Description
1 Introduction
Understanding the frequency of characters used in a body of text can be useful for various purposes. For example given an encrypted text, character frequency analysis is often used to determine the type of encryption
used. For this lab you will write a program to determine the character frequency of a text file.
1.1 Character Frequency
We are interested in determining the character frequency (a count of the number of times a character is
used) of a text file. The program will first prompt the user for the name of the file. If the file does not exist,
re-prompt until a legitimate file name is provided. Afterwards, read the file and process every character in
the file. Once the the frequency has been determined print out the top 5 letters, top 5 numbers, and top 5
other characters used in the file, as seen below (character in brackets then the count). Also note, the total
number of characters per type is displayed. Consider upper case and lower case letters as the same; therefore,
convert uppercase letters to lowercase. The other category consists of all non-alphanumeric characters.
✷ Terminal ✷✷
> ./lab5
Enter the name of the text file -> roll.txt
alpha top 5 out of 26
———————
[e] 2324
[t] 1502
[a] 1470
[r] 1444
[o] 1408
digit top 5 out of 10
———————
[0] 457
[2] 312
[8] 202
[1] 185
[9] 52
other top 5 out of 28
———————
[ ] 4137
[.] 458
[“] 236
[,] 207
[
] 189
2 Program Design
Although character frequency is a simple task to describe, there are several lists that must be properly
managed. Your program must adhere to the following program design requirements.
CSC 112
Spring 2015
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2.1 A struct and Arrays for Character Counts
As characters are read from the file, you will keep track of the number of times a character appears in the
file. Although we do know all the possible characters that could appear, your lists should only store the
unique characters encountered. The character list should store two items per element, the character and the
count. Use the following struct to store the two items.
1 # include < iostream >
2 # include < fstream >
3 // maximum string size
4 # define MAX_STRING_SIZE 20
5 // maximum character list size
6 # define MAX_LIST_SIZE 50
7 using namespace std ;
8
9 // === data type to store a character and its count ================
10 struct CharFreq
11 {
12 char ch ; /// < the character
13 int count ; /// < the number of times the character occurred
14 };
As seen above, the struct definition should be placed immediately after the using namespace std; statement in your program. This placement will allow it to be used everywhere in your program. For example,
you should create an array for the alphabet (letter) characters as follows.
1 CharFreq alphaList[ MAX_LIST_SIZE ]; // list of unique letters
2 int alphaNum = 0; // number of unique letters
You should create similar lists for numbers (digits) and the other characters.
2.2 C-Strings and Reading the File Name
Strings are multiple characters in the form of a list, or array in C/C++. However, strings in C/C++ are
considered special char arrays called “C-strings.” What is special about C-strings compared to simple char
arrays is that C-strings always end with the null terminator character. In C/C++ the null terminator is the
character ’\0’. Why do we care? We will read the file name from the user as a C-string. The following code
will read characters from the keyboard until a space or newline is entered, or until the maximum string size is
exceeded (MAX STRING SIZE). These characters will be stored in the array fileName and the null terminator
will be added to the end.
1 void readFileName(char fileName [])
2 {
3 int i = 0; // / < local counter
4 char ch ; // / < a character from the keyboard
5 cout << ” Enter the name of the text file -> “;
6
7 // read in characters until a space , newline , or out of room
8 ch = cin . get ();
9 while( ch != ’\n ’ && ch != ’ ’ && i < ( MAX_STRING_SIZE – 1))
10 {
11 fileName[i] = ch ;
12 ch = cin . get ();
13 i ++;
14 }
15 // add null terminator to the end to make it an official C – string
16 fileName[i] = ’\0 ’;
17 }
2.3 Reading the File and Processing Characters
Reading the file and processing characters must be done in a separate function. The function will first
attempt to open the file and if the file does not exist, then return false and read another file name from the
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user (call the readFileName function above). Once a valid file name has been entered, read each character
one at a time from the file and process as seen below.
1 fstream inFile ( fileName , ios :: in ); // / < fstream for file
2 // if the file could not be opened , return false
3 if( inFile . fail ()) return false;
4
5 char ch ; // / < character from the file
6
7 // read characters until the end of file or list is full
8 ch = inFile . get ();
9 while (! inFile . eof () && alphaNum < MAX_LIST_SIZE )
10 {
11 if( isalpha( ch ))
12 addCharToList( tolower( ch ) , alphaList , alphaNum );
13 // get the next character from the file
14 ch = inFile . get ();
15 }
16 inFile . close ();
17 return true;
There are three additional functions referenced in the function above. The function isalpha(ch) will return
true if the character ch is a letter, false otherwise. The function tolower will return the lowercase version
of a letter. Both functions isalpha(ch) and tolower(ch) are already provided for you (thanks glibc).
The function addCharToList is a function you must write, but the code below should give you are start.
1 bool addCharToList( char ch , CharFreq list [] , int& num)
2 {
3 int loc = search (ch , list , num );
4 if( loc != -1)
5 {
6 // ch already in list , update the count
7 }
8 else if( num < MAX_LIST_SIZE)
9 {
10 // ch not in the list , so add it
11 num ++;
12 }
13 else
14 {
15 cout << ” list out of space \n “;
16 return false; // some badness happened Danny , return false
17 }
18 return true; // process correctly , return true
19 }
The function does reference search(), which you must write.
3 Programming Points
You must adhere to all of the following points to receive credit for this lab assignment.
1. Create a directory Lab5 off of your CSC112 directory to store your program in a file called lab5.cpp
2. Your program must be modular in design.
3. Your main function can only consist of variable declarations, function calls, and control structures (no
input or output in the main function).
4. Your program must compile cleanly, no errors or warnings are allowed.
5. Your program must adhere to documentation style and standards. Don’t forget function headers and
variable declarations.
6. Turn-in a print-out of your program source code (lab5.cpp). In addition, copy your program source
code to your Gade/Lab5 directory.
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