Chapter 7
C Language Preliminaries
Chapter 7: C Language
Preliminaries
1
CS1101C Textbook
C: How To Program
by H.M. Deitel and P.J. Deitel
Prentice Hall, 2nd Edition
ISBN 0-13-288333-3
Chapter 7
CS1101C Textbook
2
Introduction
1972 by Dennis Ritchie at Bell Lab.
ALGOL60  CPL  BCPL  B  C
ANSI C: standardised.
C is a high-level language.
C is a procedural language.
C is baffling to look at, easy to mess up, and
tricky to learn.
Chapter 7
Introduction
3
Introduction
Some articles (more in CS1101C web site,
under ‘CA Activities/Readings & Preparation’):
 The Ten Commandments for C Programmers
 The Top 10 Ways to get screwed by the C
programming language
Chapter 7
Introduction
4
Algorithmic Problem Solving
Understanding
the problem
Writing an
algorithm
Verifying the
algorithm
Converting to
code
Chapter 7
Algorithmic Problem Solving
5
From Problem to Algorithm
Conversion from inches to centimetres.
 Step 1: Understanding the problem.
What is the the data (input)?
Length in inches. Let’s call it inches.
What is the unknown (output)?
Length in centimetres. Let’s call it cm.
Chapter 7
From Problem to Algorithm
6
From Problem to Algorithm
 Step 2: Devising a plan.
First draft of plan (algorithm):
Get length in inches
Compute length in centimetres
Report length in centimetres
What is the relationship between the data and
the unknown?
1 inch = 2.54 centimetres
Chapter 7
From Problem to Algorithm
7
From Problem to Algorithm
 Step 2: Devising a plan. (continue…)
Do I know how to compute the length in cm?
Yes.
Length in cm is: 2.54 times length in inches
Complete plan (algorithm):
Get length in inches
Compute length in centimetres, using
2.54 * length in inches
Report length in centimetres
Chapter 7
From Problem to Algorithm
8
From Problem to Algorithm
 Step 2: Devising a plan. (continue…)
Is the plan correct?
Walk through the plan.
Work out on some examples: 1 inch, 2 inches,
3.5 inches, 0 inch, etc.
Chapter 7
From Problem to Algorithm
9
From Problem to Algorithm
Step 3: Carrying out the plan.
Convert algorithm to program.
Compile and run.
Step 4: Looking back.
Check the output. Is the output correct?
In general, an algorithm may need to go
through a few rounds of refinements.
Chapter 7
From Problem to Algorithm
10
From Algorithm to Program
 inch2cm.c
/*
* Converts length from inches to centimetres
* Author: Garfield
Date: 25 July 1997
* Note: This program contains a hard-to-spot error!
*/
#include <stdio.h>
/* printf, scanf definitions */
#define CM_PER_INCH 2.54 /* conversion constant */
int main (void)
{
float inches,
cm;
/* length in inches
length in centimetres */
/* get the length in inches */
printf ("Enter a length in inches: ");
scanf ("%f", &inches);
/* convert to length in centimetres */
cm = CM_PER_INCH * inches;
/* display the length in centimetres */
printf ("That equals %.2f centimetres.\n\n", cm);
return 0; /* indicate program ends successfully */
}
Chapter 7
From Algorithm to Program
11
Sample Runs
garfield@decunx:~/c[xx]: cc -o inch2cm inch2cm.c
garfield@decunx:~/c[xx]: inch2cm
Enter a length in inches: 2
That equals 5.08 centimetres.
garfield@decunx:~/c[xx]: inch2cm
Enter a length in inches: 5.9
That equals 14.99 centimetres.
garfield@decunx:~/c[xx]: inch2cm
Enter a length in inches: 0
That equals 0.00 centimetres.
garfield@decunx:~/c[xx]: inch2cm
Enter a length in inches: –2
That equals –5.08 centimetres.
garfield@decunx:~/c[xx]: inch2cm
Enter a length in inches: 1.2e4
That equals 30480.00 centimetres.
Chapter 7
Sample Runs
12
Preprocessor Directives
C system = C language + preprocessor +
libraries
Preprocessor modifies program before it is
compiled.
Common directives: #include and #define
# must be first non-blank character on the line.
Chapter 7
Preprocessor Directives
13
Preprocessor Directives
#include <stdio.h>
Includes header file stdio.h from /usr/include
directory into program.
stdio.h contains functions prototypes of scanf(),
printf() and others, so that inch2cm.c can use
these functions.
#include "file" assumes file in current
directory.
Chapter 7
Preprocessor Directives
14
Preprocessor Directives
#define CM_PER_INCH 2.54
Defines constant macro (or symbolic constant)
CM_PER_INCH as 2.54
Preprocessor replaces each occurrence of
CM_PER_INCH in the rest of program to 2.54.
cm = CM_PER_INCH * inches;
 cm = 2.54 * inches;
Purpose?
Chapter 7
Preprocessor Directives
15
Comments
/* this is a line of comments */
Comments are text enclosed in /* and */. Some
compilers accept //.
Comments provide documentation for program
readability.
Every program should have its author and date
written documented.
Every program and function should have a
statement to describe its purpose.
Chapter 7
Comments
16
Comments
Right dosage is important.
Too much:
i++;
/* increment i by 1 */
sum += num;
/* add num to sum */
/* multiply CM_PER_INCH by inches */
/* and store result in cm
*/
cm = CM_PER_INCH * inches;
Chapter 7
Comments
17
Comments
Comments add values to readers.
Should describe what the steps do and explain
difficult logic, instead of translating C statements
into English.
Comments explain the purpose; program
statements show the logic.
Self-explanatory codes do not need much
comments. Name variables appropriately.
Examples: ‘width’ instead of ‘w’.
Chapter 7
Comments
18
Comments
Do not fill in comments as an after-thought.
Documentation should begin at design, and
follows through every phase.
Spot the error:
float inches,
cm;
Chapter 7
/* length in inches
length in centimetres */
Comments
19
The main() Function
C program consists of functions.
At least one function must be present: main().
Execution of the program starts at this function.
Example:
int main (void)
{
/* body of function */
}
Chapter 7
The main() Function
20
The main() Function
Function consists of header and body.
Function header:
 The type of the function -- eg: int.
 The name of the function -- eg: main
 The parameter list enclosed in parentheses -eg: void
Chapter 7
The main() Function
21
The main() Function
Function type: the type of value this function
produces.
If function type is not ‘void’, then there should be
a return statement to return the result of the
function. Example: return 0;
Execution of the function ends when the return
statement is executed, or when the last
statement is executed.
Where is the result returned to?
Chapter 7
The main() Function
22
The main() Function
A function returns its result (and control) to its
caller. The caller for the main() function is the
operating system.
Zero is usually returned to UNIX to signal the
successful completion of the program.
Chapter 7
The main() Function
23
The main() Function
A function parameter lists the arguments which
the function takes from its caller.
For example:
printf ("Enter a length in inches: ");
scanf ("%f", &inches);
"Enter a length in inches: " is an argument to the
printf() function. "%f" and &inches are arguments to
the scanf() function.
A void parameter indicates no argument.
Chapter 7
The main() Function
24
The main() Function
Function body:
 Declarations
 Executable statements
Example:
int main (void)
{ int a, b, c;
b = 5; c = 3;
a = b + c;
return 0;
}
Chapter 7
declarations
statements
The main() Function
25
The main() Function
Declarations are statements that declare local
variables used in the function.
Compiler allocate memory space for all
variables.
Chapter 7
The main() Function
26
Keywords
Keywords are reserved words in C language
with special meanings, and cannot be redefined.
auto
break
case
char
const
continue
Chapter 7
default
do
double
else
enum
exterm
float
for
goto
if
int
long
register
return
short
signed
Keywords
sizeof
static
struct
switch
typedef
union
unsigned
void
volatile
while
27
Identifiers
Identifiers are names given to objects (variables
and functions).
Two types of identifiers: standard and userdefined.
Standard identifiers are predefined names, like
printf and scanf. They may be redefined, but
preferably not.
User-defined identifiers are created by
programmers to name variables and functions.
Chapter 7
Identifiers
28
Identifiers
Syntax for an identifier: consists of letters, digits
and underscore (_), but may not begin with digit.
Avoid naming an identifier starting with
underscore (_).
Choose appropriate identifiers for variable
names and function names. Compare these
identifiers for a particular variable:
sum_of_list_of_numbers
s
sum
Chapter 7
Identifiers
29
Identifiers
Identifiers are case sensitive.
By convention, identifiers for constant macros
(symbolic constants) are usually in upper-case
letters. Example: CM_PER_INCH
Chapter 7
Identifiers
30
Constants
Constants are often used in programs for
values that do not change. Examples:
perimeter = 2 * (length + breadth);
rem = count % 7;
printf ("Hello\n");
Types of constants: numeric constants,
character constants, and string constants.
Chapter 7
Constants
31
Constants
Numeric contants: integers (decimal, octal, and
hexadecimal) and floating-point numbers.
Examples:
123, -345, +12
0123, 05
0x2A, 0x88
1.234, -0.00987, 1.2e-4, -3.5E3
Chapter 7
Constants
32
Constants
Character contants: a single character enclosed
in single quotes. Examples:
'A', 'b', '^', '9', '+'
String contants: a sequence of characters
enclosed in double quotes. Examples:
"Hello\n", "ABC", "34.56"
A null character, \0, is appended at the end of
the string.
'K' and "K" are different.
Chapter 7
Constants
33
Variable Declarations
Variables must be declared before they can be
used.
Variables are declared in the declaration
statements in a function. The declaration of a
variable consists of its name and its type of
value.
float inches;
float cm;
or
float inches, cm;
Chapter 7
Variable Declarations
34
Data Types
A data type defines a set of values and a set of
operations permissible on those values.
Basic data types: int, float, double, char.
int: integer data type. Example of values of this
type: 123, -900, 987654321, +31. An integer
occupies a fixed number of bytes, so there is a
range of values that can be represented.
Chapter 7
Data Types
35
Data Types
float: floating-point data type. Example of
values of this type: 4.3, -123.00, +0.02, 1.2e4, 86e3, 3.9e-12.
A floating-point representation consists of
mantissa (fraction part) and exponent, with an
assumed base.
double: double-precision floating-point data
type. It uses twice the number of bits in the
mantissa, hence it may represent more precise
values.
Chapter 7
Data Types
36
Data Types
char: character data type. Example of values of
this type: 'a', 'P', '3', '#', ' '.
Chapter 7
Data Types
37
Programming Style
Indentation: to show the hierarchy of logic.
if (x < y) {
printf ("x is smaller than y\n");
if (y < z)
printf ("y is smaller than z\n");
else
printf ("y is larger than or equal to z\n");
}
else
printf ("x is larger than or equal to y\n");
Chapter 7
Programming Style
38
Programming Style
Blank lines: to separate functions, or logical
blocks.
Naming of identifiers.
int main (void)
{
float inches,
cm;
/* length in inches
length in centimetres */
/* get the length in inches */
printf ("Enter a length in inches: ");
scanf ("%f", &inches);
/* convert to length in centimetres */
cm = CM_PER_INCH * inches;
/* display the length in centimetres */
printf ("That equals %.2f centimetres.\n\n", cm);
return 0; /* indicate program ends successfully */
}
Chapter 7
Programming Style
39
Data Validation
Examine this run:
garfield@decunx:~/c[xx]: inch2cm
Enter a length in inches: –2
That equals –5.08 centimetres.
Should -2 be accepted?
Data validation: to validate input data and take
necessary actions.
A rugged program should not terminate abruptly
under all circumstances -- ultimate aim.
Chapter 7
Data Validation
40
Data Validation
But this is too ambitious. An easy way out is to
make assumptions, and document them in the
program as pre-conditions.
Compromise: include simple data validation
checks, whenever possible.
Chapter 7
Data Validation
41
Variables and Memory
A variable has name, type and value.
Each variable is allocated some memory
location(s) to hold its value. Such allocation
occurs at the variable declaration statements.
Every memory location has an address.
However, programmer only needs to refer to the
variable by its name, and leaves it to the system
to work out the address.
Chapter 7
Variables and Memory
42
Variables and Memory
Example: float inches, cm;
Address
Chapter 7
Memory
1000
1001
1002
:
:
Program
1500
inches
1508
cm
Variables and Memory
inch2cm
43
Variables and Memory
Initially, the value of a variable is undefined. A
variable must be initilialised before its value is
used.
3 ways to assign a value into a variable
 Initialising the variable at declaration
 Use the assignment statement
 Use the input function
Chapter 7
Variables and Memory
44
Variables and Memory
Initialisation at declaration:
float inches = 3.2;
Using assignment statement:
inches = 3.2;
Using an input function:
scanf ("%f", &inches);
1500 (inches)
?
before
Chapter 7
Variables and Memory
User
enters
3.2
1500 (inches)
3.2
after
45
Variables and Memory
Destructive write, non-destructive read.
before
after
int a, b;
a = 40;
b = 50;
b = a + b;
Chapter 7
2340 (a)
2340 (a)
?
40
2344 (b)
2344 (b)
?
50
2340 (a)
2340 (a)
40
40
2344 (b)
2344 (b)
50
90
Variables and Memory
46
Homework
Try exercises behind chapter 7.
Chapter 7
Homework
47