Content:

Reading: Textbook Chapter 2

Lecture 2:

Structure of a C program

The first program

Comments and Program Documentation

Rules for naming identifiers

Data Types

Lecture 3:

Variables

Constants

Ways of coding constants in a program

Brief introduction of formatted I/O

Lecture 2

Structure of a C program

Powerpoint file 02 slide 3 (fig. 2-2)

• Preprocessor directives:
  • Special instructions to the preprocessor that tell it how to prepare the program for compilation.
  • One of the most important preprocessor directives is include, which tells the preprocessor that you need information from selected libraries known as header files.

  Global declaration section:

  • If exists, comes at the beginning of the program.
  • Visible to all parts of the program.

  Functions:

  • There exists 1 or more in each program.
  • Exactly 1 function in the program is called main, which is the starting point of the program.
  • Every function in a program is divided into 2 sections: the declaration section and the statement section.
  • The declaration section is at the beginning of a function, and describes the data that will be used in the function.
  • These declarations are called local declarations, because they are visible only to the function that contains them.
  • The statement section follows the declaration section, and contains the instructions to the computer that cause it to do something.

The first program (Program 2-1)

• This program will display a greeting message on the screen.

  There are no global and local declarations in this program.

  Line 6 is the preprocessor directive which must always begin with a “#”.

  Line 6 means to include the standard input-output header file (stdio.h) in the program, in order to output a message to the monitor.

  Line 8 is the header of the function main, the only function in this program.

  Int means the function will return an integer to the operating system, and (void) means no parameter will be passed to this function.

  Lines 9 and 17 contain braces that show the beginning and the end of the function main.

  Line 14 calls the function printf from the stdio.h library and passes the message to be displayed on screen to the function; the “\n" meaning to advance to the next line.

  Line 16 terminates the program and returns control to the operating system. This line may be omitted.

Preprocessor directives

Powerpoint file 02 slide 4 (fig. 2-3)

Comments and Program Documentation

Powerpoint file 02 slide 5 (fig. 2-4)

• Comments are placed in program codes in order to help the reader, which may not necessary be the author of the program, to understand the program; these comments are called program documentation.

  The compiler ignores comments when it translates the program into machine language.

  Comments start by “/*” and end by “*/” and can appear anywhere in the program.

Powerpoint file 02 slide 6 (fig. 2-5)

• Comments cannot be nested.

  Lines 1-5, 10, 12, and 17 of program 2-1 are comments.

Rules for naming identifiers

• Identifiers let us name data and other objects in the program.

  Rules for identifiers:

  • Consists only of capital letters A – Z, lowercase letters a – z, the digits 0 – 9, or underscores.
  • First character must be alphabetic or the underscore (underscore is not recommended for first character in order to avoid duplicating identifiers in C system libraries).
  • Only the first 31 characters are significant.
  • Cannot be a reserved word.

  C is case sensitive: the same letter in uppercase and lowercase are treated as 2 different characters.

  Names with all capital letters are often used for preprocessor-defined names.

  Examples of valid names: a, a1, student_name, stdntNm, _aSystemName, TRUE.

  Examples of invalid names: $sum, 2names, stdnt Nmbr, int.

  Good identifier names are descriptive but short. To make them short, we often use abbreviations.

Data Types

• A data type defines a set of values and a set of operations that can be applied on those values.

  Standard data types in C:

  • void
  • char
  • int
  • float

  each data type has its own internal format in C.

void

• • has no values and no operations.

int

• • integer numbers.
  • 3 different sizes: short int (referred as short), int, and long int (referred as long); the actual sizes are machine dependent (refer to table 2-4).
  • 
  • You may find out the actual size of a data type by using the sizeof operator. e.g. sizeof(short int) is 2 in the Unix Solaris system.
  • 2 subtypes: signed and unsigned; for signed integers, 1 bit must be reserved for the sign, and hence an unsigned integer can store a positive integer that is twice as large as the signed integer of the same size (refer to table 2-4).
  • Logical data are supported by: use any non-zero number to represent true, and zero to represent false.

char

• characters – include all that the computer can represent; most computers use the ASCII alphabet.
• 1 byte is used to store this data type.
• A character is stored in memory as an integer representing the ASCII code of the corresponding character. Therefore C often treats a character like a 1-byte integer.

• Examples:     ASCII Character             Integer Representation

                                'A'                                     65

                                '1'                                      49

float

• Floating-point numbers: numbers with fractional parts.
• 3 different sizes: float, double, and long double; the actual sizes are machine dependent (refer to table 2-5).
• A floating-point number is always signed.
• Example: 0.007 = 7 x 10^-3

Summary

Lecture 3

Variables

• Variables are named memory locations that have a type.

Declaration and definition

Powerpoint file 02 slide 10 (fig. 2-9)

• Declaration is used to name an object, definition is used to create the object.

  A variable is usually declared and defined at the same time.

  Declaration gives variables a symbolic name and definition reserves memory for them.

  A variable can be of types char, int, or float, but cannot be of type void.

  To create a variable, specify the type and then its identifier. E.g. float price; (more examples below)

Initialization

• A variable may be initialized as it is created. E.g. int count = 0; creates a variable count and puts the value 0 in it.
• If a variable is not initialized, its value cannot be predicted before a value is assigned to it.

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Constants

• • Constants are data values that cannot be changed during the execution of the program.
  • All constants have a type: character, integer, floating-point, or string.

Integer constants

• • A string of digits which may be preceded by a + or -, and may be followed by u or U to denote an unsigned integer and / or a L to denote a long integer.

Float constants

• • Numbers with decimal parts.
  • Default form is double.
  • May be followed by f or F for float or l or L for long double.

Character constants

• • Single character enclosed in single quotes. E.g. ‘a’
  • Special characters are represented by a backslash (\) followed by a character. E.g. ‘\n’ means the newline character.

String constants

• Sequence of 0 or more characters enclosed in double quotes.

“” (null string)

“h”

“Hello World”

Ways of coding constants in a program

• • 3 different ways: literal constants, defined constants, and memory constants.

Literal constants

• • Unnamed constant to specify data.
  • The data value itself is coded in the program statements. E.g. ‘A’, 5, 3.1416, “Hello”.

Defined constant

• Using preprocessor command #define.
• E.g. #define SALES_TAX_RATE .0825
• The preprocessor will replace the value .0825 for all occurrences of SALES_TAX_RATE it finds in the program source.
• The #define command is usually placed at the beginning of the program so that it can be found and changed easily.

Memory constants

• Similar to defining variables with initial values except that the word const is put before the definition to indicate that the value cannot be changed throughout the program.
• E.g. const float pi = 3.1416;
• Refer to program 2-3 for an example on constants.

Brief introduction of formatted I/O

Powerpoint file 02 slide 13 (fig. 2-12)

• • C uses 2 functions for formatted input and output: printf to write formatted data to the monitor, and scanf to read formatted data from the keyboard.
  • In C, data must be input to and output from a program through a file. The keyboard is considered the standard input file, while the monitor is considered the standard output file.

printf

• • The function needs 2 sets of parameters: instructions for formatting the data and the actual data to be printed.
  • Format: printf (format string, data list);
  • E.g. printf(“The number is %6d and %6d.”, 23, 45); will generate the following output: The number is 23 and 45.
  • The format string is enclosed in double quotation marks; a format string contains constant text to be output and zero or more field specifications. For this example, The number is and and are the constant texts and there are 2 field specifications.
  • A field specification begins with a % and describes exactly 1 field or variable (for this example, %6d). The first field specification matches the first parameter in the data list, the second matches the second parameter, etc.
  • Format of field specification: %<flag><width><precision><size>conversion-code
  • <conversion-code>:

• • <size>:

    h used for short int: %hd

    l used for long int: %ld

    L used for long double: %Lf

  • <width> specifies minimum number of positions in output:

    e.g. %4d means at least 4 positions for the integer value.

    e.g. %2hd means short integer with at least 2 positions.

  • <precision> used for floating point numbers, to indicate the number of decimal places to be printed (default is 6), preceded by a “.”:

    e.g. %7.2f can print a maximum floating-point value of 9999.99.

    e.g. %10.3Lf can print a maximum long double value of 999999.999.

  • <flag>:

    - for left justify, e.g. %-8d.

    0 used with width for leading zeros, e.g. %08d.

printf (“%d\t%c\t%5.1f\n”, 23, ‘Z’, 14.2); printf (“%d\t%c\t%5.1f\n”, 107, ‘A’, 53.6); printf (“%d\t%c\t%5.1f\n”, 1754, ‘F’, 122.0); printf (“%d\t%c\t%5.1f\n”, 3, ‘P’, 0.1);

23 Z 14.2 107 A 53.6 1754 F 122.0 3 P 0.1

printf (“The number%dis my favorite number.”, 23);

The number23is my favorite number.

printf(“The number is %6d”, 23);

printf(“The tax is %6.2f this year.”, 233.12);

The tax is 233.12 this year.

printf(“The tax is %8.2f this year.”, 233.12);

The tax is   233.12 this year.

printf(“The tax is %08.2f====this year.”, 233.12);

The tax is 00233.12= = = = this year.

printf(“\”%8c===%d\””, ‘h’, 23);

scanf

• • The function needs 2 sets of parameters: a format string that describes the data and an address list that identifies where data are to be placed in memory.
  • Format: scanf (format string, address list);
  • The address of a variable is denoted by placing an ampersand (&) before the variable name.
  • E.g. scanf (“%c%d%f”, &a, &i, &x);
  • The format string is enclosed in double quotation marks; a format string contains one or more field specifications and character constants which must be matched exactly in the input. For this example, there are 3 field specifications: a character, an integer, and a floating-point number.
  • A field specification begins with a % and describes exactly 1 field. The format of field specification is generally the same as that of printf. The first field specification matches the first parameter in the address list, the second matches the second parameter, etc.
  • The programmer must make sure that the types of the variables in the address list and also the types of the input data match the types that the format string specifies, otherwise an error will occur.
  • The input conversion operation for each field processes until when any of the following occurs:

    End of file reached;

    Maximum number of characters (specified in field specification) reached;

    A whitespace character (space, tab, or new-line) is found after a digit in a numeric specification;

    An error is detected.

  • Except for %c, leading whitespaces will be skipped for all other field specifications.
  • Since %c reads exactly 1 character, it will read in any character including whitespace character. If you also want to skip leading whitespace before a character, place a space before the %c field specification. E.g. scanf(“ %c%d%f”, &a, &i, &x);
  • Any character other than whitespace or field specification in the format string must be exactly matched during input. If the input stream does not match the character specified, an error will occur and scanf will stop.
  • A fatal error will occur if the format string is ended with a whitespace character.

- End of Lectures 2 and 3-