The += operator adds the values of its two operands and assigns the result to the operand on the left. This implies that the left operand must be something to which you can assign a value, such as a variable, an array element, a structure member, or data you identify by dereferencing a pointer:
int k = 5;
k += 3; // ok, k set to 8
int *pa = new int[10]; // pa points to pa[0]
pa[4] = 12;
pa[4] += 6; // ok, pa[4] set to 18
*(pa + 4) += 7; // ok, pa[4] set to 25
pa += 2; // ok, pa points to the former pa[2]
34 += 10; // quite wrong
Each arithmetic operator has a corresponding assignment operator, as summarized in Table 5.1. Each operator works analogously to +=. Thus, for example, the following statement replaces the current value of k with a value 10 times greater:
k *= 10;
Table 5.1. Combined Assignment Operators
Compound Statements, or Blocks
The format, or syntax, for writing a C++ for statement might seem restrictive to you because the body of the loop must be a single statement. That’s awkward if you want the loop body to contain several statements. Fortunately, C++ provides a syntax loophole through which you may stuff as many statements as you like into a loop body. The trick is to use paired braces to construct a
Listing 5.8. block.cpp
// block.cpp -- use a block statement
#include
int main()
{
using namespace std;
cout << "The Amazing Accounto will sum and average ";
cout << "five numbers for you.\n";
cout << "Please enter five values:\n";
double number;
double sum = 0.0;
for (int i = 1; i <= 5; i++)
{ // block starts here
cout << "Value " << i << ": ";
cin >> number;
sum += number;
} // block ends here
cout << "Five exquisite choices indeed! ";
cout << "They sum to " << sum << endl;
cout << "and average to " << sum / 5 << ".\n";
cout << "The Amazing Accounto bids you adieu!\n";
return 0;
}
Here is a sample run of the program in Listing 5.8:
The Amazing Accounto will sum and average five numbers for you.
Please enter five values:
Value 1: 1942
Value 2: 1948
Value 3: 1957
Value 4: 1974
Value 5: 1980
Five exquisite choices indeed! They sum to 9801
and average to 1960.2.
The Amazing Accounto bids you adieu!
Suppose you leave in the indentation but omit the braces:
for (int i = 1; i <= 5; i++)
cout << "Value " << i << ": "; // loop ends here
cin >> number; // after the loop
sum += number;
cout << "Five exquisite choices indeed! ";
The compiler ignores indentation, so only the first statement would be in the loop. Thus, the loop would print the five prompts and do nothing more. After the loop completes, the program moves to the following lines, reading and summing just one number.
Compound statements have another interesting property. If you define a new variable inside a block, the variable persists only as long as the program is executing statements within the block. When execution leaves the block, the variable is deallocated. That means the variable is known only within the block:
#include
int main()
{
using namespace std;
int x = 20;
{ // block starts
int y = 100;
cout << x << endl; // ok
cout << y << endl; // ok
} // block ends
cout << x << endl; // ok
cout << y << endl; // invalid, won't compile
return 0;
Note that a variable defined in an outer block is still defined in the inner block.
What happens if you declare a variable in a block that has the same name as one outside the block? The new variable hides the old one from its point of appearance until the end of the block. Then the old one becomes visible again, as in this example:
#include
int main()
{
using std::cout;
using std::endl;
int x = 20; // original x
{ // block starts
cout << x << endl; // use original x
int x = 100; // new x
cout << x << endl; // use new x
} // block ends
cout << x << endl; // use original x
return 0;
}
More Syntax Tricks—The Comma Operator
