书中本章的例子的目的是来数每一个数(0-9)，空格字符(空格,制表符,换行符) 以及 其它所有字符分别出现的次数。
例子程序代码如下：

#include <stdio.h> 

/* count digits, white space, others */
main() {  
    int c, i, nwhite, nother; 
    int ndigit[10]; 

    nwhite = nother = 0; 
    for (i=0; i<10; i++) {
        ndigit[i]=0; 
    }

    while ((c=getchar()) != EOF) {
        if (c>='0' && c<='9') 
            ++ndigit[c-'0']; 
        else if (c == ' ' || c == '\n' || c == '\t')
            ++nwhite; 
        else 
            ++nother; 

        printf("digits ="); 
        for (i=0; i<10; ++i) 
            printf(" %d", ndigit[i]); 
        printf(", white space = %d, other = %d\n", nwhite, nother); 
    }
}

认真感受下应该就可以懂了。

书后练习题1-13：

Write a program to print a histogram of the lengths words in its input. It is easy to draw the histogram with the bars horizontal; a vertical orientation is more challenging.

我写的版本：

/* Histogram to prints the lengths of the words --- Horizontal */  
#include <stdio.h> 

#define INSIDE 1 // Inside a word
#define OUTSIDE 0 // Outside a word

main() {  
    int c; // Variable that receives the value from getchar()
    int length = 0; // Variable that counts the length of a word
    int state = OUTSIDE; // Variable that helps us to know the status

    while ((c = getchar()) != EOF) {
        // Check, if we are in a word now or not. 
        if ((c==' ')||(c=='\n')||(c=='\t')) {
            // We are not in a word anymore, we should print the information of the word to the console. 
            if (state == INSIDE) {
                // Print Information
                putchar('\t'); 
                int i; 
                for (i=0; i<length; i++) {
                    putchar(219); 
                }
                putchar('\n'); 

                // Initialize the Variables
                state = OUTSIDE; 
                length = 0; 
            }           
        } else {
            // We are in a word. 
            if (state == OUTSIDE) state = INSIDE; 
            putchar(c); 
            length++; 
        }
    }   
}

上面这个是横向版的，纵向版的还得自己写个List，有点懒， 不写了 就这样。

书后练习题1-14:

Write a program to print a histogram of the frequencies of different characters in its input.

我写的版本：

/* Histogram that prints the frequencies of different characters in its input */  
#include <stdio.h> 

main() {  
    int c; // Variable that receives the return value of the function getchar()
    int characters[128]; // An array that holds the numbers of occurrence of the characters. 

    // Initialize
    int i; 
    for (i=0; i<128; i++) {
        characters[i] = 0; 
    }

    while ((c=getchar()) != EOF) {
        if (c=='\n') {
            characters['\n']++; 
            for (i=0; i<128; i++) {
                printf("ASCII Dec Number： %d Frequencies: %d\n", i, characters[i]); 
            }
        } else {
            characters[c]++; 
        }
    }
}

参考： 《The C Programming Language》- Chapter 1.6