how to create barcodes in excel 2010 Page 166 in Software

Painting Quick Response Code in Software Page 166

Page 166
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printf( ) liberally during program development so that you can watch what is going on and abort execution if you see a mistake Function Prototypes In modern, properly written C programs, all functions must be declared before they are used This is normally accomplished using a function prototype Function prototypes were not part of the original C language, but were added by C89 Although prototypes are not technically required, their use is strongly encouraged (Prototypes are required by C++, however) In this book, all examples include full function prototypes Prototypes enable the compiler to provide stronger type checking, somewhat like that provided by languages such as Pascal When you use prototypes, the compiler can find and report any questionable type conversions between the arguments used to call a function and the type of its parameters The compiler will also catch differences between the number of arguments used to call a function and the number of parameters in the function The general form of a function prototype is type func_name(type parm_namel, type parm_name2, , type parm_nameN); The use of parameter names is optional However, they enable the compiler to identify any type mismatches by name when an error occurs, so it is a good idea to include them The following program illustrates the value of function prototypes It produces an error message because it contains an attempt to call sqr_it( ) with an integer argument instead of the integer pointer required
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/* This program uses a function prototype to enforce strong type checking */ void sqr_it(int *i); /* prototype */ int main(void) { int x; x = 10; sqr_it(x); return 0; }
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/* type mismatch */
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Page 167 void sqr_it(int *i) { *i = *i * *i; }
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A function's definition can also serve as its prototype if the definition occurs prior to the function's first use in the program For example, this is a valid program:
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#include <stdioh> /* This definition will also serve as a prototype within this program */ void f(int a, int b) { printf(''%d ", a % b); } int main (void) { f(10,3); return 0; }
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In this example, since f( ) is defined prior to its use in main( ), no separate prototype is required Although it is possible for a function's definition to serve as its prototype in small programs, it is seldom possible in large ones especially when several files are used The programs in this book include a separate prototype for each function because that is the way C code is normally written in practice The only function that does not require a prototype is main( ) because it is the first function called when your program begins There is a small but important difference between how C and C++ handle the prototyping of a function that has no parameters In C++, an empty parameter list is indicated in the prototype by the absence of any parameters For example,
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int f(); /* C++ prototype for a function with no parameters */
However, in C this statement means something different Because of the need for compatibility with the original version of C, an empty parameter list simply says that no parameter information is given As far as the compiler is concerned, the function could have several parameters or no parameters (Such a statement is called an old-style function declaration and is described in the following section)
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In C, when a function has no parameters, its prototype uses void inside the parameter list For example, here is f( )'s prototype as it would appear in a C program:
float f(void);
This tells the compiler that the function has no parameters, and any call to that function that has arguments is an error In C++, the use of void inside an empty parameter list is still allowed, but redundant Function prototypes help you trap bugs before they occur In addition, they help verify that your program is working correctly by not allowing functions to be called with mismatched arguments One last point: Since early versions of C did not support the full prototype syntax, prototypes are technically optional in C This is necessary to support pre-prototype C code If you are porting older C code to C++, you will need to add full function prototypes before the code will compile Remember, although prototypes are optional in C, they are required by C++ This means that every function in a C++ program must be fully prototyped Because of this, most C programmers also fully prototype their programs Old-Style Function Declarations In the early days of C, prior to the creation of function prototypes, there was still a need to tell the compiler in advance about the return type of a function so that the proper code could be generated when the function was called (Since sizes of different data types differ, the size of the return type needs to be known prior to a call to a function) This was accomplished using a function declaration that did not contain any parameter information The old-style approach is archaic by today's standards However, it can still be found in older code For this reason, it is important to understand how it works Using the old-style approach, the function's return type and name are declared near the start of your program, as illustrated here:
#include <stdioh> double div(); /* old-style function declaration */ int main(void) { printf(''%f", div(102, 200)); return 0; }
Page 169 double div(double num, double denom) { return num / denom; }
The old-style function type declaration tells the compiler that div( ) returns an object of type double This allows the compiler to correctly generate code for calls to div( ) It does not, however, say anything about the parameters to div( ) The old-style function declaration statement has the following general form: type_specifier function_name( ); Notice that the parameter list is empty Even if the function takes arguments, none are listed in its type declaration As stated, the old-style function declaration is outmoded and should not be used for new code It is also incompatible with C++ Standard Library Function Prototypes Any standard library function used by your program must be prototyped To accomplish this, you must include the appropriate header for each library function All necessary headers are provided by the C compiler In C, the library headers are (usually) files that use the h extension A header contains two main elements: any definitions used by the library functions and the prototypes for the library functions For example, <stdioh> is included in almost all programs in this book because it contains the prototype for printf( ) The headers for the standard library are described in Part Two Declaring Variable Length Parameter Lists You can specify a function that has a variable number of parameters The most common example is printf( ) To tell the compiler that an unknown number of arguments will be passed to a function, you must end the declaration of its parameters using three periods For example, this prototype specifies that func( ) will have at least two integer parameters and an unknown number (including 0) of parameters after that:
int func(int a, int b, );
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