Simple Java - Stanford Universityweb.stanford.edu/.../lectures/6-SimpleJava/6-SimpleJava.pdf · Scope Simple Java. Piech, CS106A, Stanford University George Boole. ... • Thescopeof

Piech,CS106A,StanfordUniversity

Simple JavaChris Piech

CS106A, Stanford University



Let’sfixanoldprogram


• Operations on numerical types• Operations:

+ “addition”- “subtraction”* “multiplication”/ “division” (different for int vs. double)% “remainder”

• Precedence (in order):() highest*, /, %+, - lowest

Operators in same precedence category evaluated left to right

Review: Operations


int x = 3;

x = x + 1; x += 1; x++;

x = x + 5; x += 5;

x = x – 1; x -= 1; x--;

x = x * 3; x *= 3;

x = x / 2; x /= 2;

Expressions Short Hand


• Boolean expression is just a test for a condition• Essentially, evaluates to true or false

• Value comparisons:== “equals” (note: not single =)!= “not equals” (cannot say <>)> “greater than”< “less than”>= “greater than or equal to”<= “less than or equal to”

Review: Boolean Expressions


Today’sGoal1. How to use constants

2. Basics of boolean variables3. Understand For loops4. Know variable scope


Today’sRoute

BooleansScope

Simple Java


GeorgeBoole


Booleanvariabletype


Value comparisons (in order of precidence):! “not”!p

&& “and”p && q

|| “or”p || q

Boolean Expressions

Ifpistruethen!pisfalse(andviceversa)

Evaluatestotrueifbothsidesaretrue

Evaluatestotrueifeitherporq(orboth)aretrue

boolean p = (x != 1) || (x != 2);boolean p = (x != 1) && (X != 2);


By Chris


Once upon a time…


int x = 5;if(lookingForLove()) {int y = 5;

}println(x + y);

x5

Xwaslookingforlove!



}println(x + y);

x5

Xwaslookingforlove!



}println(x + y);

x5

x was definitelylooking for love

Xwaslookingforlove!



}println(x + y);

x5

y5

Andmety



}println(x + y);

x5

y5 Hi, I’m y

Andmety


“Wow!”



}println(x + y);

x5

y5Wow

Andmety



}println(x + y);

x5

y5 We have so much

in common

Andmety



}println(x + y);

x5

y5 We both have

value 5!

Andmety



}println(x + y);

x5

y5 Maybe one day

we can…

Andmety



}println(x + y);

x5

y5 println together?

Andmety



}println(x + y);

x5

y5

Theygotalong


It was a beautiful match…


But then tragedy struck.



}println(x + y);

x5

y5

TragedyStruck



}println(x + y);

x5

y5

TragedyStruck



}println(x + y);

x5

TragedyStruck


Noooooooooooooooo!


You see…



}println(x + y);

x5

Whenaprogramexitsacodeblock…



}println(x + y);

x5

Allvariablesdeclaredinsidethatblock..



}println(x + y);

x5

Getdeletedfrommemory!



}println(x + y);

x5

Sinceywasdeclaredintheif-block



}println(x + y);

x5

Itgetsdeletedfrommemoryhere



}println(x + y);

x5

Anddoesn’texisthere



}println(x + y);

x5

Error. Undefined variable y.

Anddoesn’texisthere


The End


Sad times L


public void run(){double v = 8;if(condition){

v = 4;…somecode

}…someothercode

}

Variableshavealifetime(calledscope)



v = 4;…somecode

}…someothercode

}

Variableshavealifetime(calledscope)


v

Comestolifehere

8


v = 4;…somecode

}…someothercode

}

Vars cometoexistencewhendeclared



v = 4;…somecode

}…someothercode

}

Thisistheinnermostcodeblockinwhichitwasdeclared….

v4

Liveuntilendoftheircodeblock



v = 4;…somecode

}…someothercode

}

Stillalivehere…

v4




v = 4;…somecode

}…someothercode

}v4


Itdieshere(attheendofitscodeblock)



v = 4;…somecode

}…someothercode

}Itdieshere(attheendofitscodeblock)




public void run(){…somecodeif(condition){

int w = 4;…somecode

}…someothercode

}

Example2

Thisisthescopeofw


public void run(){…somecodeif(condition){

int w = 4;…somecode

}…someothercode

}

wdieshere(attheendofitscodeblock)

Example2

wcomestolifehere


Chapter 2


The programmer fixed her bug


int x = 5;if(lookingForLove()) {int y = 5;println(x + y);

}

x5

x waslookingforlove!



}

x5

x was definitelylooking for love

x waslookingforlove…



}

x5

y5

x mety



}

x5

y5

Sincetheywereboth“inscope”


The story had a happy ending!


• The scope ofavariablereferstothesectionofcodewhereavariablecanbeaccessed.

• Scopestartswherethevariableisdeclared.• Scopeendsattheterminationoftheinner-most

codeblockinwhichthevariablewasdefined.

• Acodeblockisachunkofcodebetween{}brackets

Scope Formally


Game Show


Choose a Doorint door = readInt("Door: ");// while the input is invalidwhile(door < 1 || door > 3) {

// tell the user the input was invalidprintln("Invalid door!");// ask for a new inputdoor = readInt("Door: ");

}

|| or&& and


int prize = 3;if(door == 1) {

prize = 2 + 9 / 10 * 100;} else if(door == 2) {

boolean locked = prize % 2 != 1;if(!locked) {

prize += 7;}

} else if(door == 3) {prize++;

}

The Door Logic


Howwouldyouprintln “Nickrockssocks”100times


println(“Nick rocks socks!”);println(“Nick rocks socks!”);println(“Nick rocks socks!”);println(“Nick rocks socks!”);println(“Nick rocks socks!”);println(“Nick rocks socks!”);println(“Nick rocks socks!”);println(“Nick rocks socks!”);println(“Nick rocks socks!”);println(“Nick rocks socks!”);println(“Nick rocks socks!”);println(“Nick rocks socks!”);println(“Nick rocks socks!”);println(“Nick rocks socks!”);println(“Nick rocks socks!”);println(“Nick rocks socks!”);println(“Nick rocks socks!”);println(“Nick rocks socks!”);


ForLoopRedux

for(int i = 0; i < 100; i++) {println(“Nick rocks socks!”);

}

Enters the loop if this condition

passes


ForLoopRedux


}

For Loop Redux


ForLoopRedux


}

i 0

For Loop Redux


ForLoopRedux


}

i 0

For Loop Redux


ForLoopRedux


}

i 0

For Loop Redux

Nick rocks socks


ForLoopRedux


}

i 0

For Loop Redux

Nick rocks socks


ForLoopRedux


}

i 1

For Loop Redux

Nick rocks socks


ForLoopRedux


}

i 1

For Loop Redux

Nick rocks socks


ForLoopRedux


}

For Loop Redux

Nick rocks socksNick rocks socks

Nick rocks socks


Youcanusetheforloopvariable


Howwouldyouprintln thefirst100evennumbers?


PrintingEvenNumbers


PrintingEvenNumbers

for(int i = 0; i < NUM_NUMS; i++) {println(i * 2);

}


PrintingEvenNumbers

for(int i = 0; i < 3; i++) {println(i * 2);

}

For Loop Redux


PrintingEvenNumbers


}

For Loop Redux


PrintingEvenNumbers


}

i 0

For Loop Redux


PrintingEvenNumbers


}

i 0

For Loop Redux


PrintingEvenNumbers


}

i 0

For Loop Redux

0


PrintingEvenNumbers


}

i 1

For Loop Redux

0


PrintingEvenNumbers


}

i 1

For Loop Redux

0


PrintingEvenNumbers


}

i 1

For Loop Redux

0

2


PrintingEvenNumbers


}

i 2

For Loop Redux

0

2


PrintingEvenNumbers


}

i 2

For Loop Redux

0

2


PrintingEvenNumbers


}

i 2

For Loop Redux

0

2

4


PrintingEvenNumbers


}

i 3

For Loop Redux

0

2

4


PrintingEvenNumbers


}

i 3

For Loop Redux

0

2

4


PrintingEvenNumbers


}

i 3

For Loop Redux

0

2

4


PrintingEvenNumbers


}

For Loop Redux

0

2

4



Milestone1


Milestone2


Milestone3


Today’sRoute

ReviewGraphics

Simple Java

Youarehere


Today’sGoal1. How to use constants

2. Basics of boolean variables3. Understand For loops4. Know variable scope

Documents

Simple Java - Stanford Universityweb.stanford.edu/.../lectures/6-SimpleJava/6-SimpleJava.pdf · Scope Simple Java. Piech, CS106A, Stanford University George Boole. ... • Thescopeof