1. User Interaction and Variables
Tarik Booker
CS 242
California State University, Los Angeles

2. What we will cover…
Review
Data Types
Standard Input

3. Variables Can store values to be used (later) in a program
Values can be changed
Remember algebra:
x + y = 34
y = 5
x =?
Similar Concept in Programming

4. Variables (2)
You must declare a variable in a program before you use it
Format:
datatype variableName;
A variable can be of any name
Follow identifier rules (later…)
Descriptive names are best

5. Variables (3) Datatype variableName; Datatype
Datatype variableName1, variableName2, …,variableName3;
Datatype
Specific classification of variables
Different types / different memory sizes
int, double (main ones)
Discuss later

6. Variables (4) Variables are Case-Sensitive in C
Uppercase and Lowercase
Not the Same
“Variable1” is not the same as “variable1”

7. Identifiers To name variables or functions Called identifiers
Should obey rules:
Consists of letters (upper/lower), digits, underscores
Must start with a letter (alphabetic character)
Must NOT start with a digit!!!
Cannot be a reserved word (int, return, main, etc.)
Can be up to 31 characters

8. Identifiers (2) Which are legal? helloworld
aadzxvcna thjke456hftg3sd
HappyHappy
34joy56
$Dollarsand
return
mains
#love
Why / Why not?

9. Char
There is another type of variable other than integers or floating-point
Char
Allows you to store a single character
Uses ASCII
American Standard Code for Information Interchange
Each character is assigned to a particular number
ASCII Table
‘A’ is 65
‘a’ is 97
Lowercase is higher than Uppercase
Add 32 to change an uppercase letter to lowercase
Subtract 32 to change a lowercase letter to uppercase

10. ASCII Table

11. Reading in a Value from Standard In
Before, we used a function to send information to Standard out
What was it?
Now we can get information from Standard in
Let’s get a character from standard in
Use the scanf() function

12. Scanf A way to input data into the computer is to use scanf
This can “scan” in characters from the console
Format similar to printf
int x;
scanf(“%d”, &x);
This puts the value of %d into x
Different types for % (called format specifiers)
%d = decimal (integer)
%c = character
We will talk about others later…

13. Scanf (2) Note the & sign before the x int x; scanf(“%d”, &x);
This is a pointer reference
Outside the scope of this class
Passes to scanf the memory address of the variable x
Keep the same format – don’t worry about pointers

14. Data Types Char is considered a data type
A data type is just the way information is stored in the program
char, int, float, double, void (will discuss) are called primitive data types or primitives
Other types of primitive data types

15. Data Types Char Character Int Integer
Double Double Precision Floating-Point
Void no type (or empty)
Void
Use this data type when no type is expected
Primarily used for functions to specify not a return type
Look at main

16. Data Types (2)
C has many many different types of basic (or primitive) data types
Different versions of char, int, long, double
Char
Signed char uses signed version of char
Unsigned char uses unsigned version of char

17. Data Types (3) Short 16-bit integer type Short int Signed short
Signed short int
Unsigned short
Unsigned short int

18. Data types (4) Int Signed int Unsigned Unsigned int
Long long integer (at least 32 bits)
Long int
Signed long
Signed long int
Unsigned long
Unsigned long int

19. Data types (5) Long long 64-bit integer Long long int Signed long long
Signed long long int

20. Data Types (6) Float Double Long double There are also other types:
Boolean (next week)
True/False

21. We will do in class work today!
Let’s do some problems!