CS 106A, Lecture 8 - Stanford University
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This document is copyright (C) Stanford Computer Science and Marty Stepp, licensed under Creative Commons Attribution 2.5 License. All rights reserved. Based on slides created by Keith Schwarz, Mehran Sahami, Eric Roberts, Stuart Reges, and others. CS 106A, Lecture 8 Characters and Strings suggested reading: Java Ch. 8.1-8.4
Transcript of CS 106A, Lecture 8 - Stanford University
Thisdocumentiscopyright(C)StanfordComputerScienceandMartyStepp,licensedunderCreativeCommonsAttribution2.5License.Allrightsreserved.BasedonslidescreatedbyKeithSchwarz,MehranSahami,EricRoberts,StuartReges,andothers.
CS106A,Lecture8CharactersandStrings
suggestedreading:JavaCh.8.1-8.4
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Learning Goals•Beabletoconfidentlywriteandcallmethodsthatuseparametersandreturnvalues.
•Beabletogeneraterandomvaluesinyourprograms.
•Beabletouseandmanipulatechars.•Beabletowritestringalgorithmsthatoperateoneachcharacter.
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Plan For Today•Announcements•Recap–Parameters–Return
•RandomNumbers•TextProcessing–Characters–Strings
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Plan For Today•Announcements•Recap–Parameters–Return
•RandomNumbers•TextProcessing–Characters–Strings
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Parameters
Parametersletyouprovideamethodsomeinformationwhenyouarecallingit.
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Return
Returnvaluesletyougivebacksomeinformationwhen
amethodisfinished.
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Methods = Toasters
parameter
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Methods = Toasters
parameter
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Methods = Toasters
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Methods = Toasters
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Methods = Toasters
return
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Example: readInt
int x = readInt(”Your guess? ");
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Example: readInt
int x = readInt(”Your guess? ");
We give readInt some information (the text to
print to the user)
We call readInt
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Example: readInt
int x = readInt(”Your guess? ");
When we include values in the parentheses of a method call, this means we are passing them as parameters to this
method.
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Example: readInt
int x = readInt(”Your guess? ");
When finished, readInt gives us information back (the user’s number) and we put it in x.
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Example: readInt
int x = readInt(”Your guess? ");
When we set a variable equal to a method, this tells Java to save the return value of the method in that variable.
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Plan For Today•Announcements•Recap–Parameters–Return
•RandomNumbers•TextProcessing–Characters–Strings
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Parameters Example: drawBox
private void drawBox(int width, int height) {// use width and height variables// to draw a box
}
Tells Java this method needs two ints in order to
execute.
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Parameters Example: drawBox
private void drawBox(int width, int height) {// use width and height variables// to draw a box
}
Inside drawBox, refer to the first parameter value
as width…
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Parameters Example: drawBox
private void drawBox(int width, int height) {// use width and height variables// to draw a box
}
…and the second parameter value as height.
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drawBox
drawBox(10, 4);
We give drawBox some information (the size of
the box we want)
We call drawBox
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drawBox
int width = readInt("Width? ");int height = readInt("Height? ");...
drawBox(width, height);
We give drawBox some information (the size of
the box we want)
We call drawBox
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drawBox
int width = readInt("Width? "); 7int height = readInt("Height? "); 4...
drawBox(width, height);
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drawBox
int width = readInt("Width? "); 7int height = readInt("Height? "); 4...
drawBox(width, height);7 4
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drawBox
int width = readInt("Width? "); 7int height = readInt("Height? "); 4...
drawBox(7, 4);
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drawBox
7 4
Second parameter to
drawBox
First parameter to drawBox
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drawBox
private void drawBox(int width, int height) {
7 4
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drawBox
private void drawBox(int width, int height) {// use width and height variables// to draw a box
}
7 4
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drawBox
private void drawBox(int width, int height) {...println(width); // prints 7println(height); // prints 4...
}
7 4
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Parameter Names
Parameternamesdonotaffectprogrambehavior.
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Parameter Namespublic void run() {
int width = readInt("Width? "); 7int height = readInt("Height? "); 4drawBox(width, height);
}
private void drawBox(int width, int height) {...
}
run
width height
7 4drawBox
width height
7 4
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Parameter Namespublic void run() {
int width = readInt("Width? "); 7int height = readInt("Height? "); 4drawBox(width, height);
}
private void drawBox(int w, int h) {...
}
run
width height
7 4drawBox
w h
7 4
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Plan For Today•Announcements•Recap–Parameters–Return
•RandomNumbers•TextProcessing–Characters–Strings
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Return Example: metersToCm
private double metersToCm(double meters) {...
}
When this method finishes, it will return a double.
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Return Example: metersToCm
private double metersToCm(double meters) {double centimeters = meters * 100;return centimeters;
}
Returns the value of this expression (centimeters).
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Return Example: metersToCm
public void run() {double cm = metersToCm(10);...
}
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Return Example: metersToCm
public void run() {double cm = metersToCm(10);...
}
Setting a variable equal to a method means we save the method’s return
value in that variable.
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Return Example: metersToCmpublic void run() {
double meters = readDouble("# meters? ”);...
double cm = metersToCm(meters);println(cm + " centimeters.");
}
private double metersToCm(double meters) {double centimeters = meters * 100;return centimeters;
}
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Return Example: metersToCmpublic void run() {
double meters = readDouble("# meters? ”);...
double cm = metersToCm(meters);println(cm + " centimeters.");
}
private double metersToCm(double meters) {double centimeters = meters * 100;return centimeters;
}
7
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Return Example: metersToCmpublic void run() {
double meters = readDouble("# meters? ”);...
double cm = metersToCm(meters);println(cm + " centimeters.");
}
private double metersToCm(double meters) {double centimeters = meters * 100;return centimeters;
}
7
7
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Return Example: metersToCmpublic void run() {
double meters = readDouble("# meters? ”);...
double cm = metersToCm(meters);println(cm + " centimeters.");
}
private double metersToCm(double meters) {double centimeters = meters * 100;return centimeters;
} 700
7
7
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Return Example: metersToCmpublic void run() {
double meters = readDouble("# meters? ”);...
double cm = metersToCm(meters);println(cm + " centimeters.");
}
700
7
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Return Values and Expressionspublic void run() {
double meters = readDouble("# meters? ”);println(metersToCm(meters) + " cm.");
}
private double metersToCm(double meters) {...
}
7
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Return Values and Expressionspublic void run() {
double meters = readDouble("# meters? ”);println(metersToCm(meters) + " cm.");
}
private double metersToCm(double meters) {...
}
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You can use a method’s return value directly in an expression.
7
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Buggy Example!public void run() {
double meters = readDouble("# meters? ”);...
metersToCm(meters); // Does nothing!...
}
7
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Buggy Example!public void run() {
double meters = readDouble("# meters? ”);...
metersToCm(meters); // Does nothing!...
}
7
700
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private int max(int num1, int num2) {if(num1 >= num2) {
return num1;}return num2; // here only if num1 < num2
}
Return Stops Method Execution
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Returning Booleans
private boolean isEven(int number) {return number % 2 == 0;
}
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Returning Booleans
private boolean isEven(int number) {return number % 2 == 0;
}
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Returning Booleans
private boolean isEven(int number) {return number % 2 == 0;
}
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true
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Returning Booleans
private boolean isEven(int number) {return number % 2 == 0;
}
// Examplepublic void run() {
if (isEven(2)) {...
}}
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Returning Booleans
private boolean isDivisibleBy(int a, int b) {return a % b == 0;
}
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Returning Booleans
private boolean isDivisibleBy(int a, int b) {return a % b == 0;
}
24 9
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Returning Booleans
private boolean isDivisibleBy(int a, int b) {return a % b == 0;
}
24 9
false
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Returning Booleans
private boolean isDivisibleBy(int a, int b) {return a % b == 0;
}
// Examplepublic void run() {
if (isDivisibleBy(4, 2)) {...
}}
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Factorial Code Walkthrough
Seenextslides
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public void run() {for(int i = 0; i < MAX_NUM; i++) {
println(i + "! = " + factorial(i));}
}i
4
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public void run() {for(int i = 0; i < MAX_NUM; i++) {
println(i + "! = " + factorial(i));}
}0i
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public void run() {for(int i = 0; i < MAX_NUM; i++) {
println(i + "! = " + factorial(i));}
}0i
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public void run() {for(int i = 0; i < MAX_NUM; i++) {
println(i + "! = " + factorial(i));}
}0i
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public void run() {for(int i = 0; i < MAX_NUM; i++) {
println(i + "! = " + factorial(i));}
}0i
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public void run() {for(int i = 0; i < MAX_NUM; i++) {
println(i + "! = " + factorial(i));}
}0
i
private int factorial(int n) {int result = 1;for (int i = 1; i <= n; i++) {
result *= i;}return result;
}
result0n i
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public void run() {for(int i = 0; i < MAX_NUM; i++) {
println(i + "! = " + factorial(i));}
}0
i
private int factorial(int n) {int result = 1;for (int i = 1; i <= n; i++) {
result *= i;}return result;
}
1result0n i
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public void run() {for(int i = 0; i < MAX_NUM; i++) {
println(i + "! = " + factorial(i));}
}0
i
private int factorial(int n) {int result = 1;for (int i = 1; i <= n; i++) {
result *= i;}return result;
}
1result0n 1i
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public void run() {for(int i = 0; i < MAX_NUM; i++) {
println(i + "! = " + factorial(i));}
}0
i
private int factorial(int n) {int result = 1;for (int i = 1; i <= n; i++) {
result *= i;}return result;
}
1result0n 1i
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public void run() {for(int i = 0; i < MAX_NUM; i++) {
println(i + "! = " + factorial(i));}
}0
i
private int factorial(int n) {int result = 1;for (int i = 1; i <= n; i++) {
result *= i;}return result;
}
1result0n 1i
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public void run() {for(int i = 0; i < MAX_NUM; i++) {
println(i + "! = " + factorial(i));}
}0i
1
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public void run() {for(int i = 0; i < MAX_NUM; i++) {
println(i + "! = " + factorial(i));}
}0i
1
0! = 1
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public void run() {for(int i = 0; i < MAX_NUM; i++) {
println(i + "! = " + factorial(i));}
}1i
0! = 1
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public void run() {for(int i = 0; i < MAX_NUM; i++) {
println(i + "! = " + factorial(i));}
}1i
0! = 1
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public void run() {for(int i = 0; i < MAX_NUM; i++) {
println(i + "! = " + factorial(i));}
}1i
0! = 1
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public void run() {for(int i = 0; i < MAX_NUM; i++) {
println(i + "! = " + factorial(i));}
}1i
0! = 1
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public void run() {for(int i = 0; i < MAX_NUM; i++) {
println(i + "! = " + factorial(i));}
}0
i
private int factorial(int n) {int result = 1;for (int i = 1; i <= n; i++) {
result *= i;}return result;
}
result1n i
0! = 1
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public void run() {for(int i = 0; i < MAX_NUM; i++) {
println(i + "! = " + factorial(i));}
}0
i
private int factorial(int n) {int result = 1;for (int i = 1; i <= n; i++) {
result *= i;}return result;
}
1result1n i
0! = 1
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public void run() {for(int i = 0; i < MAX_NUM; i++) {
println(i + "! = " + factorial(i));}
}0
i
private int factorial(int n) {int result = 1;for (int i = 1; i <= n; i++) {
result *= i;}return result;
}
1result1n 1i
0! = 1
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public void run() {for(int i = 0; i < MAX_NUM; i++) {
println(i + "! = " + factorial(i));}
}0
i
private int factorial(int n) {int result = 1;for (int i = 1; i <= n; i++) {
result *= i;}return result;
}
1result1n 1i
0! = 1
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public void run() {for(int i = 0; i < MAX_NUM; i++) {
println(i + "! = " + factorial(i));}
}0
i
private int factorial(int n) {int result = 1;for (int i = 1; i <= n; i++) {
result *= i;}return result;
}
1result1n 1i
0! = 1
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public void run() {for(int i = 0; i < MAX_NUM; i++) {
println(i + "! = " + factorial(i));}
}0
i
private int factorial(int n) {int result = 1;for (int i = 1; i <= n; i++) {
result *= i;}return result;
}
1result1n 2i
0! = 1
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public void run() {for(int i = 0; i < MAX_NUM; i++) {
println(i + "! = " + factorial(i));}
}0
i
private int factorial(int n) {int result = 1;for (int i = 1; i <= n; i++) {
result *= i;}return result;
}
1result1n 2i
0! = 1
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public void run() {for(int i = 0; i < MAX_NUM; i++) {
println(i + "! = " + factorial(i));}
}0
i
private int factorial(int n) {int result = 1;for (int i = 1; i <= n; i++) {
result *= i;}return result;
}
1result1n 2i
0! = 1
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public void run() {for(int i = 0; i < MAX_NUM; i++) {
println(i + "! = " + factorial(i));}
}1i
1
0! = 1
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public void run() {for(int i = 0; i < MAX_NUM; i++) {
println(i + "! = " + factorial(i));}
}1i
1
0! = 11! = 1
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public void run() {for(int i = 0; i < MAX_NUM; i++) {
println(i + "! = " + factorial(i));}
}2i
0! = 11! = 1
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public void run() {for(int i = 0; i < MAX_NUM; i++) {
println(i + "! = " + factorial(i));}
}2i
0! = 11! = 1
85
public void run() {for(int i = 0; i < MAX_NUM; i++) {
println(i + "! = " + factorial(i));}
}2i
0! = 11! = 1
86
public void run() {for(int i = 0; i < MAX_NUM; i++) {
println(i + "! = " + factorial(i));}
}2i
0! = 11! = 1
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public void run() {for(int i = 0; i < MAX_NUM; i++) {
println(i + "! = " + factorial(i));}
}2i
2
0! = 11! = 1
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public void run() {for(int i = 0; i < MAX_NUM; i++) {
println(i + "! = " + factorial(i));}
}2i
2
0! = 11! = 12! = 2
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public void run() {for(int i = 0; i < MAX_NUM; i++) {
println(i + "! = " + factorial(i));}
}3i
0! = 11! = 12! = 2
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public void run() {for(int i = 0; i < MAX_NUM; i++) {
println(i + "! = " + factorial(i));}
}3i
0! = 11! = 12! = 2
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public void run() {for(int i = 0; i < MAX_NUM; i++) {
println(i + "! = " + factorial(i));}
}3i
0! = 11! = 12! = 2
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public void run() {for(int i = 0; i < MAX_NUM; i++) {
println(i + "! = " + factorial(i));}
}3i
0! = 11! = 12! = 2
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public void run() {for(int i = 0; i < MAX_NUM; i++) {
println(i + "! = " + factorial(i));}
}3i
6
0! = 11! = 12! = 2
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public void run() {for(int i = 0; i < MAX_NUM; i++) {
println(i + "! = " + factorial(i));}
}3i
6
0! = 11! = 12! = 23! = 6
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public void run() {for(int i = 0; i < MAX_NUM; i++) {
println(i + "! = " + factorial(i));}
}4i
0! = 11! = 12! = 23! = 6
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public void run() {for(int i = 0; i < MAX_NUM; i++) {
println(i + "! = " + factorial(i));}
}4i
0! = 11! = 12! = 23! = 6
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Plan For Today•Announcements•Recap–Parameters–Return
•RandomNumbers•TextProcessing–Characters–Strings
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RandomGenerator• import acm.util.*;
// random number from 0-9 inclusiveint digit = RandomGenerator.getInstance().nextInt(0, 9);println(digit);
// prints "hello!" between 3-6 timesint times = RandomGenerator.getInstance().nextInt(3, 6);for (int i = 0; i < times; i++) {
println("hello!");}
Method DescriptionRandomGenerator.getInstance().nextInt(min, max) arandomintegerinthe
givenrange,inclusive
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RandomGeneratorThe RandomGenerator class defines the following methods:
int nextInt(int low, int high)Returns a random int between low and high, inclusive.
int nextInt(int n)Returns a random int between 0 and n-1.
double nextDouble(double low, double high)Returns a random double d in the range low ≤ d < high.
double nextDouble()Returns a random double d in the range 0 ≤ d < 1.
boolean nextBoolean()Returns a random boolean value, which is true 50 percent of the time.
boolean nextBoolean(double p)Returns a random boolean, which is true with probability p, where 0 ≤ p ≤ 1.
Color nextColor()Returns a random color.
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Extra: Dice exercise• WriteaconsoleprogramRollTwoDice thatrepeatedlyrollstwo6-sideddiceuntiltheyarriveatagivendesiredsum.
Desired sum? 93 and 4 = 72 and 1 = 35 and 5 = 106 and 2 = 86 and 5 = 114 and 5 = 9
• TrysolvingthisonyourownonCodeStepByStep!
RollTwoDice
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Plan For Today•Announcements•Recap–Parameters–Return
•RandomNumbers•TextProcessing–Characters–Strings
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Text Processing
103
Goal
After learning text processing, we will be able to write our own encryption program!
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Plan For Today•Announcements•Recap–Parameters–Return
•RandomNumbers•TextProcessing–Characters–Strings
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CharAchar isavariabletypethatrepresentsasinglecharacteror“glyph”.
char letterA = 'A';char plus = '+';char zero = '0';char space = ' ';char newLine = '\n';char tab = '\t';char singleQuote = '\'';char backSlash = '\\';
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CharUnderthehood,Javarepresentseachchar asaninteger (its“ASCIIvalue”).
•Uppercaselettersaresequentiallynumbered•Lowercaselettersaresequentiallynumbered•Digitsaresequentiallynumbered
char uppercaseA = 'A'; // Actually 65char lowercaseA = 'a'; // Actually 97char zeroDigit = '0'; // Actually 48
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Char Math!WecantakeadvantageofJavarepresentingeachchar asaninteger (its“ASCIIvalue”):
boolean areEqual = 'A' == 'A'; // trueboolean earlierLetter = 'f' < 'c'; // falsechar uppercaseB = 'A' + 1;int diff = 'c' - 'a'; // 2int numLettersInAlphabet = 'z' – 'a' + 1;// orint numLettersInAlphabet = 'Z' – 'A' + 1;
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Char Math!WecantakeadvantageofJavarepresentingeachchar asaninteger (its“ASCIIvalue”):
// prints out every characterfor (char ch = 'a'; ch <= 'z'; ch++) {
print(ch);}
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Char Math!Noteveryintegermapstoacharacter.Sowhenyouhaveanexpressionwithints andchars,Javapicksint asthemostexpressivetype.
'A' + 1 // evaluates to 66 (int)'c' + (2*5) – 1 // evaluates to 108
Wecanmakeitacharbyputtingitinacharvariable.char uppercaseB = 'A' + 1;// orchar uppercaseB = 66;
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Side Note: Type-castingIfwewanttoforceJavatotreatanexpressionasaparticulartype,wecanalsocastit tothattype.
'A' + 1 // evaluates to 66 (int)(char)('A' + 1) // evaluates to 'B' (char)
1 / 2 // evaluates to 0 (int)(double)1 / 2 // evaluates to 0.5 (double)1 / (double)2 // evaluates to 0.5 (double)
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Character MethodsTherearesomehelpfulbuilt-inJavamethodstomanipulatechars.
char lowercaseA = 'a';char uppercaseA = Character.toUpperCase(lowercaseA);
char plus = '+';if (Character.isLetter(plus)) {
...}
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Character MethodsMethod Description
Character.isDigit(ch) trueifch is'0' through'9'
Character.isLetter(ch) trueifch is'a' through'z' or'A' through'Z'
Character.isLetterOrDigit(ch) trueifch is'a' through'z','A' through'Z' or '0' through'9'
Character.isLowerCase(ch) trueifch is'a' through'z'
Character.isUpperCase(ch) trueifch is'A' through'Z'
Character.toLowerCase(ch) returnslowercaseequivalentofaletter
Character.toUpperCase(ch) returnsuppercaseequivalentofaletter
Character.isWhitespace(ch) trueifch isaspace,tab,newline,etc.
Remember: these returnthe new char, they cannot modify an existing char!
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Character MethodsRemembertoalwayssavethereturnvalueofCharactermethods!
char lowercaseA = 'a';Character.toUpperCase(lowercaseA); // Does nothing!println(lowercaseA); // prints ’a’!
char uppercaseA = Character.toUpperCase(lowercaseA); // OK
println(uppercaseA); // prints ’A’!
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Plan For Today•Announcements•Recap–Parameters–Return
•RandomNumbers•TextProcessing–Characters–Strings
115
StringsAString isavariabletyperepresentingasequenceofcharacters.
String text = "Hi parents!";
– Eachcharacterisassignedanindex,goingfrom0tolength-1– Thereisachar ateachindex
index 0 1 2 3 4 5 6 7 8 9 10
character 'H' 'i' ' ' 'p' 'a' 'r' 'e' 'n' 't' 's' '!'
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String str = "Hello, world!";String empty = "";println(str);
// Read in text from the userString name = readLine("What is your name? ");
// String concatenation (using “+”)String message = 2 + " cool " + 2 + " handle";int x = 2;println("x has the value " + x);
Creating Strings
117
String str = "Hello, world!";
// Lengthint strLength = str.length(); // 13
// Access individual characterschar firstLetter = str.charAt(0);char lastLetter = str.charAt(strLength – 1);char badTimes = str.charAt(strLength); // ERROR
Common String Operations
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SubstringsAsubstring isasubsetofastring.
String str = "Hello, world!";String hello = str.substring(0, 5);
0 1 2 3 4 5 6 7 8 9 10 11 12'H' 'e' 'l' 'l' 'o' ',' ' ' 'w' 'o' 'r' 'l' 'd' '!'
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SubstringsAsubstring isasubsetofastring.
String str = "Hello, world!";String worldExclm = str.substring(7, 13);
0 1 2 3 4 5 6 7 8 9 10 11 12'H' 'e' 'l' 'l' 'o' ',' ' ' 'w' 'o' 'r' 'l' 'd' '!'
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SubstringsAsubstring isasubsetofastring.
String str = "Hello, world!";String worldExclm = str.substring(7); // to end
0 1 2 3 4 5 6 7 8 9 10 11 12'H' 'e' 'l' 'l' 'o' ',' ' ' 'w' 'o' 'r' 'l' 'd' '!'
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String MethodsMethodname Description
s.length() numberofcharactersinthisstring
s.charAt(index) charatthegivenindex
s.indexOf(str) indexwherethestartofthegivenstringappearsinthisstring(-1ifnotfound)
s.substring(index1, index2)ors.substring(index1)
thecharactersinthisstringfromindex1 (inclusive)toindex2 (exclusive);ifindex2 isomitted,goesuntilend
s.toLowerCase() anewstringwithalllowercaseletterss.toUpperCase() anewstringwithalluppercaseletters
• Thesemethodsarecalledusingdotnotation:
String className = "CS 106A yay!";println(className.length()); // 12
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Strings are ImmutableOnceyoucreateaString,itscontentscannotbechanged.
// Cannot change individual chars in the stringString typo = "Hello, warld!”;
TochangeaString,youmustcreateanew Stringcontainingthevalueyouwant(e.g.usingStringmethods).
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Strings are ImmutableString className = "cs 106a";className.toUpperCase(); // does nothing!
className = className.toUpperCase(); // ✔println(className); // CS 106A
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Comparing StringsString greeting = "Hello!”;if (greeting == "Hello!") { // Doesn’t work!
...}
// Instead:if (greeting.equals("Hello!")) {
...}
Always use .equals instead of == and !=
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Comparing StringsMethod Description
s1.equals(s2) whethertwostringscontainthesamecharacters
s1.equalsIgnoreCase(s2) whethertwostringscontainthesamecharacters,ignoringuppervs.lowercase
s1.startsWith(s2) whethers1 containss2’scharactersatstart
s1.endsWith(s2) whethers1 containss2’scharactersatend
s1.contains(s2) whethers2 isfoundwithins1
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Looping Over StringsAcommonStringprogrammingpatternisloopingoverastringandoperatingoneachcharacter.
String str = "Hello!";for (int i = 0; i < str.length(); i++) {
char ch = str.charAt(i);// Do something with ch here
}
127
Looping Over StringsAcommonStringprogrammingpatternisloopingoverastringandoperatingoneachcharacter.
// Prints out each letter on a separate lineString str = "Hello!";for (int i = 0; i < str.length(); i++) {
char ch = str.charAt(i);println(ch);
}
128
Looping Over StringsAcommonStringprogrammingpatternisloopingoverastringandoperatingoneachcharacter.
// Creates a new String in all capsString str = "Hello!";String newStr = "";for (int i = 0; i < str.length(); i++) {
char ch = str.charAt(i);newStr += Character.toUpperCase(ch);
}println(newStr); // HELLO!
129
Recap•Recap–Parameters–Return
•RandomNumbers•TextProcessing–Characters–Strings
Nexttime:problem-solvingwithStrings
