The String Data Type CS110: Introduction to Text is represented in programs by the Computer Science string data type.
A string is a sequence of characters enclosed within quotation marks (") or Dianna Xu apostrophes (').
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The String Data Type The String Data Type
When you enter a name, it ’s doing the One way to fix this is to enter your same thing as: string input with quotes around it. firstName = Dianna
The way Python evaluates expressions Even though this works, this is is to look up the value of the variable cumbersome! Dianna and store it in firstName.
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The String Data Type The String Data Type
There is a better way to handle We can access the individual characters text – the raw_input function. in a string through indexing . The positions in a string are numbered raw_input is like input , but it from the left, starting with 0. doesn ’t evaluate the expression The general form is that the user enters.
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Python Programming, 1/e 1 The String Data Type The String Data Type
H e l l o B o b H e l l o B o b 0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8
In a string of n characters, the last character is at position n-1 since we start counting with 0. We can index from the right side using negative indexes.
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The String Data Type The String Data Type
Indexing returns a string containing a Slicing: single character from a larger string.
We can also access a contiguous start and end should both be int s
sequence of characters, called a The slice contains the substring substring , through a process called beginning at position start and runs up to slicing . but doesn ’t include the position end.
If either expression is missing, then the start or the end of the string are used.
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The String Data Type The String Data Type
Can we put two strings together into a Operator Meaning longer string? + Concatenation Concatenation “glues ” two strings together ( +) * Repetition
Python Programming, 1/e 2 Simple String Processing Simple String Processing
>>> Usernames on a computer system Please enter your first name (all lowercase): john First initial, first seven characters of last Please enter your last name (all lowercase): doe name uname = jdoe
# get user ’s first and last names >>> first = raw_input( “Please enter your first name (all lowercase): ”) Please enter your first name (all lowercase): donna last = raw_input( “Please enter your last name (all lowercase): ”) Please enter your last name (all lowercase): rostenkowski uname = drostenk # concatenate first initial with 7 chars of last name uname = first[0] + last[:7]
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Simple String Processing Simple String Processing
Another use – converting an int that Month Number Position stands for the month into the three letter Jan 1 0 abbreviation for that month. Feb 2 3 Store all the names in one big string: “JanFebMarAprMayJunJulAugSepOctNovDec” Mar 3 6 Use the month number as an index for Apr 4 9 slicing this string: monthAbbrev = months[pos:pos+3] To get the correct position, subtract one from the month number and multiply by three 15 16
Strings, Lists, and Sequences Strings, Lists, and Sequences
Strings are really a special kind of We can use the idea of a list to make sequence , so these operations also our previous month program even apply to sequences! simpler!
We change the lookup table for months Strings are always sequences of to a list: characters, but lists can be sequences of arbitrary values. months = ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", Lists can have numbers, strings, or "Dec"] both!
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Python Programming, 1/e 3 Strings, Lists, and Sequences Strings, Lists, and Sequences
To get the months out of the sequence, This version of the program is easy to do this: extend to print out the whole month monthAbbrev = months[n-1] name rather than an abbreviation!
Rather than this: months = ["January", "February", "March", "April", "May", monthAbbrev = months[pos:pos+3] "June", "July", "August", "September", "October", "November", "December"]
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Strings, Lists, and Sequences Strings and Secret Codes
Lists are mutable , meaning they can be Inside the computer, strings are not changed. Strings can be changed. represented as sequences of 1 ’s and 0’s, just like numbers.
A string is stored as a sequence of numbers, one number per character.
It doesn ’t matter what value is assigned as long as it ’s done consistently.
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Strings and Secret Codes Strings and Secret Codes
In the early days of computers, each 0 – 127 are used to represent the characters manufacturer used their own encoding typically found on American keyboards. of numbers for characters. 65 – 90 are “A”–“Z” 97 – 122 are “a”–“z” Today, American computers use the 48 – 57 are “0”–“9” ASCII system (American Standard Code The others are punctuation and control codes for Information Interchange). used to coordinate the sending and receiving of information.
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Python Programming, 1/e 4 Strings and Secret Codes Strings and Secret Codes
One major problem with ASCII is that The ord function returns the it ’s American-centric, it doesn ’t have numeric (ordinal) code of a single many of the symbols necessary for character. other languages. The chr function converts a Newer systems use Unicode , an numeric code to the corresponding alternate standard that includes support character. for nearly all written languages.
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Other String Operations Other String Operations
There are a number of other string count( s, sub ) – Count the number of processing functions available in the occurrences of sub in s
string library. Try them all! find( s, sub ) – Find the first position capitalize( s) – Copy of s with only the where sub occurs in s
first character capitalized join( list ) – Concatenate list of strings capwords( s) – Copy of s; first character into one large string of each word capitalized ljust( s, width ) – Like center, but s is center( s, width ) – Center s in a field of left-justified given width
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Other String Operations Other String Operations
lower( s) – Copy of s in all lowercase letters rstrip( s) – Copy of s with trailing lstrip( s) – Copy of s with leading whitespace removed
whitespace removed split( s) – Split s into a list of substrings replace( s, oldsub, newsub ) – Replace upper( s) – Copy of s; all characters occurrences of oldsub in s with newsub converted to uppercase rfind( s, sub ) – Like find, but returns the right-most position rjust( s, width ) – Like center, but s is right- justified
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Python Programming, 1/e 5 From Encoding to Encryption
The process of encoding information for the purpose of keeping it secret or transmitting it privately is called encryption .
Cryptography is the study of encryption methods.
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Python Programming, 1/e 6