “)
new_string = “ “
for i in user_input:
new_string =+ i
print new_string
Chapter 4: For Loops, Strings, and Tuples

18. Slicing Sequences
Used to copy continuous sections of elements (slices) in a sequence.
Copy a single element like indexing
Copy all elements or the entire sequence
Copy partial elements within a range
Elements are now blocked in by two points
A beginning slice point
An ending slice point
Slice points may be positive or negative
Chapter 4: For Loops, Strings, and Tuples

19. Slicing Sequences Example of slicing begin and end points:
Both may be positive
Both may be negative
One may be positive and one may be negative
Element 1
Element 2
Element 3
Element 4
Element 5
Chapter 4: For Loops, Strings, and Tuples

20. Slicing Sequences Avoid the impossible slice
Results from starting point > ending point
Results from backward references
Produces an empty slice (no error generated)
Examples: [4:3], [-1,-3], [1:-5], [-2:0]
Element 1
Element 2
Element 3
Element 4
Element 5
Chapter 4: For Loops, Strings, and Tuples

21. Slicing Sequences: Shorthand
Omit the start point [:3]
Starts at the first element of the sequence
Omit the end point [1:]
Ends at the last element of the sequence
Omit both the start point and the end point [:]
Returns a “copy” of the entire sequence
Useful for quick copying and efficient programming
Chapter 4: For Loops, Strings, and Tuples

22. Tuples
Another type of sequence where the elements may be any of the following:
Strings
Numbers (integer or float)
Strings and numbers combined
Any type represented by a variable
Chapter 4: For Loops, Strings, and Tuples

23. Tuples Empty tuple Tuple with values
Create by naming the tuple and enclosing the values within ( )
Tuple with values
Create by separating values by commas and enclosing them within ( )
Example: empty_tuple = ( )
Example: string_tuple = (“value1”, “value2”)
number_tuple = (7,10)
combo_tuple = (1,”text”)
Chapter 4: For Loops, Strings, and Tuples

24. Tuples
Print tuple elements directly and the result will appear on a single line.
Example: tuple = (1, “For”, “you and me”)
print tuple
will generate the result of
(1, ‘For’, ‘you and me’)
Chapter 4: For Loops, Strings, and Tuples

25. Tuples
Print tuple elements using a loop and the elements will print on separate lines.
Example: tuple = (“I plan”, “to program”,
“in Python”)
for i in tuple:
print i
will generate the result of
I plan
to program
in Python
Chapter 4: For Loops, Strings, and Tuples

26. Tuples What else may be done with tuples?
Everything that is done with sequences
Obtain a length
Check for a condition by using the in operator
Index sequentially or randomly
Slice
Concatenate
Final note: Tuples are immutable so copying a tuple requires creating a new tuple.
Chapter 4: For Loops, Strings, and Tuples