Overview

ASSIGNMENT OPERATORS

On the face of it, in many ways, C++ seems to favour lazy typists as there are numerous short-cuts designed to save typing.

Among the most common are the assignment operators :

int num = 123;	// assigns the value 123 to the variable num
num = num + 10;	// adds 10 to num

The above can be written:

num += 10;		// adds to to num

Here is a list of these shortcuts:

num += 10;		// same as num = num + 10
num -= 10;		// same as num = num - 10
num *= 2;		// same as num = num * 2;
num /= 4;		// same as num = num / 4;
num %= 3;		// same as num = num % 3

The % is the modulus or remainder operator, so the following :

int num = 10;
num %= 3;

would result in 10 being divided by 3 and the remainder 1 would be stored in num.

EXERCISE 5

• Using the += assignment operator write a program that will list the eight times table to the screen.
• Add the -= operator to the program so that after listing the last value, it then prints the table backwards.
• Add the *= operator to the program so that it prints the powers of two to the screen i.e. 2, 4, 8, 16 etc.
• Add the /= operator so that after printing the last power of two the list is printed backwards.
• Add the modulus operator to the program in such as way as to list all the table results of the first table that will divide by 3 without leaving a remainder.

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DATA VALIDATION

When dealing with "user input" in a program, there is a need to check the user's response for validity. If this is not done, an invalid response could crash the entire system. Thankfully the following libraries come to the rescue with the following macros :

MACRO             Tests for
isalpha()         Alphabetic character
isupper()         Uppercase letter
islower()         Lowercase letter
isdigit()         Decimal digit
isalnum()         Alphanumeric character
isspace()         Whitespace character i.e. Space, form 
                  feed (\f),(\n), carriage return (\r), 
                  horizontal tab (\t) and vertical tab (\v)
ispunct()         Punctuation character
iscntrl()         Control character
isascii()         An ASCII value i.e. 0 - 127

All of the above macros take an integer (2 bytes) as an argument and returns an integer. This is because some keys (e.g. Function keys) produce an integer value. The macros can however still be used with char values (1 byte).

EXERCISE 6

Write a program that asks the user to press a number key. If a letter key is pressed, print an "Access denied" message and loop again, otherwise, print a thank you message, wait for a keypress and exit gracefully. Notice what happens when the user presses a function key. Try to work out why. Is there a way to make the program work with function keys?

Hint:

cin ignores whitespace and will wait for a carriage return, so is not suitable for this type of progam. Try experimenting with the getch() function prototyped in conio.h. More on prototyping later.

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EXERCISE 7

Modify the previous program to work with letter keys.

FUNCTION PROTOTYPES

When the ANSI C standard was implemented from C, it was the C functions that underwent the greatest change. The ANSI C standard for functions is based upon the function prototype that has already been used extensively in C++.

Under the ANSI C standard, all functions must be prototyped. The prototyping can take place in the C or C++ program itself or in a header file. The function prototype is simple, and is usually included at the start of the program code to notify the compiler of the type and number of arguments that a function will use. Prototyping enforces stronger type checking than was previously possible when the C standards were less strongly enforced.

Assume that a function called sqr has to be written. The criteria for the function is to be

• The function is to take a copy of an integer as an argument and print the square of the number to the screen.
• The original integer is not to be changed in anyway, and a return value from the function is not needed.

First the prototype needs to be coded. The prototype can take one of two forms:

void sqr(int);
void sqr(int number);

Notice the difference? Either of the above will work, but the second is preferred, as it is more descriptive and will be used for the remainder of this booklet.

The return type comes first in the prototype. In this case it is void, which means there is no return value.

Next comes the function name, followed by it's argument type and name.

Finally, a semi-colon must be added. The semi-colon is crucial, as it is the only way the compiler can tell the difference between the function prototype and the start of the function.