microsoft reporting services qr code The swap ( ) Function in Software

Draw QR Code in Software The swap ( ) Function

EXAMPLE 4.15 The swap ( ) Function
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This little function is widely used in sorting data:
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// Swaps x and y so that each ends up with the other's value: void swap(float& x, float& y) { float temp = x; x = y; y = temp;
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Its sole purpose is to interchange the two objects that are passed to it. This is accomplished by declaring the formal parameters x and y as reference variables: f loa t & x , f 1 oat& y. The reference operator SC makes x and y synonyms for the actual parameters passed to the function.
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FUNCTIONS
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Here is a test driver and output from a sample run:
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void swap(float&, float&);
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// Test driver for the swap function: main0 float a = 27, b = -5.041; tout << a << fl ' << b << endl; swap(a,b); tout << a << ' ' CC b << endl;
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When the call swap ( a, b ) executes, the function creates its local references x and y, so that x is the function s local name for a, and y is the function s local name for b. Then the local variable temp is declared and initialized with the value of a, a is assigned the value of b, and b is assigned the value of temp. Consequently, a ends up with the value -5.041, and b ends up with the value 27.0. Note that the function declaration:
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void swap(float&, float&);
includes the reference operator & for each reference parameter, even though the parameters themselves are omitted. Some C++ programmers write the reference operator SC as a prefix to the parameter, like this:
void swap(float &x, float &y)
instead of as a suffix to its type as we do. The compiler will accept float& x, float &x, float & x, or even float&x. It s mostly a matter of taste.
EXAMPLE 4.16 Passing By Value and Passing By Reference
This example shows the difference between passing by value and passing by reference.
void f(int x, int& y) { x = 88; y = 99; }
main0 -t int a = 22, b = 33; tout << "a = M -CC a C-C II, b = ' -CC b -CC endl;
f (ah) ;
tout << "a = ' << a CC ", b = ' <=< b << endl;
a = 22, 53 = 33
3.L 22,a = 9 9
The call f ( a, b ) passes a by value to x and b by reference to y. So x is a local variable which is assigned a s value of 22, while y is an alias for the variable b whose value is 33. The function assigns 88 to x, but that has no effect on a. But when it assigns 99 to y, it is really assigning 99 to b. Thus, when the function terminates, a still has its original value 22, while b has the new value 99. The actual parameter a is read-only, while the actual parameter b is read-write.
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FUNCTIONS
This table summarizes the differences between passing by value and passing by reference:
Table 4.3 Passing By Value Versus Passing By Reference
Passing By Value
int x; Formal parameter x is a local variable. It is a duplicate of the actual parameter. It cannot change the actual parameter. Actual parameter may be a constant, a variable, or an expression. Actual parameter is read-only. int &x;
Passing By Reference
Formal parameter x is a local reference. It is a svnonvm for the actual parameter. It can change the actual parameter. Actual parameter must be a variable. Actual parameter is read-write.
A common situation where reference parameters are needed is where the function has to return more than one value. It can only return one value directly with a return statement. So if more than one value must be returned, reference parameters can do the job.
EXAMPLE 4.17 Computing the Area and Circumference of a Circle This function returns through its two reference parameters the area and the circumference of a circle whose radius has the given length r: void { computeCircle(double& area, double& circumference, double r)
const double pi = 3.141592653589793; area = pi*r*r; circumference = 2*pi*r; > Here is a test driver and output from a sample run: void computeCircle(double&, double&, double);
main0 { double r, a, c; tout << "Enter radius: "; tin >> r; computeCircle(a, c, r); tout << "area = ' << a << ',
circumference = ' -KC c <c endl;
Note that the output parameters area and circumference are listed first in the parameter list, to the left of the input parameter r. This standard C style is consistent with the format of assignment statements: q = p, where the information (the value) flows from the read-only variable p on the right & the read-write variable q on the left.
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