Introduction to Strings - PowerPoint PPT Presentation

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Introduction to Strings. CSIS 1595: Fundamentals of Programming and P roblem Solving 1. Strings as a Single Value. Basic operators: Assignment: name = “Fred” name1 = name Comparison: if name == “Fred” if name1 != “Barney” Concatenation: fullname = name + “ Flintstone”

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Introduction to Strings

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Introduction to Strings

CSIS 1595: Fundamentals of Programming and Problem Solving 1

Strings as a Single Value

• Basic operators:

• Assignment:name = “Fred”name1 = name

• Comparison: if name == “Fred”if name1 != “Barney”

• Concatenation: fullname = name + “ Flintstone”

• Repetition: name3 = name * 3

String Literals

• Quotes used to indicate string literal

• x is a variable, “x” is a string

• 123 is a number, “123”is a string

• Can use singlequote ‘ or doublequote “

• x = “Larry”

• y = ‘Curley’

• Notz = “Moe’

• Should use one form consistently

Escape Characters

• Syntax: \somecharacter

• Spacing characters:

\t inserts tab

\n inserts new line

• Inserting quotes into strings without having them treated like beginning/end of string literal:

\’inserts single quote

\” inserts doulequote

Example: print(“He said \”Hello\””)

He said “Hello”

Strings as Lists of Characters

• Strings can be broken down into their individual characters

• Each character in a string has an index

• First index is 0

index

Strings as Lists of Characters

• Accessing individual characters:Syntax: stringvariable[index]

• Example: greeting = “Hello world!”print(greeting[7])  ‘r’

• Note: Index must be legalprint(greeting[12])  error!

• Can use len function to find length of stringlen(greeting)  12

• Note: Highest index = length – 1 since first index is 0

Strings as Lists of Characters

• Accessing substring of characters

• All characters between startindex and end index

• Syntax: stringvar[start:end]

• Example: print(greeting[3:7])  ‘lo wor’

• Default syntax:

stringvar[start:]– All chars from start to end of string

stringvar[:end]– All chars from start of string to end

Strings and Loops

• Loops often used to process strings

• Loop counter = index in string

• Example: Printing all leading substrings of a given word

• Strategy:

• Use stringvar[:end]to print first end characters

• Use for loop to vary end from 1 to length of word

• Use len function to find length of word

For Loops and Strings

• Special form of for loop for strings

• for charvariable in stringvariable:

• Each time through loop, charvariableis the next character in stringvariable

• Example:

• char = “H”

• char = “e”

• char = “l”

• char = “l”

• char = “o”

• char = “!”

For Loops and Strings

• Example: “Exploding” a word by inserting a space between each letter

• Strategy:

• Use loop to get each letter

• Print it followed by a space (instead of a return)

String Mutability

• Can index be used to change a string?

name = “Larry”

name[1] = “o”  name now “Lorry”

• Legal in languages with mutablestrings

• C, C++,…

• Not legal in languages with immutablestrings

• Python, Java,…

• Usually related to efficiency of string representation