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String and Character Operations in CSCI-1200 Computer Science II, Assignments of Data Structures and Algorithms

Rensselaer Polytechnic Institute (RPI)Data Structures and Algorithms

The lecture on string and character operations in csci-1200 computer science ii. The lecture covers the use of maps, string manipulation, character operations, and an exercise to write a palindrome detection program. The document also includes an outline for solving text analysis problems.

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Uploaded on 08/09/2009

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CSCI-1200 Computer Science II — Fall 2006
Lecture 10 — String and Character Operations
Review from Lectures 8 & 9
•Maps are associations between keys and values.
•Maps store pairs; map iterators refer to these pairs.
•Maps have fast insert, access and remove operations: O(log n).
•The choice between maps, vectors and lists is based on naturalness, ease of programming, and efficiency of the
resulting program.
•Classes can be used as map keys if a well-behaved operator< is available.
•Maps can store more complicated values, such as vectors or classes. The syntax of using maps in this way can
become a bit complicated, you’ll get more comfortable with practice.
•We saw how to use maps to solve the-word-to-line-number indexing problem. Using a vector or a list would be
significantly more difficult and the solution is less natural.
•Maps should be used when there is an association — natural or easily created — between two classes. As a
larger example we looked at the MP3 database.
Homework 4: Using maps to build an efficient library database
•What operations must be sublinear? With respect to which data size?
Today’s Class — String and Character Operations
•Motivating problem: input text analysis
•String operations: input a line at a time; substring.
•Character operations: checking character types
•Solving the motivating problem
Koenig & Moo: Sections 5.6-5.9
10.1 Motivation
•Problem: analyzing an input text file to find
–Number of lines
–Number of words
–Number of letters
–Number of occurrences of letters and words
•Challenges:
–Distinguishing lines
–Ignoring whitespace characters
–Avoiding punctuation
–Mixture of upper and lower case letters
•Assumptions:
–A word is a sequence of uninterrupted letters.
–Whitespace should not be included in the character count, but punctuation should.
pf3
pf4
pf5

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CSCI-1200 Computer Science II — Fall 2006

Lecture 10 — String and Character Operations

Review from Lectures 8 & 9

  • Maps are associations between keys and values.
  • Maps store pairs; map iterators refer to these pairs.
  • Maps have fast insert, access and remove operations: O(log n).
  • The choice between maps, vectors and lists is based on naturalness, ease of programming, and efficiency of the resulting program.
  • Classes can be used as map keys if a well-behaved operator< is available.
  • Maps can store more complicated values, such as vectors or classes. The syntax of using maps in this way can become a bit complicated, you’ll get more comfortable with practice.
  • We saw how to use maps to solve the-word-to-line-number indexing problem. Using a vector or a list would be significantly more difficult and the solution is less natural.
  • Maps should be used when there is an association — natural or easily created — between two classes. As a larger example we looked at the MP3 database.

Homework 4: Using maps to build an efficient library database

  • What operations must be sublinear? With respect to which data size?

Today’s Class — String and Character Operations

  • Motivating problem: input text analysis
  • String operations: input a line at a time; substring.
  • Character operations: checking character types
  • Solving the motivating problem

Koenig & Moo: Sections 5.6-5.

10.1 Motivation

  • Problem: analyzing an input text file to find
    • Number of lines
    • Number of words
    • Number of letters
    • Number of occurrences of letters and words
  • Challenges:
    • Distinguishing lines
    • Ignoring whitespace characters
    • Avoiding punctuation
    • Mixture of upper and lower case letters
  • Assumptions:
    • A word is a sequence of uninterrupted letters.
    • Whitespace should not be included in the character count, but punctuation should.

10.2 String and Character Manipulation

  • Until now, we’ve been reading strings from the input separated by whitespace. Some of you may have experi- mented with other operations for homework, but they weren’t necessary to solve the problems.
  • We can also read a whole line of input (including whitespace) with the function getline. This function reads characters until a newline character (or the end-of-file) is encountered. Here’s the prototype:

istream& getline(istream &, string &);

Returning the istream reference may seem a bit strange, but it is common practice. It allows the state of the stream to be tested in a conditional. We’ve seen this already with loops to read integers and strings, for example:

std::string name; while (std::cin >> name) { ... }

  • The string class has a substr member function that extracts a substring starting at a given location. For example:

std::string s = "My name is Sally Jones"; std::string t = s.substr(11,5); // Starting at location 11, extract the next 5 chars. std::cout << t << std::endl; // Outputs: Sally

  • The header file provides prototypes for character functions from the C library (hence the ’c’ in front of ’ctype’). Here are some examples: - isspace(c) - isalpha(c) - isdigit(c) - ispunct(c) - isupper(c) - tolower(c)

Each of these functions takes a character and returns true or false.

  • The type char is a special case of the type integer. As such, we can do simple math with values of type character. When we do this, the compiler automatically converts the char value to be of integer type. We can cast the value back to type char as illustrated below:

’c’ - ’a’ == 2 // this is true char(’B’ + 4) == ’F’ // this is true std::cout << ’a’ + 10 << std::endl; // outputs the integer 107 std::cout << char(’a’ + 10) << std::endl; // outputs the letter k

10.3 Exercise

  • For the last expression in the fragment of code below, give the type and the value.

char c = ’P’ + 2; tolower(c); c

10.6 Problem Solving Approach

Now let’s address the text analysis posed at the beginning of the lecture. Here’s an outline of how you might approach solving problems like this, which do not involve the design of classes:

  1. Outline the flow and the major steps of the program.
  2. Make note of the information that must be kept by the main function. This will dictate (most of) the variables.
  3. Make a list of the functions that the main function needs.
  4. Write these functions (and test them). If necessary, repeat the above process for these functions.
  5. Write the main program and test it.

10.7 Returning to the Text Analysis Problem

We want to analyzing an input text file to find:

  • Number of lines
  • Number of words
  • Number of letters
  • Number of occurrences of letters and words

10.8 Text Analysis Algorithm Outline

Here’s one outline (others are certainly possible). Each of the (*) corresponds to a helper function.

  • Main function:
    1. For each line, (a) Increment line counter (b) Count characters () and add to character count (c) Add to letter counters () (d) Break up into words of small letters only (*) (e) Save all words
    2. Sort words (including repetitions) and count occurrences (*)
  • Variables:
    • Counter: lines, words, letters
    • Vector of 26 individual letter counts
    • Vector of strings to represent words

10.9 Exercise: Write the helper functions!

unsigned int count_characters(const string& a_line) {

}

void add_to_letter_counts(const string & a_line, vector& letter_counters) {

}

vector break_up_line(const string& a_line) {

}

void count_word_occurrences(vector& words) {

}