countlet.c

c语言开发方面的经典问题,包括源代码.c语言开发所要注意的问题,以及在嵌入式等各方面的应用

/*
 * File: countlet.c
 * ----------------
 * This program counts the occurrences of individual letters
 * that appear in text read in from the user.  This program
 * might be useful as a tool in solving cryptograms.
 */

#include <stdio.h>
#include <ctype.h>
#include "simpio.h"
#include "strlib.h"
#include "genlib.h"

/*
 * Constants
 * ---------
 * MaxLines -- Maximum number of input lines
 * NLetters -- Number of letters
 */

#define MaxLines 100
#define NLetters  26

/* Private function declarations */

static void CountLetters(int letterCounts[]);
static void CountLettersInString(string str, int letterCounts[]);
static void RecordLetter(char ch, int letterCounts[]);
static void DisplayLetterCounts(const int letterCounts[]);
static int LetterIndex(char ch);
void ClearIntegerArray(int array[], int n);

/* Main program */

main()
{
    int letterCounts[NLetters];

    printf("This program counts letter frequencies.\n");
    printf("Enter a blank line to signal end of input.\n");
    ClearIntegerArray(letterCounts, NLetters);
    CountLetters(letterCounts);
    DisplayLetterCounts(letterCounts);
}

/*
 * Function: CountLetters
 * Usage: CountLetters(letterCounts);
 * ----------------------------------
 * This function updates the values in the letterCounts array
 * by scanning through a series of strings read in from the
 * user.  A blank line is used to signal the end of the input
 * text.
 */

static void CountLetters(int letterCounts[])
{
    string line;

    while (TRUE) {
        line = GetLine();
        if (StringLength(line) == 0) break;
        CountLettersInString(line, letterCounts);
    }
}

/*
 * Function: CountLettersInString
 * Usage: CountLettersInString(str, letterCounts);
 * -----------------------------------------------
 * This function updates the values in the letterCounts array for
 * each character in the string str.
 */

static void CountLettersInString(string str, int letterCounts[])
{
    int i;

    for (i = 0; i < StringLength(str); i++) {
        RecordLetter(IthChar(str, i), letterCounts);
    }
}

/*
 * Function: RecordLetter
 * Usage: RecordLetter(ch, letterCounts);
 * --------------------------------------
 * This function records the fact that the character ch has
 * been seen by incrementing the appropriate element in the
 * letterCounts array.  Non-letters are ignored.
 */

void RecordLetter(char ch, int letterCounts[])
{
    int index;

    index = LetterIndex(ch);
    if (index != -1) letterCounts[index]++;
}

/*
 * Function: DisplayLetterCounts
 * Usage: DisplayLetterCounts(letterCounts);
 * -----------------------------------------
 * This function displays the letter frequency table, leaving
 * out any letters that did not occur.
 */

void DisplayLetterCounts(const int letterCounts[])
{
    char ch;
    int num;

    for (ch = 'A'; ch <= 'Z'; ch++) {
        num = letterCounts[LetterIndex(ch)];
        if (num != 0) printf("%c  %4d\n", ch, num);
    }
}

/*
 * Function: LetterIndex
 * Usage: index = LetterIndex(ch);
 * -------------------------------
 * This function converts a character into the appropriate index
 * for use with the letterCounts array.  In this implementation,
 * LetterIndex converts characters in either case to an integer
 * in the range 0 to 25.  If ch is not a valid letter, LetterIndex
 * returns -1.  Clients should check for a -1 return value unless
 * they are able to guarantee that the argument is a letter.
 */

int LetterIndex(char ch)
{
    if (isalpha(ch)) {
        return (toupper(ch) - 'A');
    } else {
        return (-1);
    }
}

/*
 * Function: ClearIntegerArray
 * Usage: ClearIntegerArray(array, n);
 * -----------------------------------
 * This function sets the first n elements in the array to 0.
 */

void ClearIntegerArray(int array[], int n)
{
    int i;

    for (i = 0; i < n; i++) {
        array[i] = 0;
    }
}