CS100J, Spring 2001
Tues 1/30
Lecture 3
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Announcements:
+ newcomers: see Overview on website; see Announcements for what to do about P1
+ P2 due Thurs 2/8
+ T1 (prelim 1) Tues 2/13
+ CS100J sections start this week
+ don't go to Section 5 (use 6 or another instead)! (temporarily cancelled)
+ group sessions starrt Sun 1-3pm
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Topics:
+ statements
+ arithmetic
+ variables and assignments
+ conditions
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Summary from lecture 2:
+ need written computer language to communicate with computer
+ written language makes tokens from characters
+ write statements (instructions) from tokens
+ assignment as important statement
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Statements:
+ combine tokens to create "sentences"
+ each "sentence" forms an instruction/command
+ "sentences" are called statements
+ end statements with semicolon
+ going to a new line does NOT mean you have a new statement!
+ Java executes each statement top-down, left-to-right
(within a statement, operator precedence determines direction of execution)
+ see Java in Nutshell pp43-44 for full list
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+ Empty statement
ex) ;
- doesn't actually do anything
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+ Expression:
- combine constants with operators
- precedence: some operators come before others
- associativity: operators "work" left to right or right to left
ex) 1 + 1;
2 * 2 - 4; // technically means (2*2) - 4 because of precedence
2 - 1 - 3; // technically means (2-1) - 3 due to associativity
- use () to force some expressions to evaluate first
ex) (1 + 1); // () not needed
2 * (2 - 4);
2 - (1 - 3);
- beware of mixing types!
ex) System.out.println(1/2) yields 0, not 0.5
- mixing doubles and ints makes doubles
ex) System.out.println(1.0/2) yields 0.5
- use CAST to force different value
- syntax: (type) expression
- ex) System.out.println( (int) 9.8) yields 9 (an integer)
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+ Declaration:
- want identifiers to represent other values
- variables (identifiers) can hold values
- values can be primitive types
- can also be references, but that deals with objects, so we'll skip this
for now
- Java is strongly typed, so must tell Java about variables before using
- syntax: variabletype var ;
- ex) char c;
int value;
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+ Assignment:
- special kind of expression statement
- store value in a variable that's been declared
- cannot use variable until it's declared!
- syntax: declaredvar = value
- the value's type MUST match the variable's type
- ex) int x;
x = 1;
- now, x has the value 1 until the current METHOD (we use main) ends
- think x <- 1 because assign operator associates right-to-left
- can say in English "x gets 1"
- later in code, Java "knows" x means 1
- cannot redeclare variable unless "elsewhere" in code
(too hard to explain for now -- so, just don't redeclare!)
- shortcut:
o can combine syntax for shortcut:
o syntax: type var = value
o ex) int y = 7;
o called INITIALIZING a variable (declaring and assigning initial value)
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+ Style issues:
- usually one statement per line
- leave space between operators and operands (values operated upon)
- use descriptive variable names
- avoid hard coding values (using specific values that)
o use named constants
o variables initialized "early" in code and used instead of the constants
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+ Swap example
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Conditions:
+ execution of program goes in order of operators, starting from 1st statement
+ some say "top-down" execution
+ some statements can change FLOW OF CONTROL
(order of which statements execute)
+ sometimes need to make choices (example: comparison)
+ tree concept
+ use if, if-else (and if-else if)
+ syntax:
if (c) // if c is true
s; // do s
// otherwise, skip this statement
if (c) // if c is true
s1; // do s1
else // otherwise
s; // do s2
+ other varieties:
if (c1)
s1
else if (c2)
s2
if (c1)
s1;
else if (c2)
s2;
else
s3;
+ can have as many else-ifs as you wish
+ indent statements, but entire body counts as SINGLE statement!
(indenting does not make a new statement, just improves clarity)
+ can do multiple statements under each condition:
- use statement block
- { statements }
+ language elements:
- if, else are keywords
- conditions? must evaluate to true or false (Boolean)
o relations: < (less than) > (greater than)
<= (less than or equal) > (greater than or equal)
== (equal, but do NOT use =)
!= (not equal)
o logic: && (and) || (or) ! (not)
o values: true false
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Loops:
+ statement for repeated actions