expressions, Data, and variables
1
How the computer sees the world
Internally, the computer stores everything in terms of 1s and 0s
Example: h 0110100 "hi" 01101000110101 104 0110100
How can the computer tell the difference between an h and 104?
2
Data types
type: A category of data values
Example: integer, real number, character
C has many types (below is only a partial list)
integers: char, short, int, long real numbers: float, double
3
Data types
Though the book uses many different types in the example programs, we will only concern ourselves with int and double for now. Name int double Description integers real numbers Examples 42, -3, 0, 926394 3.4, -2.53, 91.4e3
Numbers with a decimal point are treated as real numbers. Question: Isnt every integer a real number? Why bother?
4
Integer or real number?
Which category is more appropriate?
integer (int)
real number (double)
1. 2. 3. 4. 5. 6.
Temperature in degrees Celsius The population of lemmings Your grade point average A person's age in years A person's weight in kilograms A person's height in meters
7. Number of kilometers traveled 8. Number of dry days in the past month 9. Your locker number 10. Number of seconds left in a game 11. The sum of a group of integers 12. The average of a group of integers
credit: Kate Deibel, http://www.cs.washington.edu/homes/deibel/CATs/
5
Manipulating data via expressions
expression: A data value or a set of operations that produces a value.
Examples:
1 + 4 * 3 3 "CSE142" (1 + 2) % 3 * 4
6
The operators
Arithmetic operators we will use:
+ * / %
addition subtraction or negation multiplication division modulus, a.k.a. remainder
7
Evaluating expressions
When C executes a program and encounters an expression, the expression is evaluated (i.e., computed).
Example:
3 * 4 evaluates to 12
8
Evaluating expressions: Integer division
When dividing integers, the result is also an integer. Example: 14 / 4 evaluates to 3, not 3.5 (truncate the number)
3 4 ) 14 12 2
Examples:
52 27 ) 1425 135 75 54 21
1425 / 27 is 52 35 / 5 is 7 84 / 10 is 8 156 / 100 is 1 24 / 0 is illegal
9
Evaluating expressions: The modulus (%)
The modulus computes the remainder from a division of integers.
Example: 14 % 4 is 2 1425 % 27 is 21
52 27 ) 1425 135 75 54 21
3 4 ) 14 12 2
What are the results of the following expressions?
45 % 6 2 % 2 8 % 20 11 % 0
10
Applying the modulus
What expression obtains
the last digit (units place) of a number? Example: From 230857, obtain the 7. the last 4 digits of a Social Security Number? Example: From 658236489, obtain 6489. the second-to-last digit (tens place) of a number? Example: From 7342, obtain the 4.
11
Applying the modulus
How can we use the % operator to determine whether a number is odd? How about if a number is divisible by, say, 27?
12
Precedence
precedence: Order in which operations are computed in an expression. Operators on the same level are evaluated from left to right. Example: 1 - 2 + 3 is 2 (not -4) Spacing does not affect order of evaluation. Example: 1+3 * 4-2 is 11
Parentheses Multiplication, Division, Mod Addition, Subtraction
() * / + -
%
13
Precedence examples
1 * 2 + 3 * 5 / 4 \_/ | 2 + 3 * 5 / 4 \_/ | 2 + 15 / 4 \___/ | 2 + 3 \________/ | 5
1 + 2 / 3 * 5 - 4 \_/ | 1 + 0 * 5 - 4 \___/ | 1 + 0 - 4 \______/ | 1 - 4 \_________/ | -3
14
Precedence exercise
Evaluate the following expressions:
9 / 5 695 % 20 7 + 6 * 5 7 * 6 + 5 248 % 100 6 * 3 - 9 (5 - 7) * 6 + (18 %
/ 5 / 4 4 (17 - 12))
Which parentheses are unnecessary?
15
Real numbers (double)
The operators also work with real numbers.
The division operator produces an exact answer.
Examples:
15.0 / 2.0 is 7.5 15.3 + 2.5 is 17.8 1.23 + 15.0 * 2.0 is 31.23
The same precedence rules apply.
16
Real numbers example
2.0 * 2.4 + 2.25 * 4.0 / 2.0 \___/ | 4.8 + 2.25 * 4.0 / 2.0 \___/ | 4.8 + 9.0 / 2.0 \_____/ | 4.8 + 4.5 \____________/ | 9.3
17
Mixing integers and real numbers
When an operator is used on an integer and a real number, the result is a real number.
Examples: 4.2 * 3 is 12.6 1 / 2.0 is 0.5
7 / 3 * 1.2 + 3 / 2 \_/ | 2 * 1.2 + 3 / 2 \___/ | 2.4 + 3 / 2 \_/ | 2.4 + 1 \________/ | 3.4
The conversion occurs on a per-operator basis. It affects only its two operands.
Notice how 3 / 2 is still 1 above, not 1.5.
18
Mixed types example
2.0 + 10 / 3 * 2.5 - 6 / 4 \___/ | 2.0 + 3 * 2.5 - 6 / 4 \_____/ | 2.0 + 7.5 - 6 / 4 \_/ | 2.0 + 7.5 1 \_________/ | 9.5 1 \______________/ | 8.5
19
Type casting
type cast: A conversion from one type to another.
Common uses: To promote an int into a double to achieve exact division. To truncate a double from a real number to an integer.
General syntax:
(<type>)<expression>
Examples:
(double)19 / 5 (int)3.8 // 3.8 // 3
20
Type casting
Type casting has high precedence and only casts the item immediately next to it.
(double)1 + 1 / 2 (double)1 / 2; // 1.0 // 0.5
You can use parentheses to force evaluation order.
(double)(7 + 3 + 4) / 3
A conversion to double can be achieved in other ways.
1.0 * (7 + 3 + 4) / 3
21
Printing numerical expressions
Syntax:
printf("%d", <integer expression>); printf("%lf", <double expression>);
Example:
printf("%d", 3 * 5 / 4); printf("%lf", 3.14);
22
More on printf
Can have multiple %d's and %lf's along with text in the string (called the format string) Example:
printf("%d and %lf are bigger than %d\n", 10, 3.14, 1);
Output:
10 and 3.140000 are bigger than 1
23
More on printf
Lets print more complicated messages with computed values.
printf("Your grade was %lf\n", ((95.1 + 71.9 + 82.6) / 3.0)); printf("There are %d students in the course.\n", (11 + 17 + 4 + 19 + 14));
24
What was the answer again?
Using the data from the last slide, what if we wanted to print the following?
Your grade was 83.2 Summary: Course grade: 83.2
Answer?
printf("Your grade was %lf\n", ((95.1 + 71.9 + 82.6) / 3.0))); printf("Summary:\n"); printf("Course grade: %lf\n", ((95.1 + 71.9 + 82.6) / 3.0)));
25
Variables
variable: A piece of your computer's memory that is given a name and type can and store a value.
Usage:
compute an expression's result store that result into a variable use that variable later in the program
Variables are a bit like preset stations on a car stereo:
26
Declaring variables
To use a variable, first it must be declared. Variable declaration syntax:
<type> <name>;
Convention: Variable identifiers are written in all lowercase with words separated by underscores. Examples:
int x; double my_gpa; int var_name;
27
Declaring variables
Declaring a variable sets aside a piece of memory in which you can store a value.
int x; int y;
Inside the computer: x: ? y: ?
(The variables' values are unknown.)
28
Setting variables
assignment statement: A C statement that stores a value into a variable.
Variables must be declared before they can be assigned a value.
Assignment statement syntax: <variable> = <expression>; Examples: x = 2 * 4; my_gpa = 3.25;
x: 8
my_gpa: 3.25
29
Setting variables
A variable can be assigned a value more than once. Example:
int x; x = 3; printf("%d\n", x); x = 4 + 7; printf("%d\n", x);
// 3 // 11
30
Using variables
Once a variable has been assigned a value, it can be used in any expression. int x; x = 2 * 4; printf("%d", x * 5 - 1);
The above has output equivalent to: printf("%d", 8 * 5 - 1);
What happens when a variable is used on both sides of an assignment statement? int x; x = 3; x = x + 2; // what happens?
31
C99
In 1999, there were some changes made to C that made it easier to use. The result was C99. Some changes:
Can comment with // Don't have to declare all variables at the very beginning
The textbook does not fully use C99 and thus says that you must declare all variables at the beginning.
Ignore thatwe will be using C99.
In Quincy, to enable C99, go to ToolsOptions and check "C99 Support for C Programs"
32
Errors in coding
It is a good idea to be familiar with different types of errors, so when your program does not compile or runs improperly, you will know how to fix it. Types of errors:
A compiler error is an error that the compiler will catch, though the error message may not always be helpful. A logic error (also known as a bug) is an error that the compiler will not catch. Your code will compile, but your program will not always run properly (it may work sometimes which makes beginning programmers believe that the computer is behaving randomly).
33
Errors in coding
COMPILER ERROR: Declaring two variables with the same name
Example:
int x; int x; // ERROR: x redeclared
LOGIC ERROR: Reading a variables value before it has been assigned
Example:
int x; printf("%d\n", x); // LOGIC ERROR: x value unknown
34
Assignment vs. algebra
The assignment statement is not an algebraic equation! <variable> = <expression>; means:
"store the value of <expression> into <variable>"
Some people read x = 3 * 4; as
"x gets the value of 3 * 4"
ERROR: 3 = 1 + 2; is an illegal statement, because 3 is not a variable.
35
Assignment and types
A variable should be assigned a value of its own type, otherwise there could be a loss of information. Example: int x; x = 2.5; // x will contain 2 (0.5 is lost) An int value can be stored in a double variable with no loss of information. Why? The value is converted into the equivalent real number. Example: double my_gpa; my_gpa: 2.0 my_gpa = 2;
36
Assignment exercise
What is the output of the following C code?
int x; x = 3; int y; y = x; x = 5; printf("%d\n", x); printf("%d\n", y);
37
Assignment exercise
What is the output of the following C code?
int number; number = 2 + 3 * 4; printf("%d\n", number - 1); number = 16 % 6; printf("%d\n", 2 * number);
What is the output of the following C code?
double average; average = (11 + 8) / 2; printf("%lf\n", average); average = (5 + average * 2) / 2; printf("%lf\n", average);
38
Shortcut: Declaring and initializing
A variable can be declared and assigned an initial value in the same statement. Declaration/initialization statement syntax:
<type> <name> = <expression>;
Examples: double my_gpa = 3.95; int x = (11 % 3) + 12;
39
Shortcut: Declaring many variables at once
It is legal to declare multiple variables on one line: <type> <name>, <name>, ..., <name>; Examples:
int a, b, c; double x, y;
It is also legal to declare/initialize several at once:
<type> <name> = <expression> , ..., <name> = <expression>;
Examples:
int a = 2, b = 3, c = -4; double grade = 3.5, delta = 0.1;
NB: The variables must be of the same type.
40
Shortcut: Modify and assign
C has several shortcut operators that allow you to quickly modify a variable's value.
Shorthand <variable> <variable> <variable> <variable> <variable> += -= *= /= %= <exp>; <exp>; <exp>; <exp>; <exp>; Equivalent longer version <variable> = <variable> <variable> = <variable> <variable> = <variable> <variable> = <variable> <variable> = <variable> + * / % (<exp>); (<exp>); (<exp>); (<exp>); (<exp>);
Examples:
x += 3 - 4; gpa -= 0.5; number *= 2;
// x = x + (3 - 4); // gpa = gpa (0.5); // number = number * (2);
41
Shortcut: Increment and decrement
Incrementing and decrementing 1 is used often enough that they have a special shortcut operator! Shorthand <variable>++; <variable>--; Equivalent longer version <variable> = <variable> + 1; <variable> = <variable> - 1;
Examples: int x = 2; x++; double gpa = 2.5; gpa++;
// x = x + 1; // x now stores 3 // gpa = gpa + 1; // gpa now stores 3.5
42
Putting it all together: Exercise
Write a program that stores the following data:
Section AA has 17 students. Section AB has 8 students. Section AC has 11 students. Section AD has 23 students. Section AE has 24 students. Section AF has 7 students. The average number of students per section.
Have your program print the following:
There are 24 students in Section AE. There are an average of 15 students per section.
43