Question

Background:

The first step towards helping someone 'decode' a message is often to count how many times each letter of the alphabet appears in the message. Then divide each count by the total number of letters to compute the relative 'frequency'.   From these frequencies, it may be easier to recognize which letters are assigned to the vowels.

For your own reference, here is a table of standard letter frequencies from typical English text. (You do NOT need to display this to the user.)

letter frequency

• e      12.702%
• t       9.056%
• a       8.167%
• o       7.507%
• i       6.966%
• n       6.749%
• s       6.327%
• h       6.094%
• r       5.987%
• d       4.025%

For example, given this (encoded) message:

message:    "K ukd, k yckd, k fkdkc, ykdkuk"

The letter counts and frequencies are as follows:

letter count frequency

•    a      0     0.0%
•    b      0     0.0%
•    c      2     9.5%
•    d      4    19.0% (4/21)
•    e      0     0.0%
•    f      1     4.8% (1/21)
•    g      0     0.0%
•    h      0     0.0%
•    i      0     0.0%
•    j      0     0.0%
•    k     10    47.6% (10/21)       
•    l      0     0.0%
•    m      0     0.0%
•    n      0     0.0%
•    ......
•    y      2     9.5%
•    z      0     0.0%

Application:

Write a program in C or C++ to repeatedly ask the user for a file name, reads the file, then displays the original text, and displays the number of instances of each letter from a to z and the letter frequencies. Repeat that process until the user chooses to stop.

Implementation:

You must use a c-string (an array of characters with a null character on the end) to represent the message. You must also use an array (of exactly 26 integers) to represent the letter counters.   These arrays must be declared in the main program and shared with the subprograms by passing them as parameters. For the message, declare an array of 501 characters.

Write functions for the following:

    (1) A function that reads text from a file one character at a time (including white-space) and returns a string containing the text. To see the new-line characters, you will need to use the 'get' function or scanf function (instead of the more complicated >> operator). You cannot 'return' an array in C++, so the text string must be allocated externally and passed as a parameter. Ask the user to enter the file name. (Use the 'getLine' or scanf function.)

    (2) A function that is given a string and an array of 26 character counters that computes how many times each letter appears in the string. Start by initializing the counters to zero. To access the array of character counters, compute the index from each character (first convert the letter to lower-case, if necessary, and then the index is just the letter 'a' subtracted from the given character). If the character is not a letter, just ignore it.

    (3) A function given a character that returns the lower-case version if it is upper case, or the same character.

    (4) A function given an array of character counters that displays the contents in a table, with letter frequencies, as described above. Hold the screen (wait for user input) after displaying the first thirteen letters (a-m), then display the rest and hold the screen again.

You must use the letters of the message to compute indexes into the array of letter counters. (You may NOT use an 'if' statement with 26 branches.) Most of all, do not create code using 'if' statements with large numbers of branches (more than 5).

When computing the letter counts, compute the index into the array of counters as follows:

            index = letter - 'a';

The difference between 'b' and 'a' is 1, and the difference between 'c' and 'a' is 2, and so on. So you can take each letter and compute the index, then add one to the counter at that index.

When you make the table of letters, letter counts, and frequencies, you can also compute the letter from the array index, as follows:

            char letter = (char) ('a' + index);

Do not use any library functions. Write your own versions.

Notes:

(a) A character is upper-case if its ASCII code is between 'A' and 'Z' inclusive.

(b) The difference between the ASCII code of a lower-case letter and the ASCII code for the corresponding lower-case letter is a constant. That constant difference can be represented as 'a' - 'A'. Adding that constant to the upper-case ASCII code will produce the corresponding lower-case ASCII code.

Answer 1

#include"stdio.h"
#include"string.h"

/* read from file */

char readingfile(char *_file,char *msg){
FILE *in;
char *ret;
char ch;
int i=0;

/* opening file in read mode */
in=fopen(_file,"r");
  
/* if file not found */
if(in==NULL){
printf("Could not open the file.\n");
strcpy(msg,"");
}
else{
  
/* read file by each character */
while((ch = fgetc(in)) != EOF){
msg[i]=ch;
i++;
}
  
/* add null character at end */
msg[i]='\0';
}
  
/* close file */
fclose(in);
}

/* change upper to lower case letter */

char lowerCase(char ch){
   int dis=0;
  
   /* check character */
if((ch>='a' && ch<='z') || (ch>='A' && ch<='Z')){
  
/* lower case character */
  
if(ch>='a' && ch<='z') {
dis=ch-'a';   
}
  
/* upper case character */
  
else{
dis=ch-'A';
}

return ((char)('a'+dis));
}

return ch;
}

/* get number of characters */

int charCount(char *s,char ch){
   int len=strlen(s);
   int count=0;
   int a;
  
   for(a=0;a<len;a++){
      
       /* check with lowercase letter */
       if(lowerCase(s[a])==ch){
          
           /* increase count */
           count++;
       }
   }
  
   return count;
}

/* count update */

void setCharsCount(char *s,int *chCount){
int i;
char ch;
int dis;
int len=strlen(s);
  
/* set each character count to 0 */
  
for(i=0;i<26;i++){
chCount[i]=charCount(s,(char)('a'+i));
}
}

/* print characters count and percentage */

void printFreq(int *count){
int i;
int total=0;
  
/* calculate total */
  
for(i=0;i<26;i++){
total+=count[i];
}
  
/* character exist in string*/
  
if(total>0){
for(i=0;i<26;i++){
printf("%6c",((char)(i+'a')));
printf("%5d",count[i]);

   /* calculate percentage */
printf("%5.1f%%",(count[i]*100/(float)total));
  
if(count[i]!=0){
printf("\t(%d/%d)",count[i],total);
}
  
printf("\n");
}
}
}

int main(){
  
int chCount[26];
char ignoreCh,msg[501],_file[30];
  
/* infinite loop */
  
while(1){
printf("Enter file name : ");
scanf("%s",_file);
  
/* read file */
readingfile(_file,msg);

/* set total character from msg */
setCharsCount(msg,chCount);

/* print */
printFreq(chCount);
  
printf("Want to read file again? [y/n]: ");
  
/* ignore enter */
  
ignoreCh=getchar();
  
/* user choice next file read */
  
ignoreCh=getchar();
  
if(ignoreCh=='N' || ignoreCh=='n'){
break;
}
  
printf("\n");
}

printf("\nGood Bye!\n");
return(0);
}