Programming in Python Part III Dr. Fatma Cemile Serçe Atılım University

Loops Loop is a statement or group of statements that execute repeatedly until a terminating condition is satisfied Infinite loop: A loop in which the terminating condition is never satisfied

while Statement def countdown(n): while n > 0: print n n = n-1 print “End!“ Explanation: “While n is greater than 0, continue displaying the value of n and then reducing the value of n by 1. When you get to 0, display the word End!”

while Statement(cont.) Exercise: write the function nLines to display a given amount of number times (n) empty lines def nLines(n): while n > 0: print n = n-1

Strings Strings are made up of smaller pieces—characters. The bracket operator selects a single character from a string. >>> fruit = "banana" >>> letter = fruit[1] >>> print letter Output: a The expression in brackets is called an index Index starts at zero

Length The len function returns the number of characters in a string: >>> fruit = "banana" >>> len(fruit) Output: 6 length = len(fruit) last = fruit[length] # ERROR! That won’t work. It causes the runtime error IndexError: string index out of range. The reason is that there is no 6th letter in "banana“ the six letters are numbered 0 to 5 length = len(fruit) last = fruit[length-1]

Exercise As an exercise, write a function that takes a string as an argument and outputs the letters backward, one per line.

for Loop for loop: for char in fruit: print char The loop continues until no characters are left.

for Loop(cont.) Example: prefixes = "JKLMNOPQ" suffix = "ack" for letter in prefixes: print letter + suffix The output of this program is: Jack Kack Lack Mack Nack Oack Pack Qack

String Slices A segment of a string is called a slice. Selecting a slice is similar to selecting a character: >>> s = "Peter, Paul, and Mary" >>> print s[0:5] Peter >>> print s[7:11] Paul >>> print s[17:21] Mary The operator [n:m] returns the part of the string from the “n- eth” character to the “m-eth” character, including the first but excluding the last

String Slices(cont.) If you omit the first index (before the colon), the slice starts at the beginning of the string. If you omit the second index, the slice goes to the end of the string. >>> fruit = "banana" >>> fruit[:3] ’ban’ >>> fruit[3:] ’ana’ What do you think s[:] means?

String Comparison The comparison operators work on strings. if word == "banana“ print "Yes, we have no bananas!“ if word < "banana": print "Your word," + word + ", comes before banana." elif word > "banana": print "Your word," + word + ", comes after banana." else: print "Yes, we have no bananas!" In Python, uppercase letters come before all the lowercase letters. Zebra, comes before banana.

The string Module The string module contains useful functions ( import string) find function: >>> fruit = "banana" >>> index = string.find(fruit, "a") >>> print index 1

The string Module(cont.) >>> string.find("banana", "na") 2 >>> string.find("banana", "na", 3) 4 >>> string.find("bob", "b", 1, 2)

Lists A list is an ordered set of values, where each value is identified by an index. the elements of a list can have any type [10, 20, 30, 40] ["spam", "bungee", "swallow"] ["hello", 2.0, 5, [10, 20]]

Lists(cont.) Loop horsemen = ["war", "famine", "pestilence", "death"] i = 0 while i < 4: print horsemen[i] i = i + 1

List Length The function len returns the length of a list horsemen = ["war", "famine", "pestilence", "death"] i = 0 while i < len(horsemen): print horsemen[i] i = i + 1

List Membership in is a boolean operator that tests membership in a sequence >>> horsemen = [’war’, ’famine’, ’pestilence’, ’death’] >>> ’pestilence’ in horsemen True >>> ’debauchery’ in horsemen False >>> ’debauchery’ not in horsemen True

Lists and for Loops Syntax for VARIABLE in LIST: BODY for number in range(20): if number % 2 == 0: print number for fruit in ["banana", "apple", "quince"]: print "I like to eat " + fruit + "s!"

List Operations The + operator concatenates lists: >>> a = [1, 2, 3] >>> b = [4, 5, 6] >>> c = a + b >>> print c [1, 2, 3, 4, 5, 6] The * operator repeats a list a given number of times: >>> [0] * 4 [0, 0, 0, 0] >>> [1, 2, 3] * 3 [1, 2, 3, 1, 2, 3, 1, 2, 3]