C programming language (2nd Edition) -- Reading Notes -- 1.5.2

1.5.2 Character count

  The next program counts characters , Similar to the replication program .

#include <stdio.h>

 /* Count characteristics in input, first version*/
main()
{
	long nc;

	nc=0;
	while (getchar() != EOF)
		++nc;
	printf("%ld\n",nc);
}

Its

++nc;

Introduce new operator ++, Add 1 operation . You can use nc = nc + 1 Instead of . but ++nc More refined , And often more efficient . And ++ The corresponding is the self subtraction operator --.++ And -- It can be used as prefix operator （ Such as ++nc）, It can also be used as suffix operator （ Such as nc++）. The first 2 These two forms have different values in the expression , but ++nc And nc++ All make nc The value of the increase 1. At present, only prefix form .

        This procedure use long Type variables store count values , Instead of using int Type variable .long Integer numbers （ Long integer ） At least occupy 32 Bit storage unit . On some machines int And long Integer has the same length , But on some machines ,int The value of type may only be 16 The length of the bit storage unit （ The maximum value is 32767）, A fairly small input can make int Count variable overflow of type . Conversion instructions ——%ld—— tell printf Function whose corresponding arguments are long integer .

        use double The double precision floating-point number type can handle larger numbers . Here it is , no need while, While using for Loop to write another method of this loop ：

#include <stdio.h>

 /* Count characteristics in input, first version*/
main()
{
#if 0            /* Put this to use , Conditional compilation */
	long nc;

	nc = 0;
	while (getchar() !=  EOF)
		++nc;
	printf("%ld\n",nc);
#else 
	/* Count characteristics in input second version*/
	double nc;

	for(nc = 0; getchar() != EOF; ++nc)
		;      // Emphasis ";"
	printf("%.0f\n",nc);
#endif
}

about float and double type , printf Functions are used %f To illustrate .%.0f Force not to print Decimal point and decimal part , Therefore, the number of decimal places is 0.

        In the program for The loop body of the loop statement is empty , Because all the work is being tested （ Conditions ） And increasing the step length is partially completed . but C Language Rule of grammar requirement for A loop statement must have a loop body , So use a separate semicolon instead . Separate semicolons are called empty statements , It just meets for Statement . Put it on a separate line to make it stand out .

        Before the end of the , Consideration ： If the input does not contain characters , That's the first call getchar Function time , while or for The conditional test in the statement is false from the beginning , The result of program execution is 0, This is also the right result . That's important . while And for One of the advantages of statements ： Test the condition before executing the loop body , If the conditions are not met , The loop body... Is not executed , It may happen that the loop body is not executed once . In the presence of 0 When inputting the length , Program processing should be more flexible , When boundary conditions occur , while And for Statements help ensure that the program performs reasonable operations .