Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Intro to: Computers & Programming: Loops in PythonV22.0002-001Adam MeyersNew York University Introduction to: Computers & Programming: Loops in PythonIntro to: Computers & Programming: Loops in PythonV22.0002-001Outline• What is a Loop?•While Loops• For Loops•Examples• Nested LoopsIntro to: Computers & Programming: Loops in PythonV22.0002-001What is a Loop?• Loops are control structures– A block of code repeats– The extent of the repetition is usually limited in some way• Two kinds of Loops in Python–while loops• The evaluation of a boolean expression determines when the repetition stops• Changes in values of variables lead to different evaluations of the boolean expression on each repetition•When the expression is evaluated as False, the loop halts•If the expression can never evaluate as False, the loop is endless– for loops• The length of a sequence determines how many times the body executes• The loop uses one member of the sequence at a time, until there are no more membersIntro to: Computers & Programming: Loops in PythonV22.0002-001An Endless Loop•Exampledef endless_timer ():import timenow = 0while (True):time.sleep(1)now = now + 1print(now)•This loop will keep counting seconds until stopped with a Control-CIntro to: Computers & Programming: Loops in PythonV22.0002-001What is a while Loop?• A while loop consists of:–The word while–A boolean expression (True on the last slide)–A colon :–The body: an indented block of instructions•The body of the loop repeats–until the boolean expression is False•The loop on the previous slide is endless–because boolean expression is never False. –Any program can be stopped using Control-CIntro to: Computers & Programming: Loops in PythonV22.0002-001What is a while Loop? 2•A loop that iterates a limited number of timesdef seconds_stop_watch (total_seconds):import timenow = 0while (now < total_seconds):time.sleep(1)now = now + 1print(now)•If we call seconds_stop_watch with 5 as an argument– The variable now is initialized to 0– The loop iterates 5 times– Each time: a second passes, 1 is added to now and now is printed– In this way, 1 to 5 is printed over 5 seconds• How many times would a loop beginning while (False): repeat?Intro to: Computers & Programming: Loops in PythonV22.0002-001A sample for loop•This function simulates a 60 second timerdef one_minute_timer ():print(0)for second in range(60):time.sleep(1)print(second + 1)• The function prints 0, then enters a for loop–The loop iterates through a list of numbers from 0 to 59•The variable second is assigned that number as a value•The system waits one second• The system prints second + 1Intro to: Computers & Programming: Loops in PythonV22.0002-001New Material Introduced in the one_minute_timer function • The range function – range takes one or two integers m and n as an arguments–when m is left out, it is (by default) set to 0 –creates a sequence of numbers from m to n• A for loop–The first line – for variable in sequence:•for and in are keywords• variable can be any legal variable name• sequence is an ordered set of items–Python sequences includes data types like: range, list, string, …– The body of the loop repeats once for each item in the sequence– On each iteration, the variable is bound to the next item in the sequenceIntro to: Computers & Programming: Loops in PythonV22.0002-001Looping Through a String• Using a for loopdef for_string_loop (string):for letter in string:print(letter)–for-string-loop('Downward')• Using a while loopdef while_string_loop (string):position = 0while(position < len(string))print(string[position])position = 1 + positionIntro to: Computers & Programming: Loops in PythonV22.0002-001Lengths and elements of Sequences• The function len returns a sequence's length–The number of characters – len('Downward')–The number of integers in a range – len(range(60))–Etc.• Elements in a range can be identified by their position, beginning with 0 and ending in one less than the length.–'Downward'[0], range(5,10)[0]–'Downward'[7], range(5,10)[4]–'Downward'[8], range(5,10)[5] --- these are errorsIntro to: Computers & Programming: Loops in PythonV22.0002-001for loops vs. while loops• With some code modification, it is always possible to replace a for loop with a while loop, but not the other way around• for loops are used for situations where you know the number of iterations ahead of time– e.g., looping through sequences•There is no significant efficiency difference• The difference relates to ease in which humans can read/write codeIntro to: Computers & Programming: Loops in PythonV22.0002-001Example: Drawing an asterisk triangle•def draw_n_asterisks(n):for current_length in range(n):print('*',end='')– print can take a named argument• End='' indicates what to print at the end of the string– the character in between the single quotes• In this case, nothing• The default is a newline character•def asterisk_triangle(base_size):for current_length in range(base_size):draw_n_asterisks(current_length)print()Intro to: Computers & Programming: Loops in PythonV22.0002-001Drawing an asterisk triangle 2• Nested Loops – a single functiondef asterisk_triangle2(base_size):for current_length in range(base_size):for n in range(current_length):print('*',end='')print()•Python indicates depth of nesting via indentation–Suppose the last line was indented onceIntro to: Computers & Programming: Loops in PythonV22.0002-001Printing a Multiplication table•def multiplication_table (high_num):for num1 in range(1, 1+high_num):for num2 in range(1, 1+high_num):print(num1,'X',num2, '=', num1*num2)• How does this work?Intro to: Computers & Programming: Loops in PythonV22.0002-001Sample Problem for Class• Write a function that:–Takes three arguments: • base_size • repetitions• hour_glass_or_diamond–This function makes a pattern of asterisks that repeats the number of times indicates by repetitions– Each cycle consists of two triangles, one the upside down version of each other, both of which have a base of size base_size–If