بانک مقالات علمی

%#f or

%#e    ensures decimal point is printed

%#g    displays trailing zeros

Strings will be discussed later but for now remember: if you print a string using the %s specifier then all of the characters stored in the array up to the first null will be printed. If you use a width specifier then the string will be right justified within the space. If you include a precision specifier then only that number of characters will be printed.

For example:

printf("%s,Hello")

will print Hello,

printf("%25s ,Hello")

will print 25 characters with Hello right justified and

printf("%25.3s,Hello")

will print Hello right justified in a group of 25 spaces.

Also notice that it is fine to pass a constant value to printf as in printf("%s,Hello").

Finally there are the control codes:

\b       backspace

\f       formfeed

\n       new line

\r       carriage return

\t       horizontal tab

\'       single quote

\0       null

If you include any of these in the control string then the corresponding ASCII control code is sent to the screen, or output device, which should produce the effect listed. In most cases you only need to remember \n for new line.

scanf

Now that we have mastered the intricacies of printf you should find scanf very easy. The scanf function works in much the same way as the printf. That is it has the general form:

scanf(control string,variable,variable,...)

In this case the control string specifies how strings of characters, usually typed on the keyboard, should be converted into values and stored in the listed variables. However there are a number of important differences as well as similarities between scanf and printf.

The most obvious is that scanf has to change the values stored in the parts of computers memory that is associated with parameters (variables).

To understand this fully you will have to wait until we have covered functions in more detail. But, just for now, bare with us when we say to do this the scanf function has to have the addresses of the variables rather than just their values. This means that simple variables have to be passed with a preceding >&. (Note for future reference: There is no need to do this for strings stored in arrays because the array name is already a pointer.)

The second difference is that the control string has some extra items to cope with the problems of reading data in. However, all of the conversion specifiers listed in connection with printf can be used with scanf.

The rule is that scanf processes the control string from left to right and each time it reaches a specifier it tries to interpret what has been typed as a value. If you input multiple values then these are assumed to be separated by white space - i.e. spaces, newline or tabs. This means you can type:

3 4 5

or

3

4

5

and it doesn't matter how many spaces are included between items. For example:

scanf("%d %d",&i,&j);

will read in two integer values into i and j. The integer values can be typed on the same line or on different lines as long as there is at least one white space character between them.

The only exception to this rule is the %c specifier which always reads in the next character typed no matter what it is. You can also use a width modifier in scanf. In this case its effect is to limit the number of characters accepted to the width.

For example:

scanf("%lOd",&i)

would use at most the first ten digits typed as the new value for i.

There is one main problem with scanf function which can make it unreliable in certain cases. The reason being is that scanf tends to ignore white spaces, i.e. the space character. If you require your input to contain spaces this can cause a problem. Therefore for string data input the function getstr() may well be more reliable as it records spaces in the input text and treats them as an ordinary characters.

Custom Libraries

If you think printf and scanf don't seem enough to do the sort of job that any modern programmer expects to do, you would be right. In the early days being able to print a line at a time was fine but today we expect to be able to print anywhere on the screen at any time.

The point is that as far as standard C goes simple I/O devices are stream-oriented - that is you send or get a stream of characters without any notion of being able to move the current position in the stream. If you want to move backwards and forwards through the data then you need to use a direct access file. In more simple terms, C doesn't have a Tab(X,Y) or Locate(X,Y) function or command which moves the cursor to the specified location! How are you ever going to write your latest block buster game, let alone build your sophisticated input screens?

Well you don't have to worry too much because although C may not define them as standard, all C implementations come with an extensive graphics/text function library that allows you to do all of this and more. Such a library isn't standard, however the principles are always the same. The Borland and Microsoft offerings are usually considered as the two facto standards.

Summing It Up

Now that we have arithmetic, a way of reading values in and a way of displaying them, it's possible to write a slightly more interesting program than "Hello World". Not much more interesting, it's true, but what do you expect with two instructions and some arithmetic?

Let's write a program that adds two numbers together and prints the result. (I told you it wasn't that much more interesting!) Of course, if you want to work out something else like Fahrenheit to centigrade, inches to centimetres or the size of your bank balance, then that's up to you - the principle is the same.

The program is a bit more complicated than you might expect, but only because of the need to let the user know what is happening:

#include

main()

 {

   int a,b,c;

   printf("\nThe first number is ");

   scanf("%d",&a);

   printf("The second number is ");

   scanf("%d",&b);

   c=a+b;

   printf("The answer is %d \n",c);

 }

[program]

The first instruction declares three integer variables: a, b and c. The first two printf statements simply display message on the screen asking the user for the values. The scanf functions then read in the values from the keyboard into a and b. These are added together and the result in c is displayed on the screen with a suitable message. Notice the way that you can include a message in the printf statement along with the value.