1. Doing Arithmetic Today’s lecture is in chapter 2 Assignment statement:
double area, radius;
area = 3.14*radius*radius;
operator
expression
assignment
statement
Expression:
anything that has a value
e.g. radius, 3.14
or a combination e.g.
3.14*radius*radius
The value of an expression depends on
_ the data: operands radius,
_ the operators: *, + ...
142 C-1

2. Processing an assignment statement
Consider: area = 3.14*radius*radius; 1
Evaluate the right hand side (=expression)
3.14*radius*radius 2
The value is stored in the left hand side,
the assignment variable
area
depends on:
types in the right hand side
operators in the right hand side
type of the left hand side
Note: can have
year = year + 1;
(NOT a mathematical equation!)
142 C-2

3. Expressions with doubles
Reminder:
double = type for a floating point variable
e.g. 2.15, -1.28e+28, NOT 3 or -4
Operators on doubles
unary: -
acts on one piece of data
-23.4
-temperature
binary: +, -, /, *
acts on two pieces of data
1.609 * miles
add
subtract
divide
multiply
All operators in C are unary or binary
If so, what happens in
3.14*radius*radius ?
142 C-3

4. Expressions with ints Reminder: int = type for an integer variable
e.g. 3, -216, NOT 3.0
Operators on ints
unary: -
(same as for doubles)
binary: +, -, *,
add
subtract
multiply
as usual
integer
division
modulus
/, %
BUT
/ is the integer division
2/3 is 0, 3/2 is 1
% is the modulus operator:
a%b is the remainder of the division of a by b
114%50 is
2%3 is 14 2
142 C-4

5. Examples convert inches to feet and inches e.g. 74’’ = 6’ 2’’
int total_inches, feet, inches;
total_inches = 74;
feet = total_inches/12;
inches = total_inches%12;
Beware:
double half_distance, distance;
half_distance = 1/2 * distance;
always 0!
Why using ints and not always doubles?
int sometimes makes more sense
int number_of_children;
computations with ints are faster
doubles may be inaccurate:
mathematically = 2 not 3 1
142 C-5

6. Operator Precedence What is a + b*c ? Is it (a+b)*c or a+(b*c) ?
precedence rules:
1. evaluate expressions in parentheses:
start with the innermost set of parentheses
2. do unary -
3. do *, /, %
4. do binary +, -
a + b*c is a + (b*c)
When coding, do not hesitate to use
parentheses
avoid errors and clearer!
142C-6

7. Associativity What is a*b/c? Is it a*(b/c) or (a*b)/c ?
C binary arithmetic operators associate left to
right within the same precedence level
a*b/c is (a*b)/c
(but,
unary - is right associative: write -2 not 2-)
Example
Add parentheses to the following expression
to show how it is evaluated:
a + b - c + d
Ans: ((a+b) - c)+d
There are also C operators that associate
right to left
e.g. the assignment operator =
(see table on the inside cover of the text)
142C-7

8. What happens in expressions with
ints and doubles
When adding an int and a double,
the compiler automatically converts the
int to a double
int + double
double + double
(also with -, * and /)
Beware: it can be tricky!
2*3*6.2
6*6.2
6.0*6.2
37.2
conversion occurs here
NOT before
Thus:
2/3*6.2
0*6.2
0.0*
AVOID MIXED TYPES
142C-8

9. Conversions in assignments
int total, count;
double avg;
total = 96;
count = 10;
avg = total/count;
What is the value of avg?
avg = total/count;
implicit conversion
of int to double
avg is 9.0
DO NOT DO double TO int
may not do what you expect!
int val1;
double val2;
val2 = 4.0e+28;
val1 = val2; /* BAD */
val1 is not 40….0
28 zeros
142C-9

10. Explicit conversions To perform a conversion explicitly, use a cast
(type) expression
e.g. (double) number_of_children
declared as an int
Example
int total, count;
double avg1, avg2, avg3, avg4;
total = 96; count = 10;
avg1 = total/count;
avg2 = (double)total/(double)count;
avg3 = (double)(total/count);
avg4 = (double)total/count;
printf(“%.1f, %.1f, %.1f, %.1f”,
avg1, avg2, avg3, avg4);
9.0, 9.6, 9.0, 9.6
142C-10

11. A few words of advice know the type of the variables in your program
C cares about types
… so should you!
There are lots of cases where types have to
match up (e.g. in functions as we will see)
Style counts !!!
_ Be clear
_ KIS: Keep It Simple
(don’t write huge expressions,
break them up)
_ use parentheses and casts
142C-11