Overview of a C Program
Programming with C
CSCI 112, Spring 2015
Patrick Donnelly
Montana State University
C Language Components
• Preprocessor Directives
• Comments
• The “main” function
• Variable Declarations and Data Types
• Executable Statements
• Reserved Words
• Identifiers
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General Outline of a C Program
preprocessor directives
main function heading {
declarations
executable statements
}
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Example C Program
# include < stdio .h >
# define KMS_PER_MILE 1.609
// printf , scanf definitions
// conversion constant
int main ( int argc , char ** argv )
{
double miles ;
// distance in miles
double kms ;
// equivalent distance in kilometers
// get the distance in miles
printf ( " Enter the distance in miles > " );
scanf ( " % lf " , & miles );
// convert the distance to kilometers
kms = KMS_PER_MILE * miles ;
// display the distance in kilometers
printf ( " That equals % f kilometers .\ n " , kms );
return (0);
}
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Anatomy of a C Program
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Example C Program: Preprocessor Directives
#include <stdio.h>
#define KMS PER MILE 1.609
int main ( int argc , char ** argv )
{
double miles ;
// distance in miles
double kms ;
// equivalent distance in kilometers
// get the distance in miles
printf ( " Enter the distance in miles > " );
scanf ( " % lf " , & miles );
// convert the distance to kilometers
kms = KMS_PER_MILE * miles ;
// display the distance in kilometers
printf ( " That equals % f kilometers .\ n " , kms );
return (0);
}
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Example C Program: Declarations
# include < stdio .h >
# define KMS_PER_MILE 1.609
// printf , scanf definitions
// conversion constant
int main ( int argc , char ** argv )
{
double miles;
double kms;
// get the distance in miles
printf ( " Enter the distance in miles > " );
scanf ( " % lf " , & miles );
// convert the distance to kilometers
kms = KMS_PER_MILE * miles ;
// display the distance in kilometers
printf ( " That equals % f kilometers .\ n " , kms );
return (0);
}
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Example C Program: Comments
# include < stdio .h >
# define KMS_PER_MILE 1.609
// printf, scanf definitions
// conversion constant
int main ( int argc , char ** argv )
{
double miles ;
// distance in miles
double kms ;
// equivalent distance in kilometers
// get the distance in miles
printf ( " Enter the distance in miles > " );
scanf ( " % lf " , & miles );
// convert the distance to kilometers
kms = KMS_PER_MILE * miles ;
// display the distance in kilometers
printf ( " That equals % f kilometers .\ n " , kms );
}
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Example C Program: Executable Statements
# include < stdio .h >
# define KMS_PER_MILE 1.609
// printf , scanf definitions
// conversion constant
int main ( int argc , char ** argv )
{
double miles ;
// distance in miles
double kms ;
// equivalent distance in kilometers
// get the distance in miles
printf("Enter the distance in miles> ");
scanf("%lf", &miles);
// convert the distance to kilometers
kms = KMS PER MILE * miles;
// display the distance in kilometers
printf("That equals %f kilometers.\n", kms);
return(0);
}
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Our C Program
# include < stdio .h >
/* lab01 . c ( name of your file )
* Philip J . Fry ( your name ) , CSCI112 , Lab 01
* 08/28/2014 ( current date )
*/
int main ( void ) {
printf ( " Hello World \ n " );
return (0);
}
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Preprocessor Directives
Definition: Preprocessor Directives
Preprocessor Directives are commands that give instructions to
the C preprocessor.
Definition: Preprocessor
The Preprocessor is a program that modifies your C program
before it is compiled.
Preprocessor Directives start with a “#” symbol.
• #include
• #define
• #if
• #else
• #endif
• #pragma
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#include
#include is used to include other source files into your source file.
#include gives a program access to a library/header file.
• stdio.h
• math.h
• time.h
Libraries are a collection of useful functions and symbols.
Standard libraries are predefined by the ANSI C language.
• You must include stdio.h if you want to use printf and
scanf library functions.
• “#include <stdio.h>” inserts their definitions into your
program before compilation.
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#define
The #define directive instructs the preprocessor to replace every
occurance of a particular text with a given constant value. The
replacement occurs before compilation.
Your source code:
#d e f i n e P I 3 . 1 4 1 5 9
#d e f i n e EULERS NUMBER 2 . 7 1 8 2 8
i n t main ( v o i d ) {
d o u b l e x = 5 ∗ PI ;
d o u b l e y = EULERS NUMBER ∗ 7 ;
return (0);
}
What actually gets compiled:
i n t main ( v o i d ) {
double x = 5 ∗ 3.14159;
double y = 2.71828 ∗ 7;
return (0);
}
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Constants
#define Constants
• Consists of 3 parts:
◦ “#define”
◦ constant name
◦ constant value
• There is no “=” sign or “;” at the end
# define M Y _ C O N S T A N T _N A M E 123
# define ANSWER 42
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Comments
Comments provide extra information, making it easier for humans
to understand the program.
These comments are completely ignored by the compiler.
Comments take two forms:
• /*
• //
*/ - anything between asterisks is a comment.
- anything after is a comment, but only until EOL.
Comments are used for documentation that helps other
programmers read and understand the program.
Programs should contain a comment at the top with
• the programmer’s name,
• the date, and
• a brief description of what the program does.
COMMENT YOUR CODE!
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The “main” function
The heading “int main(void)” marks the beginning of the main
function where your program execution begins.
Every C program has a main function.
Braces/Curly brackets: { } mark the beginning and end of the
body of the function.
A function body has two parts:
• declarations: (names of variables and data types) tell the
compiler what memory cells are needed in the function and
how they should be processed.
• executable statements: they reflect “actions” of your
algorithm. These are translated into machine language by the
compiler and later executed.
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Declarations of Variables
Variable: A memory cell used to store program data.
• A variables value can change with time.
• We use its name as a simple reference (instead of the actual
address in memory).
Variable declaration: Statement that communicates to the
compiler the names of variables in the program and the data type
they represent.
Example:
double miles ;
This tells the compiler to create a space for a variable of type
double in memory and refer to it each time we use the name miles
in our program.
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Numeric Data Types
Definition: Data Type
Data Type: a set of values and a set of operations that can be
performed on those values.
Numeric Data Types:
• int: Stores integer value (whole number).
Examples: 65, -123
• double: Stores real number (uses a decimal point).
Examples: 3.14159, 1.23e5 (123000.0)
Arithmetic operations (+,-,*,/, %) can be used on ints and
doubles.
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Character Data Type
Character Data Type:
• char: An individual character value.
Examples: ‘a’, ,‘5’, ‘*’
(can be a letter, digit, or special symbol)
Character Declaration
• Use single quotes, NOT double quotes
char my_char = ’Z ’;
Comparison operations (<, >, ≤, ≥) can be used on ints, doubles
and chars.
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ASCII Code
‘A’ < ‘B’ < ‘C’ < ... < ‘X’ < ‘Y’ < ‘Z’
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Executable Statements
Executable Statements: C statements used to write or code the
algorithm.
The C compiler translates the executable statements into machine
code.
Examples:
• Input/Output Operations and Functions
◦ printf function
◦ scanf function
• Assignment Statement (=)
• return Statement/Operation
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Executable Statements: Input/Output Operations
Input operation: data transfer from outside the program into
computer memory that is usable by the program.
Input may be received from:
• A program user
• An external file
• Another program
Output operation: program results that can be displayed to the
program user.
Results can be printed to:
• The standard console output
• An external file
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Executable Statements: Functions
A function takes input parameters and produces an output.
A function definition specifies the return type, the input
parameters, and the body.
A function call is used to call or activate a function.
• Calling a function means asking another piece of code to do
some work for you.
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Executable Statements: Functions - printf()
By calling the printf() function, we are asking for the function
to print to standard output.
The first argument to printf is the formatting code (in string
form), and all following arguments are the the variables to be used
in the placeholders.
double miles = 2.7;
printf ( " That is % f miles .\ n " , miles );
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Executable Statements: Functions - printf()
Multiple placeholders can be specified, and the corresponding
arguments are then provided in order. Different types of variables
require different formatting codes.
int a = 1;
double b = 2.0;
char * c = " three " ;
printf ( " Easy as %d , %f , % s !\ n " , a , b , c );
Details on formatting codes can be found in the handout.
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Executable Statements: Functions - scanf()
Reading input data in a program is done via the scanf() function.
The concept is very much the same as the printf() function.
In this case, the format code is just the variable code for what data
will be received (%d, %c, %lf), and the following arguments are
the variables that will receive the data.
double miles ;
scanf ( " % lf " , & miles );
The “&” is the address operator in C. The “&” operator in front
of variable miles informs the scanf() function about the location
of variable miles in the computer’s memory.
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printf()/scanf() statements
printf()/scanf() statements
Use commas to deliminate printf/scanf arguments.
char a = ’! ’;
int b = 7;
float c = 3.7;
double d = 1.5;
printf ( " % c | % d | % f | % lf | % c \ n " , a , b , c , d , a );
scanf ( " % lf " , & d );
Remember that %f is for floats, %lf is for doubles
• For printf(), doubles can be implicitly converted to floats
printf ( " % f \ n " , & d );
printf ( " % lf \ n " , & d );
scanf ( " % lf " , & d );
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Assignment Statements
An assignment statement stores a value in a variable.
• The value assigned can be the result of another computation.
kms = KMS_PER_MILE * miles ;
The above assigns a value (content) to the variable kms.
The value assigned in this case is the result of the multiplication of
the constant KMS PER MILE by the variable miles.
We should interpret an assignment statement: value = expression
as: value ← expression
This should be read as “becomes”, “gets”, or “takes the value of”
rather than “equals”.
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Assignment Statements
Effect of kms = KMS PER MILE * miles;
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Assignment Statements
“=” is NOT an algebraic operator. Consider the following:
sum = sum + item ;
This is obviously not a correct algebraic equation (if item 6= 0).
This statement instructs the computer to add the current value of
sum to the value of item. Afterward the result is stored back into
sum.
Equality:
To test for equality, use the “==” operator.
Be sure to keep these concepts separate.
Remember!
Every variable has contents, even before assignment.
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Assignment Statements
Effect of sum = sum + item;
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Arithmetic Operators
Operator
+
Meaning
addition
Examples
5+2=7
5.0 + 2.0 = 7.0
−
subtraction
5 −2 = 3
5.0 −2.0 = 3.0
*
multiplication
5 * 2 = 10
5.0 * 2.0 = 10.0
/
division
5.0 / 2.0 = 2.5
5/2=2
%
remainder (mod)
5%2=1
5/2=2
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Expression Example
Area of a Circle
a = πr 2
can be written in C as:
area = PI * radius * radius ;
where PI is a constant macro set to 3.14159.
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Expression Evaluation
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Expression Example 3
Evaluation Tree and Evaluation for v = (p2 - p1) / (t2 t1);
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Expression Example 4
Evaluation Tree and Evaluation for z - (a + b / 2) + w * -y;
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The “return” statement
In your main() function, the return statement will transfer control
from your program to the operating system.
“return(0);” returns a 0 to the operating system, which indicates
that the program executed without error.
• This means that the end of the program was reached.
• It does not mean that the program did what it was supposed
to do. There still may have been logical errors.
Other functions also have return statements, which can return
error codes, data, or nothing at all.
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Punctuation & Special Symbols
Semicolons: ;
Mark the end of a statement (which may take more than one line).
Curly Braces: { }
Mark the beginning and end of the body of a function.
Mathematical Symbols: *,/,+,Are mainly used to compute numeric values.
Note that some have additional meaning.
In particular, we will talk about * later.
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Reserved Words
A reserved word is a word that has special meaning to C and can
not be used for other purposes.
These are words that C reserves for its own uses. They are often
necessary for the compiler to analyze the language’s grammar and
to understand what the programmer wants to do (declaring
variables, controlling data flow, etc).
• In example, you cannot have a variable named “return” or
“if”.
Reserved words are always lowercase.
See your book or the reference links on the website for a complete
list of reserved words.
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Standard Identifiers
An identifier is a name given to a variable or an operation.
• This is just a formal term for function names and variable
names.
A standard identifier is an identifier that is defined in the
standard C libraries and has special meaning in this standard (such
as ANSI C).
• Examples include printf and scanf
• Unlike reserved words, you can re-define a standard identifier
• It is not recommended to use standard identifiers in a
non-standard way, especially when linking to other code that
uses these standards.
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User Defined Identifiers
Variables:
We choose our own identifiers to name memory cells that will hold
data.
Functions:
We choose our own identifiers to name operations that we define.
Common Rules for Naming Identifiers:
• May consist only of letters, digits, and underscores
• cannot begin with a digit
• reserved words cannot be used
• standard identifiers should not be redefined
Valid Identifers: var1, inches, KM PER MILE
Invalid Identifiers: 1var, the*var, return
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Identifier Naming Guidelines
Some compilers only recognize the first 31 characters of identifier
names, so avoid long identifiers.
Compilers are case-sensitive.
• Aviod very similar names (differ by case, or a single letter)
• ITEM is different than Item is different than item
• Note that conventional C code uses lowercase and underscores
for variables (as opposed to camel-case used in Java)
◦ miles
◦ vehicle speed
Constants and Macros usually use all uppercase
◦ KMS PER MILE is a defined constant
Choose meaningful identifiers that are easily understood.
• distance = velocity * time is more intelligible than x = y * z.
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