Variable< Arithmetic Expressions and Input

Chapter 2
Variables, Arithmetic Expressions and
Input/Output
C Programming
a Q & A Approach
by H.H. Tan, T.B. D’Orazio, S.H. Or & Marian M.Y. Choy

2
2.1 Variables: Naming, Declaring,
Assigning and Printing Values
Question:
Calculate the area of 10,000 triangles, all of different
sizes. Suppose you have the following information:
1. the length of each of the three sides
2. the size of each three angles
Solutions:
1. representing the information with variables
2. write down the correct formula to calculate the result



1
l
2
l
3
l
1 2
* *sin( ) / 2.0
S l l 

In Algebraic

3
 For programming in C
 choose the variable names,
 consist of entire words rather than single
characters
 easier to understand your programs if given
very descriptive names to each variable

4
 We may use variable names
 Lengths: length1, length2, length3
 Angles: angle1, angle2, angle3
 Much less ambiguous than their algebraic
counterparts
 Expressions look more cumbersome
 length of expression may span over 1 line
 Disadvantage which we simply must live with
 Try your best to make the name descriptive – or
make sense

5
 What can be chosen for variable names in C?
first character must be non-digit characters a–z,
A–Z, or _
other characters must be non-digit characters
a–z, A–Z, _, or digit 0–9
 Valid examples
 apple1 interest_rate xfloat Income one_two
 Invalid
 1apple interest_rate% float In come one.two

6
 Can we use void, int, printf as a variable names?
 List of key words in C
 auto break case char const continue default
 do double else enum extern float for
 goto if int long register return short
 signed sizeof static struct switch typedef union
 unsigned void volatile while
 You could choose key words as variable names, but are strongly
suggested not to do it.
NO

7
Table 2.1
Some Constraints on Identifiers
Topic
Use of standard identifiers such
as printf
Use of uppercase or mixed-case
Comment
Standard identifiers, such as
the function name printf, can
be used as variable names.
However, their use is not
recommended because it
leads to confusion.
Allowed; however, many
programmers use lowercase
characters for variable names
and uppercase for constant
names. Differentiate your
identifiers by using different
characters rather than
different cases

8
 In C statement, we might have
 length1 = 3.0;
 length2 = 8.0;
 angle1 = 30.0;
 length1 = 30.0;
 length2 = 80.0;
 angle1 = 60.0;
1* 2*sin( 1)/2.0;
Area length length angle

length1 length2 angle1 Area

9
 But,
 Variable names must be declared before
 To define a variable, the syntaxes are:
data_type variable_name;
data_type variable_name1, variable_name2, … ;
 e.g:
int iex;
float length;
float angle1, angle2;
double length1, length2;

In C, data type categorized as:
1. Primitive Types in ANSI C (C89)/ISO C (C90) -
char, short, int, float and double.
2. Primitive Types added to ISO C (C99) - long
long
3. User Defined Types – struct, union, enum
and typedef (will be discussed in separate session).
4. Derived Types – pointer, array (will be discussed in
separate session).
C BASIC DATA TYPES
2/31

14
 Assignment operator =
 the syntax is:
variabel_name1 = value;
variable_name2 = expression;
or
variable_name1 = value, variable_name2 = value2;
 e.g:
int iex;
float length;
float angle1, angle2;
iex= 12;
angle1 = 30.0;
angle2 = angle1+30.0;

15

16
 Notes on Assignment operator =
 put the value on the right hand size of
assignment operator to the variable name on
the left;
 the precedence of operation order is from right
to left;
 the left side of “=“ must be a variable
 expressions on the right hand has to be
evaluated before assignment
 Assignment can be done with declaration
 float income=20.5, expense=8.5;
 float saving= income - expense;

17
How to print out variable area during execution?

18
How to print out variable value during execution?
printf (format_string, argument_list)
format_string:
plane_character , conversion_specification
print out directly how to display
To display integer: %[field width]d e.g. %5d
To display float: %[field width][.precision]f e.g %9.2f
argument_list:
variables/constant to be fed

19
int month;
float expense, income;
month = 12;
expense = 111.1;
income = 100.;
printf ("Month=%2d, Expense=$%9.2fn",
month, expense);
Result

20

21
int month;
float expense, income;
month = 11;
expense = 82.1;
income = 100.;
printf ("For the %2dth month of the yearn"
"the expenses were $%5.2f n"
"and the income was $%6.2fnn",
month, expense, income);
Result?

22
printf
 To display the value of a variable or constant on the screen
printf(format_string,argument_list);
 format_string
 plain characters – displayed directly unchanged on the screen, e.g. “This is C”
 conversion specification(s) – used to convert, format and display argument(s)
from the argument_list
 escape sequences – control the cursor, for example, the newline ‘n’
 Each argument must have a format specification. For example,
printf("month=%5d ",month);

23
printf
 Each argument must have a format specification. For example,
printf("month=%5d n",month);
 What will you get when:
int people = 456356;
printf(“people =%5dn",people);
printf(“people =%9dn”,people);
Question:
How is the case when we deal float numbers?

24
printf
Problem in Engineering:
float people= 0.000001;
 What will you get when:
printf(“people =%fn",people);
 Using scientific notation
printf(“people =%en", people);

28
 We can instruct the computer to retrieve
data from various input devices
 the keyboard
 a mouse
 the hard disk drive
 Programs that have input from the keyboard
usually create a dialogue between the
program and the user during execution
2.2 Reading Data from The Keyboard

29
 This is done by printing out an hint
message,e.g.,
printf(“please input the current month (int): ”)
scanf(“%d”,&month);

30
 Input data from keyboard can be done using scanf() function
 scanf(format_string, argument_list)
format_string : converts input characters into a specified type
argument_list: addresses of variables in which the input data are
stored;
 Example
float income;
double expense;
scanf(“%f %lf”,&income, &expense);
 & is required to get the address of variable

31
 scanf() function
scanf("%f%lf", &income, &expense);
 &income stands for the address of the memory cell for income
 & : “address of” operator
 &income - pass the address of variable income to function scanf
 By giving scanf the address, the program knows where in memory to put
the value typed

32
printf("what was your month salary in 2015?n");
scanf("%f", &salary);
printf("How many months have you worked?n");
scanf("%d",&work_month);
printf("what did you earn on vocation month?n");
scanf(" %f",&vocation_salary);
printf("what was month expense in 2015?n");
scanf("%f",&expense);
2.2Reading Data from The Keyboard
keyboard input
keyboard input
keyboard input
keyboard input
dialogue

33
float income;
double expense;
int month, hour, minute;
printf ("What month is it?n");
scanf ("%d", &month);
printf ("You have entered month=%5dn",month);

34
 scanf() function
scanf (format_string, argument_list);
 format_string converts characters in the input into
values of a specific type

2.2 Reading Data from The
Keyboard
 argument_list contains the address of the variable(s)
into which the input data are stored. e.g.
scanf("%f%lf",&income,&expense);
 1st
keyboard input data converted to float (%f) =>
income
 2nd
keyboard input converted to double (%lf) =>
expense
35

printf ("Please enter your income and expensesn");
scanf ("%f%lf", &income,&expense);
printf ("Please enter the time, e.g., ");
scanf ("%d : %d", &hour,&minute);
printf ("Entered Time=%d:%dn",hour,minute);
36

Input data for q2 in
homework3
40

41
2.3 Arithmetic Operators
and Expressions
 Consists of a sequence of operand(s) and
operator(s) that specify the computation of a
value
 Look much like algebraic expressions that you
write
d = x/y; // x divided by y
 Assigns the value of the arithmetic
expression(division) on the right to the variable
on the left
 Is correct to write
x/y =d ; ?
Wrong

42
and Expressions
 Calculate the time needed for a car to drive
from A to B with constant speed v
 Calculate the how many groups we have in
the class if each group has 4 students?

44
Example 1
int i,j,k,p,m,n;
float a,b,c,d,e,f,g,h,x,y;
i=5; j=5;
k=11; p=3;
x=3.0; y=4.0;
printf("...... Initial values ......n");
printf("i=%4d, j=%4dnk=%4d, p=%4dnx=%4.2f, y=
%4.2fnn", i,j,k,p,x,y);

45
Example 1
a=x+y;
b=x-y;
c=x*y;
d=x/y;
e=d+3.0;
f=d+3;
i=i+1;
j=j+1;
printf("...... Section 1 output ......n");
printf("a=%5.2f, b=%5.2fnc=%5.2f, d=%5.2fn"
"e=%5.2f, f=%5.2fni=%5d, j=%5d nn",
a,b, c,d, e,f, i,j);

46
Example 1
m=k%p;
n=p%k;
i++;
++j;
e--;
--f;
printf("...... Section 2 output ......n");
printf("m=%4d, n=%4dni=%4d, j=%4dn"
"e=%4.2f, f=%4.2fn",m,n, i,j, e,f);

47
2.3 Arithmetic Operators and
Expressions
Operators ++, --, and %
 ++ increment operator, can be placed before or after a
variable
 increase the value of the variable by 1
 i++; or ++i;
equivalent to i=i+1;
 i--; or --i;
same as i=i-1;
 Only difference between i++ and ++i is in order of increment
(addressed later)
 % is a remainder operator
 must be placed between two integer variables or constants
 e.g. 11%3 return 2

48
and Expressions
 Depending on data types, division get can you
different answers
int i=3, j = 5, k;
float x = 3.0, y=5.0,z;
k = i/j;
z = x/y;
 remainder operator only accept integer
operands
k = i%j; right
z = x%y; wrong

49
and Expressions
 Cannot write
x/y = d;
i + 1 = i;
 Left side of assignment statement can have
only single variable
 Single variables are allowed to be lvalues
(allowed to be on the left side of assignment
)
 Expressions are rvalues (allowed on the
right side of assignment )

50
Conversion Specification
 If the precision specified for a real is
 less than actual, displays only the number of
digits in the specified precision (trailing digits are
not lost from memory, they simply are not
displayed)
 greater than actual, adds trailing zeros to make
the displayed precision equal to the precision
specified
 not specified, makes the precision equal to six

51
int DAYS_IN_YEAR = 365;
//One int value displayed in different field width
printf ("CONVERSION SPECIFICATIONS FOR INTEGERS n
n");
printf ("Days in year = n"
"[[%1d]] t(field width less than actual)n"
"[[%9d]] t(field width greater than actual)
n"
"[[%d]] t(no field width specified) nnn",
DAYS_IN_YEAR, DAYS_IN_YEAR, DAYS_IN_YEAR);

52
float PI=3.14159;
//One float value displayed in different field width
printf ("CONVERSION SPECIFICATIONS FOR REAL NUMBERSnn");
printf ("Cases for precision being specified correctly
n");
printf ("PI = n"
"[[%1.5f]] tt(field width less than actual) n"
"[[%15.5f]] t(field width greater than actual)n"
"[[%.5f]] tt(no field width specified) nn",
PI,PI,PI);

53
float PI=3.14159;
//One float value displayed in different precision
printf ("Cases for field width being specified
correctly n");
printf ("PI = n"
"[[%7.2f]] tt(precision less than actual) n"
"[[%7.8f]] tt(precision greater than actual)n"
"[[%7.f]] tt(no precision specified) nn",
PI,PI,PI);

54
Conversion Specification
 Display int with %f or float with %d?
likely get nonsensical values or zeros
displayed
common error beginners make
 Why was so much attention given to the
printf statement?
will write printf statements very frequently
can substantially reduce your programming
errors

55
Constant
 constant can be done using constant macro
# define PI 3.1415926
 it is a preprocessing directive, it replace the occurrence of macro name (PI) by its value (3.1415926)
 it is not a C statement, no “;” at the end
 it is usually appear after preprocessing directives
E.g.:
# include <stdio.h>
# define DAY_OF_WEEK 7
void main(void)
{
printf(“total =%dn”,DAY_OF_WEEK);
}

56
Preprocessor
 To create a constant macro
 use a preprocessor directive
 begin with the symbol # (which must begin the line)
 semicolon must not be used at the end
 General form
#define symbolic_name replacement
 #define DAYS_IN_YEAR 365
 Draw equivalent of DAYS_IN_YEAR to 365
 Prior to translation into machine code, the preprocessor
replaces every symbolic_name in the program with the
given replacement
 printf("Days in year=%5d
n",DAYS_IN_YEAR);
after preprocessing becomes
printf("Days in year=%5dn",365);

57
2.4 Mixed Type Arithmetic,
 Giving variables their first numerical values is called
initializing them, e.g.
x = 5;
 Usually complicated expression is placed
 Order of performance of numerical operations can be
controlled
 Rules about the order of operation of +, -, *, / are
established by setting the precedence of the
operators
 Operators of higher precedence are executed first
while those of lower precedence are executed later

58
Operator Name No of operands Position Associativity Precedence
( parentheses unary prefix L to R 1
) parentheses unary postfix L to R 1
+ positive sign unary prefix R to L 2
- negative sign unary prefix R to L 2
++ post-increment unary postfix L to R 2
-- post-decrement unary postfix L to R 2
++ pre-increment unary prefix R to L 2
-- pre-decrement unary prefix R to L 2
+= addition and assignment binary infix R to L 2
-= subtraction and
assignment
binary infix R to L 2
*= multiplication and
assignment
binary infix R to L 2
/= division and assignment binary infix R to L 2
%= remainder and assignment binary infix R to L 2
% remainder binary infix L to R 3
* multiplication binary infix L to R 3
/ division binary infix L to R 3
+ addition binary infix L to R 4
- subtraction binary infix L to R 4
= assignment binary infix R to L 5

*=
Multiply the value of the first operand by the value of the second
operand; store the result in the object specified by the first operand.
A*=B; // A=A*B;
/=
Divide the value of the first operand by the value of the second
A/=B; // A=A/B;
%=
Take modulus of the first operand specified by the value of the
second operand; store the result in the object specified by the first
operand.
A%=B; // A=A%B;
+=
Add the value of the second operand to the value of the first
A+=B; // A=A+B;
–=
Subtract the value of the second operand from the value of the first
A-=B; // A=A-B;
59
Compound Assignment

60
Variable Initialization/Increment
Operator
 How do we initialize variables?
1. Uses an assignment statement, e.g.
int e ; e=3;
2. Initializes in a declaration statement, e.g.
float a=7, b=6;
– int i = 1, j =1;
int k = i++;
– int h = ++j ;
– For 1st
expression,
1. value of i is first assigned to k
2. i is then incremented (post-increment ++) from 1 to 2
– For 2nd
one,
1. value of j is first incremented (pre-increment operator ++) from
1 to 2.
2. Then the new j value, now equal to 2, is assigned to h
?i,j,k,h

61
Mixed Data Type
Calculation
 6.0/4.0 = 1.5
 6/4 = 1
 6/4.0 = 1.5
C converts integer to
real temporarily and
perform the operation

62
cast Operators
 Change the type of an expression
temporarily
 General form
(type) expression
 int aa=5, bb=2;
float xx;
xx = (float) aa / bb;
 xx = 2.5, otherwise xx = 2.0
Force a floating
Point division

63
Controlling Precedence
 Arithmetic operators located within the
parentheses() always have highest
precedence
 Example
z = ((a+b)*c/d);
 a+b evaluated first

64
Associativity
 Specifies the direction of evaluation of
the operators with the same
precedence
1+2 -3
First evaluated
Next evaluated
Direction of evaluation

65
Further Exploration
 Assign an integer value to a float variable
 convert the result to float and store to variable
 e.g.,
float p = 6 / 4;
 floating variable p will store 1.0

66
Summary
 #define symbolic_name replacement
 where symbolic_name occurred throughout the rest of
program will be replaced by replacement during compilation
by preprocessor
 Format specifications
 %[flag][field width][.precision]type
 where format string components enclosed by [ ] are optional
 Output of a float number to scientific notation is
 [sign]d.ddd e[sign]ddd
 where d represents a digit

67
Summary
 During mixed mode (different data types) arithmetic, C
will automatically convert one of the operands to facilitate
calculations
 Sometimes type casting can be used to explicitly change
data type during calculation
 (type) operand
 where type is the target type being converted into
operand is the data to be casted

68
Summary
 Basic structure of a program
# include<stdio.h>
void main()
{
statements;
}
 Constant with preprocessing
 using #define CONT_NAME value
 # define DAY_OF_YEAR 365
 Define variables
 data_type variable_name ;
 int number; float salary; double rate;
Indentation

69
Summary
 Output data on to screen
 printf(format_string, argument_lists)
 printf("You input %dn", month);
 Get data from keyboard
 scanf(format_string, address_variables)
 scanf("%d, %f", &month, &salary);
 Mathematical Operators
 +, -, *, /, %, ++, --, +=,-=, *=, …
 binary operators require its operands of same type
 % works on integers
 / depends on operands

70
Home work 4
 Calculate the average score of a students within
one semester. Suppose there are 3 courses in
total.
 Procedures
• Question description(not code in program):
• average_score = (score1+score2+score3)/3
• Steps of programming:
• define variables with right data type
• ask user to input 3 scores from keyboard
• convert the math expression into C code
• Output the results (keep 2 decimal places)

Variable< Arithmetic Expressions and Input

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