The C programming language (2nd) -- Notes -- 1.6

1.6 Array    

    Programming statistics 、 Blank symbols （ Space character 、 Box drawings and line breaks ） And the number of occurrences of all other characters . The program is of little practical significance , But it can be discussed through this procedure C Many aspects of language .

         All inputs （ character ） Can be divided into 12 class , Use an array to store the number of times each number appears than use 10 Independent variables are more convenient . Program version 1 ：

#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 );
}

When taking the program itself as input , The output is ：

        digits =  9 3 0 0 0 0 0 0 0 1,white space = 123, other = 345

         Declaration statements in the program

int ndigit[10];

Declare the variable as 10 An array of integer numbers . stay C in , Array subscripts always start from 0 Start , So... Of the array 10 The elements are ndigit[0]、ndigit[1]、…、ndigit[9], This can be done by initializing and printing two in the array for Circular statements reflect .

        The array subscript can be any integer expression , Including integer variables （ Such as i） And integer constants .

        The execution of this program depends on the number Characters represent attributes . Such as test statement

if(c >= '0' && c <= '9')

Used to judge whether the characters in are numbers . If it is , This number corresponds to

c -'0'

Only when '0'、'1'、…、'9' With continuously increasing values , This approach is feasible . Fortunately, , All character sets are like this .

        By definition ,char The characters of type are small integers , therefore char Variables and constants of type are equivalent to in arithmetic expressions int Constants and variables of type . This is natural and convenient , for example ,c - '0' Is an integer expression , If stored in c The characters in are '0'~'9', Its value will be 0~9, Therefore, it can act as an array ndigit Legal subscript of .

        Judge whether a character is a number 、 Whitespace or other characters can be completed by the following statement sequence ：

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

         The following methods are often used in programs to express and determine multiple ways ：

if( Conditions 1)
 sentence 1
else if ( Conditions 2)
 sentence 2
…
…
else
 sentence n

In this way , The conditions are evaluated from front to back , Until a condition is met , Then execute the corresponding statement part . After the execution of this part of the statement , The execution of the whole statement body ends （ Any of these statements can be several statements locked in curly braces ）. If all conditions are not met , The execution is at the last else The following statement （ If you have any ）. Similar to the previous word counting program , If there is no last else And corresponding sentences , The body of the statement will not perform any actions . At the first if With the last else Between 0 Or multiple statement sequences in the following forms ：

else if( Conditions )
 sentence 

        In terms of programming style , Readers are advised to use the indentation format shown above to reflect the hierarchical relationship of the structure , otherwise , If each if Than the previous one else Indent some distance in , Then the long decision sequence may exceed the right boundary of the page .

        The first 3 Chapter will discuss switch Statement provides another way to write multi branch programs . It is especially suitable for determining whether an integer or character expression matches an element in a constant set . In the fourth 3.4 Section for switch Statement to write another version of the program .

        practice 1-13          Programming , Print a histogram of the length of words in the input . The histogram in the horizontal direction is easier to draw , Histogram in vertical direction is more difficult .

         practice 1-14          Programming , Print the histogram of the occurrence frequency of each character in the input .