1. 16.216 ECE Application Programming
Instructor: Dr. Michael Geiger
Fall 2014
Lecture 3:
Variables
Operators
Basic variable output with printf()

2. ECE Application Programming: Lecture 3
Lecture outline
Announcements/reminders
Program 1 due today
Program 2 posted; due 9/17
Sign up for the course discussion group
Review
Comments
Data types
Today’s lecture
Constants
Variables
Operators
Basic variable output with printf()
8/8/2018
ECE Application Programming: Lecture 3

3. ECE Application Programming: Lecture 3
Review
Comments
Single-line: // This is a comment
Multi-line:
/* This is also
a comment */
Data types
Define size & format of data
Four basic types: int, float, double, char
8/8/2018
ECE Application Programming: Lecture 3

4. Using #define with constants
Often makes sense to give constant value a symbolic name for readability
Don’t want constant wasting memory space
Use #define to define a macro
Macro: named code fragment; when name is used, the preprocessor replaces name with code
Syntax: #define <name> <code>
Common use: defining constants
By convention, start constant values with capital letters
e.g. #define NumberOne 1
At compile time, all uses of “NumberOne” in your program are replaced with “1” and then compiled
8/8/2018
ECE Application Programming: Lecture 2

5. ECE Application Programming: Lecture 2
Variables
All variables have four characteristics:
A type
An address (in memory)
A value
A name
8/8/2018
ECE Application Programming: Lecture 2

6. ECE Application Programming: Lecture 2
Variables - name
must start with a-z, A-Z ( _ allowed, but not recommended)
other characters may be a-z, A-Z, 0-9, _
upper case/lower case are not equal (i.e. ECE, ece, Ece, EcE, eCe would be five different variables)
max length system dependent (usually at least 32)
By convention
Start with lowercase letter
Descriptive names improve code readability
8/8/2018
ECE Application Programming: Lecture 2

7. Variables - legal names
grossPay
carpet_Price
cArPeT_price
a_very_long_variable_name i
______strange___one_____
_ (not recommended)
8/8/2018
ECE Application Programming: Lecture 2

8. Variables - legal names (but not recommended)
l (that's lower case L)
O (that's capital O)
l1 (that's lower case L, and digit one)
O0Oll11 (oh,zero,oh,el,el,one,one)
_var (many system variables begin w/ _ )
8/8/2018
ECE Application Programming: Lecture 2

9. ECE Application Programming: Lecture 2
Variables - declaring
var name
memory loc
main() { float hours, payrate; float grosspay; int j;
hours ?
4278
payrate ?
427C
4280
grosspay ? j ?
4284
All variable declarations should be grouped together at the start of the function
8/8/2018
ECE Application Programming: Lecture 2

10. ECE Application Programming: Lecture 2
Variables - assigning
varname = expression;
Declared variable
single variable on left side of =
expression
any legal expression
Expression can be constant, variable, function call, arithmetic operation, etc.
Variable type (int, float, etc) and expression result type should match
If not, funny things can happen ...
8/8/2018
ECE Application Programming: Lecture 2

11. ECE Application Programming: Lecture 2
Variables (cont.)
var name
memory loc
main() { float hours, payrate; float grosspay; int j;
hours = 40.0;
hours
40.0
4278
payrate ?
427C
4280
grosspay ? j ?
4284
8/8/2018
ECE Application Programming: Lecture 2

12. ECE Application Programming: Lecture 2
Variables (cont.)
var name
memory loc
main() { float hours, payrate; float grosspay; int j;
hours = 40.0;
payrate = 20.00;
hours
40.0
4278
payrate
20.0
427C
4280
grosspay ? j ?
4284
8/8/2018
ECE Application Programming: Lecture 2

13. ECE Application Programming: Lecture 2
Variables (cont.)
var name
memory loc
main() { float hours, payrate; float grosspay; int j;
hours = 40.0;
payrate = 20.00;
grosspay = hours * payrate;
hours
40.0
4278
payrate
20.0
427C
4280
grosspay
800.00 j ?
4284
note: referencing a variable only "reads" it (non-destructive). Assigning to a variable overwrites whatever was there (destructive).
8/8/2018
ECE Application Programming: Lecture 2

14. ECE Application Programming: Lecture 2
Variables (cont.)
var name
memory loc
main() { float hours, payrate; float grosspay; int j;
hours = 40.0;
payrate = 20.00;
grosspay = hours * payrate;
j = 5;
hours
40.0
4278
payrate
20.0
427C
4280
grosspay
800.00 j 5
4284
note: referencing a variable only "reads" it (non-destructive). Assigning to a variable overwrites whatever was there (destructive).
8/8/2018
ECE Application Programming: Lecture 2

15. ECE 160 - Introduction to Computer Engineering I
02/09/2005
Variables (cont.)
var name
memory loc
main() { float hours, payrate; float grosspay; int j;
hours = 40.0;
payrate = 20.00;
grosspay = hours * payrate
j = 5;
j = j + 1;
hours
40.0
4278
payrate
20.0
427C
4280
grosspay
800.00 j
5 6
4284
note: referencing a variable only "reads" it (non-destructive). Assigning to a variable overwrites whatever was there (destructive).
8/8/2018
ECE Application Programming: Lecture 2
(c) 2005, P. H. Viall

16. ECE Application Programming: Lecture 3
Example: Variables
What values do w, x, y, and z have at the end of this program?
int main() {
int w = 5;
float x;
double y;
char z = ‘a’;
x = 8.579;
y = -0.2;
w = x;
y = y + 3;
z = w – 5;
return 0; }
8/8/2018
ECE Application Programming: Lecture 3

17. ECE Application Programming: Lecture 3
Example solution
int main() {
int w = 5;
float x;
double y;
char z = ‘a’;
x = 8.579;
y = -0.2;
w = x;
y = y + 3;
z = w – 5;
return 0; }
w = 5
z = ‘a’ (ASCII value 97)
x = 8.579
y = -0.2
w = 8 (value is truncated)
y = (-0.2) + 3 = 2.8
z = 8 – 5 = 3
8/8/2018
ECE Application Programming: Lecture 3

18. ECE Application Programming: Lecture 3
Final notes
Next time
Operators
Output using printf()
Reminders:
Sign up for the course discussion group on Piazza!
Program 1 due today
Program 2 posted; due 9/17
8/8/2018
ECE Application Programming: Lecture 3