letters.cc From Astrachan, pp 369-370
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#include <iostream.h>
#include <fstream.h>          // for ifstream
#include <stdlib.h>           // for exit()
#include <ctype.h>            // for tolower()
#include <limits.h>           // for CHAR_MAX
#include "CPstring.h"
#include "prompt.h"
#include "vector.h"

// count # occurrences of all characters in a file
// written: 8/5/94
// by:      Owen Astrachan

void Print(const Vector<int> & counts, int total);
void Count(istream & input, Vector<int> & counts, int & total);

int main()
{
    int totalAlph = 0;    
    string filename = PromptString("enter name of input file: ");
    ifstream input(filename);

    if (input.fail() )
    {
        cout << "could not open file " << filename << endl;
        exit(1);
    }
    Vector<int> charCounts(CHAR_MAX+1,0); // all initialized to 0
    
    Count(input,charCounts,totalAlph);
    Print(charCounts,totalAlph);
    
    return 0;
}

void Count(istream & input, Vector<int> & counts, int & total)
// precondition: input open for reading
//               counts[k] == 0, 0 <= k < CHAR_MAX
// postcondition: counts[k] = # occurrences of character k
//                total = # alphabetic characters     
{
    char ch;
    while (input.get(ch))               // read a character
    {
        if (isalpha(ch))                // is alphabetic (a-z)?
        {
            total++;
        }
        ch = tolower(ch);               // convert to lower case
        counts[ch]++;                   // count all characters
    }    
}

void Print(const Vector<int> & counts, int total)
// postcondition: all values of counts from 'a' to 'z' printed     
{
    cout.setf(ios::fixed);    // print 1 decimal place 
    cout.precision(1);
    
    char k;    
    for(k = 'a'; k <= 'z'; k++)
    {
        cout << k;
        cout.width(7);
        cout << counts[int(k)] << "  ";
        cout.width(4);
        cout << 100 * double(counts[int(k)])/total << "%" << endl;
    }
}

Sample output:

letters
enter name of input file: letters.cc [a C++ program]
a     71   7.0%
b      5   0.5%
c     86   8.5%
d     28   2.8%
e     72   7.1%
f     25   2.5%
g      5   0.5%
h     37   3.6%
i     86   8.5%
j      0   0.0%
k     11   1.1%
l     53   5.2%
m     19   1.9%
n     85   8.4%
o     89   8.8%
p     32   3.2%
q      0   0.0%
r     64   6.3%
s     41   4.0%
t    125  12.3%
u     48   4.7%
v     12   1.2%
w      8   0.8%
x      6   0.6%
y      2   0.2%
z      4   0.4%

letters
enter name of input file: plan.tex [an English document]
a   4013   7.8%
b    552   1.1%
c   2114   4.1%
d   1885   3.6%
e   6517  12.6%
f   1185   2.3%
g   1178   2.3%
h   1808   3.5%
i   3870   7.5%
j     75   0.1%
k    254   0.5%
l   1882   3.6%
m   1566   3.0%
n   3735   7.2%
o   3328   6.4%
p   1581   3.1%
q     82   0.2%
r   3782   7.3%
s   3550   6.9%
t   4784   9.3%
u   1876   3.6%
v    552   1.1%
w    617   1.2%
x    118   0.2%
y    720   1.4%
z     51   0.1%