1. Introduction to Computing Science and Programming I
Loops
Introduction to Computing Science and Programming I

2. Loops
To this point you’ve learned how to write code that executes from top to bottom as well as how to use conditional (if) statements to select or skip parts of the code.
Loop structures allow the programmer to have sections of code executed multiple times. This allows many programs that couldn’t be written without loops as well as making many programs that can be written without loops, more simple.

3. Algorithms With Loops
Before getting into how to write the loops themselves, let’s look at some problems that require loops.

4. Prime Number Algorithm
Here’s an algorithm we looked at previously to test whether a user input number is prime.
set divisor to 2
write “Enter an integer greater than 2”
read userNumber
repeat while divisor2 < userNumber
if userNumber % divisor is equal to 0
write “The number is not prime, it is divisible by :”
write divisor
exit program
set divisor to divisor +1
write “The number is prime.”

5. Prime Number Algorithm
Notice that we have a section of the algorithm we want to repeat
repeat while divisor2 < userNumber
if userNumber % divisor is equal to 0
write “The number is not prime, it is divisible by :”
write divisor
exit program
set divisor to divisor +1
The algorithm wants to repeat the indented section of code while (divisor2 < userNumber)

6. Algorithms A couple simpler algorithms that will need to repeat code.
Factorial
n! = n * (n-1) * (n-2) *…*2 * 1
5! = 5 * 4 * 3 * 2 * 1 = 120
Algorithm
write “Enter a nonnegative integer:”
read n
set factorial to 1
do this for i equal to each number from 1 to n:
set factorial to factorial * i
write factorial
Notice the code that will be repeated, but this time it is stated differently. Instead of repeat while this is true, it says do this this many times.

7. A Couple More Algorithms
Repeatedly prompting a user until they give proper input
set number to 0
repeat while number < 1
write “Enter a positive integer: “
read number
Printing out the powers of 2
write “How many powers of 2 should be printed?”
read max
set value to 1
do this max times
write value
set value to value * 2

8. Two Kinds of Loop while loops for loops
These are called indefinite loops because they are often used to repeat code without knowing how many times it will be repeated beforehand.
for loops
These are called definite loops because they usually will repeat a set number of times

9. while Loops while condition: code block
Similar to an if statement, a while loop is given a condition (boolean expression). The code block associated with the while loop will repeat over and over again until the condition becomes false.

10. while Loops A simple example that prints the number 1 through 3.
while num <= 3:
print num
num = num + 1
print “Finished”
When the while statement is reached the first time, num is 1 which is less than five. Therefore execution enters the code block and repeats the code block until num is greater than 5

11. while Loops Unwinding the loop
To understand what’s going on we can ‘unwind’ the loop
num = 1
while num <= 3:
print num
num = num + 1
print “Finished”
Check the condition, is num <=3? Yes, so enter the code block
num = num (num is 2)
Check the condition, is num <= 3? Yes, so repeat the code block
num = num (num is 3)
num = num (num is 4)
Check the condition, is num <= 3? No, so skip the code block

12. while Loops
Remember that the condition is checked before the block of code is executed for the first time.
Therefore, the code in a while loop may never be executed.
You also need to make sure the condition eventually becomes false, if not you will have an infinite loop. The program will never leave the while loop and never finish

13. while Loops
while loops are an easy way to wait for proper input from the user
number = 0
while number < 1:
number = int(raw_input(“Enter a positive integer: “))
print “Your number is “ + str(number)

14. for Loops
For the time being all of the for loops we look at will look something like this
for x in range(5):
print x
We’ll learn more general uses of it, but for now the for statement will be of the form
for varname in range(..):
Here we just have the single statement ‘print x’ inside the loop, but you have any sized code block you like

15. range Function
range is a Python function that returns a list of numbers
If you give range one argument, it returns a list of integers starting at 0, ending at one less than the argument
range(4) = [0, 1, 2, 3]
range(10) = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
You can also give range a beginning and ending number. The list starts at the first argument and ends at one less than the second argument.
range(1,5) = [1, 2, 3, 4]
range(-2,2) = [-2, -1, 0, 1]
range(0,4) = range(4)
We will learn about the list data type and how to create our own lists later

16. for Loops
What the for loop does is assign the values returned by range to the variable one by one and executes the code block for each value
for x in range(5):
print x
x will be assigned the 5 values in range, and those values will be printed

17. for Loops Unwinding a for loop for x in range(5): print x
Remember, range(5) = {0,1,2,3,4}
x=0 (first value in range(5))
x=1 (second value in range(5))
x=2 (third value in range(5))
x=3 (fourth value in range(5))
x=4 (final value in range(5))
No more values remain in range(5) so we are finished

18. for Loops
You can just use the structure to repeat code a certain number of times, ignoring the value assigned to the variable in the for statement
Printing out the powers of 2
max = int(raw_input("How many powers of 2 should be printed? "))
num = 1;
for i in range(max):
print num
num = num * 2
Notice that the values in range(max) will be assigned to i, but we don’t use i inside the loop. We’re just using the structure to repeat the code block max times.

19. for Loops Unwinding a for loop
Assume a user entered 3 in the last example
num = 1
for i in range(max): //max previously set to 3
print num
num = num * 2
Remember, range(3) = {0,1,2}
i=0 (first value in range(3))
print num (num is 1)
num = num * 2 (num is 2)
i=1 (second value in range(3))
num = num * 2 (num is 4)
i=2 (third and last value in range(3))
num = num * 2 (num is 8)
No values remain in range(3) so the loop is complete

20. for Loops Factorial Example
n = int( raw_input("Enter a nonnegative integer: ") )
factorial = 1
for i in range(n):
factorial = factorial * (i+1)
print factorial

21. for Loops
range(n) returns {0,1,2…,n}, but you may want to use the integers starting at 1
Print the first 5 positive integers
for num in range(5):
print num+1
num is set to 0,1,2… but we offset it by one in the for loop to get the numbers we desired
We could also have used range(1,6) in this case

22. for Loops Extending that idea Print the first 5 positive even numbers
for num in range(5):
print 2*(num+1)
Print the first 5 positive odd numbers
print 2*num + 1
Print the first 5 positive numbers squared
print num**2
Print the first 5 powers of 2
print 2**num

23. Nesting
The code block inside a loop (or if structure) can contain loops (or if structures) itself. This is called nesting loops.
As you get into more complex programs these nested structures become commonplace

24. Nesting Nested for loops to print a block of 0s Output
for row in range(4):
for col in range(5):
print 0,
print “”
Output
If you put a comma at the end of a print statement ‘print 0,’ it will not go to the next line after printing

25. Nesting
Another nested for loop example that prints out the coordinates (row,col) of each location in a grid with (0,0) being the upper left-hand corner..
for row in range(4):
for col in range(6):
print "(" +str(row) + "," + str(col) + ")",
print “”
Output:
(0,0) (0,1) (0,2) (0,3) (0,4) (0,5)
(1,0) (1,1) (1,2) (1,3) (1,4) (1,5)
(2,0) (2,1) (2,2) (2,3) (2,4) (2,5)
(3,0) (3,1) (3,2) (3,3) (3,4) (3,5)

26. Nesting
Another nested for loop example that prints out the coordinates (row,col) of each location in a grid.
for row in range(4):
for col in range(6):
print "(" +str(row) + "," + str(col) + ")",
print “”
Remember that the inner for loop is run from start to end every time the code block for the outer for loop is run
Each time one of the values in range(4) is assigned to row, every value in range(6) will be assigned to col

27. Nesting Nested loops to find the average of different students grades
count = 0
name = raw_input("Enter student's name, enter nothing to quit: ")
while name!= "":
count = count + 1
totalScore=0
for i in range(3):
totalScore=totalScore + int(raw_input("Assignment " + str(i+1) + " score: "))
print name + "'s average is " + str(round(totalScore/3.0)) + "."
print "You entered the grades of " + str(count) + " student(s)"

28. Deciding Which Control Structure To Use
You have code that only needs to run in certain situations
if statement
Absolute value
num = int(raw_input(“Enter number: “))
if num < 0:
num = -num
print “Absolute value is “ + str(num)

29. Deciding Which Control Structure To Use
You have two chunks of code, one or the other of which should run every time
if – else structure
num = int(raw_input(“Enter a number: “))
if (num % 2) == 0:
print “You entered an even number.”
else:
print “You entered an odd number.”

30. Deciding Which Control Structure To Use
You have more than two chunks of code, with one and only one of which that should run each time.
if-elif-else structure
temp = int(raw_input(“Enter the temperature: “))
if temp > 25:
print “It is hot today.”
elif temp > 15:
print “It is warm today.”
elif temp > 5:
print “It is cool today.”
else:
print “It is cold today.”

31. Deciding Which Control Structure To Use
You have code that needs to be repeated
for or while loop
There are many cases where either type of loop can be used, but there are general rules.
If you know how many times a loop is going to repeat, usually a for loop is best.
You may only know the repetitions based on user input or some other variable.
If you need to keep repeating something until some process is finished/goal is met

32. In Class Exercise
Write a short program that reads some user entered text, and outputs whether there is a repeated character in the string. e.g. “aa”, “speed”, but not “banana”
if text is a variable containing a string
len(text) tells you how many characters it has
text[0] gives you the first character, text[1] the second and text[len(text) – 1] the last
Write a short program that prints out an NxN multiplication table where N is entered by the user
If the user entered 3 the table should be.

33. In Class Exercise This could also be done with a while loop
#Find a repeated character
text = raw_input("Enter text: ")
found = False #will be set to true if we find a repeated character
for index in range(len(text)-1):
if text[index] == text[index+1]:
print "Repeated character found: "+text[index]
found = True
if not found:
print "No repeats found"

34. In Class Exercise
size = int(raw_input(“How many rows/colums? “))
for row in range(size):
for col in range(size):
print (row+1) * (col+1),
print
We need to use row+1 and col+1 since we want to start at 0, not 1
Remember that the comma at the end of the print statement prevents it from proceeding to the next line
print without anything after ends the line

35. In Class Exercise What does the code print out?
count = int(raw_input("Enter the max: "))
for row in range(count):
line = ""
for i in range(count-row-1):
line = line + " “
for i in range(row+1):
line = line + str(i+1)
for i in range(row):
line = line + str(row-i)
print line

36. In Class Exercise
The outer loop is pretty clearly printing out one line for each iteration, the inner loops are adding text to that line.
Take each inner loop individually
for i in range(count-row-1):
line = line + " “
Assume count is 5. For each value of row, this will add count-row-1 spaces. row will be assigned the values 0,1,2…count – 1
Therefore, on the first line this adds count-1 spaces, count-2 on the second, … , and none on the last

37. In Class Exercise for i in range(row+1): line = line + str(i+1)
For each value of row this will add the numbers 1,2,3…row+1 to the current line
for i in range(row):
line = line + str(row-i)
For each value of row this will add the numbers row,row-1,row-2…,1 to the current line
Combining these two, the first line will get 1, the second, 121, the third, 12321, and so on

38. In Class Exercise
Combining all of this, we see a pyramid of numbers printed out
The first inner for loop adds the spaces at the left, the second for loop adds the increasing number, the last adds the decreasing numbers 1
121
12321