Loops

1
Loops

• Introduction to Computing Science and Programming
  I

2
Loops

• To this point youve 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 couldnt 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, lets look at some problems that
  require loops.

4
Prime Number Algorithm

• Heres 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 lt 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 lt 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 lt 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 lt 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

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Two Kinds of Loop

• while 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.

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while Loops

• A simple example that prints the number 1 through
  3.
• num 1
• while num lt 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 whats going on we can unwind the
  loop
• num 1
• while num lt 3
• print num
• num num 1
• print Finished
• num 1
• Check the condition, is num lt3? Yes, so enter
  the code block
• print num
• num num 1 (num is 2)
• Check the condition, is num lt 3? Yes, so repeat
  the code block
• print num
• num num 1 (num is 3)
• Check the condition, is num lt 3? Yes, so repeat
  the code block
• print num
• num num 1 (num is 4)
• Check the condition, is num lt 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 lt 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
• Well 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

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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

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for Loops

• Unwinding a for loop
• for x in range(5)
• print x
• Remember, range(5) 0,1,2,3,4
• x0 (first value in range(5))
• print x
• x1 (second value in range(5))
• print x
• x2 (third value in range(5))
• print x
• x3 (fourth value in range(5))
• print x
• x4 (final value in range(5))
• print x
• No more values remain in range(5) so we are
  finished

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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 dont use i inside the
  loop. Were just using the structure to repeat
  the code block max times.

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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
• i0 (first value in range(3))
• print num (num is 1)
• num num 2 (num is 2)
• i1 (second value in range(3))
• print num
• num num 2 (num is 4)
• i2 (third and last value in range(3))
• print num
• num num 2 (num is 8)
• No values remain in range(3) so the loop is
  complete

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for Loops

• Factorial Example
• n int( raw_input("Enter a nonnegative integer
  ") )
• factorial 1
• for i in range(n)
• factorial factorial (i1)
• print factorial

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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 num1
• 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(num1)
• Print the first 5 positive odd numbers
• for num in range(5)
• print 2num 1
• Print the first 5 positive numbers squared
• for num in range(5)
• print num2
• Print the first 5 powers of 2
• for num in range(5)
• print 2num

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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

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Nesting

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

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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)

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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

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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
• totalScore0
• for i in range(3)
• totalScoretotalScore
  int(raw_input("Assignment " str(i1) " score
  "))
• print name "'s average is "
  str(round(totalScore/3.0)) "."
• name raw_input("Enter student's name, enter
  nothing to quit ")
• 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 lt 0
• num -num
• print Absolute value is str(num)

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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.

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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 gt 25
• print It is hot today.
• elif temp gt 15
• print It is warm today.
• elif temp gt 5
• print It is cool today.
• else
• print It is cold today.

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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
• text0 gives you the first character, text1
  the second and textlen(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.
• 1 2 3
• 2 4 6
• 3 6 9

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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 textindex textindex1
• print "Repeated character found
  "textindex
• 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 (row1) (col1),
• print
• We need to use row1 and col1 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(row1)
• line line str(i1)
• 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 0,1,2count 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(row1)
• line line str(i1)
• For each value of row this will add the numbers
  1,2,3row1 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
• 1234321
• 123454321