Introduction to C Input and Output

 

Text-based:  Output goes to printer or monitor as text;  Input comes from a file of text (words)

Output goes to special file, named “stdout”;  Input from another special file named “stdin”.

 

Output

  1. printf is the handiest way to create a message for display on  a monitor.  The monitor by default displays whatever is sent to file “stdout”, and printf() sends data to that file.
    1. printf(“FORMAT”);

                                                    i.     The first argument to printf is a C string of characters.  This argument “controls” everything printf() does.

WARNING:  NOTHING will display unless this string contains a “newline” character, which is represented as \n.  Usually this character should be last in the string.

                                                  ii.     In the simplest case, the entire first argument is simply copied to stdout:
            printf(“This is the test output\n”);
            When executed, this displays:
This is the test output
            on a line by itself.  The quotation marks don’t display – these simply mark the beginning and end of the string, for the C compiler.  The \n causes the line to print, and moves to the beginning of the next line.

                                                iii.     In more complicated cases, the first argument contains the character %, followed by a “conversion specifier”.  Simple conversion specifiers consist of single letters, each of which cause the next argument’s value to be converted, and substituted for the conversion specifier, while copying the first argument to stdout:
            printf(“A[%d]=%d\n”, 10, 20);   displays
A[10]=20

1.     %d – integer argument

2.     %s – string argument

3.     %f -- floating point number argument

                                                iv.     More complex specifiers allow you to control

1.     How many characters, minimum, the converted quantity will occupy on the line (useful for creating tables)

2.     Whether the extra characters should be added at the left, or the right of the number

3.     Whether the extra characters should be zeros or spaces

4.     Whether a leading plus sign should be printed

5.     How many digits should appear to the right of the decimal point, when floating point numbers are printed

For details see the Input Output document.

Exercises:

  1. Display the message “Hello world!” without the quotes on a line by itself.
    ANSWER:
    #include <stdio.h>
    int main() {
                printf(“Hello world!\n”);
                }
  2. Display a line consisting of the letter z, 5 spaces, and the numeric value of the code which represents z internally.  This code is an integer, and can be represented in C as
                ‘z’
    Display as the next line, the character and code value for Z.  Continue with lines for a and A.  Finish with lines for the digit 0, and the digit 9.
  3. The internal codes for the digits 0 through 9 are sequential.  This means that, if variable x contains the code for a digit, the value of the expression x-‘0’ will be the value of that digit.  Write a program which will read a sequence of digit sequences (separated by \n characters) from stdin, using “getchar()”, until End Of File is encountered.  For each sequence read, convert it to internal format, multiply it by 3, and print the result, on a line by itself.
    1. Declare c as an “int”:  “int c;”

                                                    i.     The declarations in C should follow the “main() {“ line immediately.

    1. Write “c = getchar();” to read the next character from stdin
    2. Use an if-test, to compare c’s value with ‘\n’

                                                    i.     Note: In C, the “equality test” is written ==, NOT =.

                                                  ii.     An “if test” is written:
if (test) {
            do something if test was “true”;
            }
else {
            do something if test was false”
            }
The “else” can be omitted.  So can the braces “{}” IF they enclose a single statement.  Do NOT follow the first “}” with a semi-colon, or C may not understand the “else”.

    1. Accumulate the value of the number in another int variable: x.
    2. As each new digit is read, multiply x by 10, and add the value of the digit to the result, putting that result back into x.
    3. When the end of the sequence of digits is found, print the value of x*3 on a line, set x back to 0, and repeat, until the value getchar returns is –1.

                                                    i.     A “while loop” may be useful here:
            while (test) {
                        do something;
                        }
repeats “do something (which can be a sequence of statements), over and over, until the “test” is false.  (If the test is false the first time, it sodes nothing.)