Download
1 / 15
180 likes | 370 Vues
Strings in Python. Computer Science 18 Feb 2011. Introduction. Prior experience Defining string variables Getting user input Printing strings Lesson Objectives Understand string structure Access characters and sub-sections from a string Concatenate strings Traverse a string.
E N D
Strings in Python Computer Science 18 Feb 2011
Introduction Prior experience Defining string variables Getting user input Printing strings Lesson Objectives Understand string structure Access characters and sub-sections from a string Concatenate strings Traverse a string
Strings so far • Strings are generally used to store text data • sample="LEVEL" • Name=input ("What is your name?") • Can also store non-natural language data e.g. telephone numbers • my_tel="+1 (301) 294 4444" • Q: What data should/should not be stored in strings?
Strings vs. Integers • Both strings and integer variables can store numbers • my_speed_str="300" • my_speed_integer=300 • Q: What is the difference between strings and integers?
Strings in use • Q: Where are strings displayed and entered?
String composition • Strings are a composite data type • Built from another data type, characters • In a string, characters arestored in sequence. • Strings can be any length (including empty). • String are enclosed in quotation marks. • str_var = "300 m/s" • empty_str=""
Length of a string • Use len(str) to return string length (# of characters) • sample="SERIES" • len(sample)= • empty_str="" • len(empty_str) = 6 0
String representation • In strings, characters are accessed by index • …like mailboxes in an apartment building. • First index is 0, not 1. • s="LEVEL" • startChar=s[ ] • just_v=ss[ ] • Python strings are immutable (can’t change individual chars) • s[0]="B" • Try it out 0 2
Slices (sections of a string) • Use a range to specify a slice (sub-string) • from start index up to but not including the last index • speed_display = "300 m/s“ • zeroZero = speed_display[ : ] • Omit the first value to select the start of a string • just_num= speed_display[: ] • Omit the second value to select the end of a string • just_unit = speed_display[ :] • Try it out 1 3 3 4
Operations: Concatenation • Combine strings using + (concatenation operator) • full_name = "Henry" + " " + "James" • print ":" + full_name + ":" • Concatenation is not addition • vision_str="20"+"20" • vision_val=20+20 • Try it out: Build a string • build="" • while len(build)<5: • build = build +"a" • print build =“2020” =40
String comparison • To test for equality, use the == operator • To compare order, use the < and > operators user_name=raw_input("Enter your name- ") if user_name==“Sam": print "Welcome back Sam“ elif user_name<"Sam": print "Your name is before Sam“ else: print "Your name is after Sam“ • These operators are case sensitive. • Upper case characters are ‘less than’ lower case
Operations: Traversing through a string • Use a loop to examine each character in a string See HTTLCSfor an alternative format: for in strng="count # of us" index=0 count=0 while index < len(strng): if strng[index] == "u“ or strng[index] == “U": count+=1 index+=1 • Try it out • How would we traverse backwards?
Summary • Strings are composed of characters • len(sample) String length • sample[i] Character at index i • sample[start:end]Slice from start up to but not including end index • sample+sample Concatenate strings • sample=="test" Test for equality • sample<test Test for order
String operations: find • find() searches for a string within a string • To use it, insert this at the top of your code: • import string • find() returns the first index of a substring • full_name = "Henry James" • string.find(full_name,"e") • You can specify the starting point of the search: • string.find(full_name,"e",2) • If the string is not found, find() returns -1 • find() is case sensitive
