Strings String: Text as a Value String are quoted characters Type - PowerPoint PPT Presentation

Mini-Lecture 5 Strings

String: Text as a Value • String are quoted characters Type : str § 'abc d' (Python prefers) § "abc d" (most languages) • How to write quotes in quotes? Char Meaning \' single quote § Delineate with “other quote” \" double quote § Example : " ' " or ' " ' \n new line § What if need both " and ' ? \t tab • Solution : escape characters \\ backslash § Format: \ + letter § Special or invisible chars 9/5/18 Strings 2

String are Indexed • s = 'abc d' • s = 'Hello all' 0 1 2 3 4 0 1 2 3 4 5 6 7 8 a b c d H e l l o a l l • What is s[3:6] ? • Access characters with [] § s[0] is 'a' A: 'lo a' § s[4] is 'd' B: 'lo' § s[5] causes an error C: 'lo ' § s[0:2] is 'ab' (excludes c ) D: 'o ' § s[2:] is 'c d' E: I do not know • Called “string slicing” 9/5/18 Strings 3

String are Indexed • s = 'abc d' • s = 'Hello all' 0 1 2 3 4 0 1 2 3 4 5 6 7 8 a b c d H e l l o a l l • What is s[3:6] ? • Access characters with [] § s[0] is 'a' A: 'lo a' § s[4] is 'd' B: 'lo' § s[5] causes an error C: 'lo ' CORRECT § s[0:2] is 'ab' (excludes c ) D: 'o ' § s[2:] is 'c d' E: I do not know • Called “string slicing” 9/5/18 Strings 4

String are Indexed • s = 'abc d' • s = 'Hello all' 0 1 2 3 4 0 1 2 3 4 5 6 7 8 a b c d H e l l o a l l • What is s[:4] ? • Access characters with [] § s[0] is 'a' A: 'o all' § s[4] is 'd' B: 'Hello' § s[5] causes an error C: 'Hell' § s[0:2] is 'ab' (excludes c ) D: Error! § s[2:] is 'c d' E: I do not know • Called “string slicing” 9/5/18 Strings 5

String are Indexed • s = 'abc d' • s = 'Hello all' 0 1 2 3 4 0 1 2 3 4 5 6 7 8 a b c d H e l l o a l l • What is s[:4] ? • Access characters with [] § s[0] is 'a' A: 'o all' § s[4] is 'd' B: 'Hello' § s[5] causes an error C: 'Hell' CORRECT § s[0:2] is 'ab' (excludes c ) D: Error! § s[2:] is 'c d' E: I do not know • Called “string slicing” 9/5/18 Strings 6

Other Things We Can Do With Strings • Operation in : s 1 in s 2 • Function len: len(s) § Tests if s 1 “a part of” s 2 § Value is # of chars in s § Say s 1 a substring of s 2 § Evaluates to an int § Evaluates to a bool • Examples : • Examples : § s = 'abracadabra' § s = 'abracadabra’ § 'a' in s == True § len(s) == 11 § 'cad' in s == True § len(s[1:5]) == 4 § 'foo' in s == False § s[1:len(s)-1] == 'bracadabr' 9/5/18 Strings 7

String Functions • The introcs module has several string functions § Installed as part of Cornell Extensions § http://cs1110.cs.cornell.edu/docs/strings.html • Use these instead of methods for now § Methods are an advanced programming feature § You will see them on Stack Overflow § Will come back to methods later • Will need these functions for Assignment 1 9/5/18 Strings 8

Examples of String Functions • introcs.index_str(s 1 ,s 2 ) • s = 'abracadabra’ § Position of the first • from introcs import * instance of s 2 in s 1 • index_str(s,'a') == 0 • index_str(s, 'rac') == 2 • introcs.count_str(s 1 ,s 2 ) • count_str(s, 'a') == 5 § Number of times s 2 appears inside of s 1 • count_str(s, 'x') == 0 • strip(' a b ') == 'a b' • introcs.strip(s) § A copy of s with white- See IntroCS space removed at ends Docs for more 9/5/18 Strings 9