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CSE 142

Lecture Notes

Week 4

User input

  • We will be using Scanner for user input
  • In order to use Scanner you need to add an import statement to the top of your code: import java.util.*;
  • In main, you will use Scanner console = new Scanner(System.in); to create a Scanner object named console that you can pass to any of your methods that need user input
  • Note: The console name is arbitrary, if it makes sense use a different name
  • Note: You should only ever construct 1 Scanner object and pass it in as a parameter to only the methods that need it
  • token: A sequence of characters that are not white space (e.g., tabs, spaces, etc)

Scanner methods

Method Description
nextInt() reads a token of user input as an int; can only read ints, otherwise error
nextDouble() reads a token of user input as a double; can read doubles and ints (converts to double)
next() reads a token of user input as a String
nextLine() reads a line of user input as a String; will include white space characters

Sample code

Scanner console = new Scanner(System.in);
System.out.print("How olde are you? ");
int age = console.nextInt();
System.out.println("You'll be 40 in " + (40 - age) + " years.");

Relational operators

Operator Description Example Result
== equals 1 + 1 == 2 true
!= does not equal 3.2 != 2.5 true
< less than 10 < 5 false
> greater than 10 > 5 true
<= less than or equal to 126 <= 100 false
>= greater than or equal to 5.0 >= 5.0 true

Logical operators

Operator Description Example Result
&& and (2 == 3) && (-1 < 5) false
|| or (2 == 3) || (-1 < 5) true
! not !(2 == 3) true
p q p && q p || q
true true true true
true false false true
false true false true
false false false false
p !p
true false
false true

Common Algorithms

These are extremely common patterns in programming, super important to know!

Cumulative Sum

// returns the sum of integers from 1 up to n
public static int calculateSum(int n) {
   int sum = 0;
   for (int i = 1; i <= n; i++) {
     sum = sum + i;
   }
   return sum;
}

Max

public static int findMax(Scanner console, int n) {
    int max = 0
    
    System.out.print("How many numbers? ");
    int nums = console.nextInt();
    
    for (int i = 0; i < nums; i++) {
        System.out.print("Enter an integer: ");
        int num = console.nextInt();
        
        if (num > max) {
            max = num;
        }
    }
}

Even or Odd

public static void evenOrOdd(int n) {
    if (n % 2 == 0) {
        System.out.println(n + " is even.");
    } else {
        System.out.println(n + " is odd.");
    }
}

Replicate

// returns a String containing n replications of s
public static String replicate(String s, int n) {
    String output = "";
    for (int i = 0; i < n; i++) {
      output = output + s;
    }
    return output;
}

Reverse String

public static String reverse(String phrase) {
    String output = "";
    for (int i = 0; i < phrase.length(); i++) {
        output = phrase.charAt(i) + output;
    }
    return output;
}

Conditionals

  • else can only be used when paired with an if

Structure

// independent tests; not exclusive
// 0, 1, or many of the statement(s) may execute
// every test in every if block is checked
if (test) {
    statement(s);
}
if (test) {
    statement(s);
}
if (test) {
    statement(s);
}
// 0, or 1 of the if blocks may execute
// at most only 1 of the if blocks execute
// it could be the case that 0 if blocks execute because their is no else
if (test) {
    statement(s);
} else if (test) {
    statement(s);
} else if (test) {
    statement(s);
}
// mutually exclusive
// exactly 1 of the if blocks will execute
if (test) {
    statement(s);
} else if (test) {
    statement(s);
} else {
    statement(s);
}

Rounding

If you want to round to 1 digit after the decimal use the below code, adjust the 10.0 to other multiples of 10 for different number of places after the decimal.

// Note: This 1 line method is okay because it does a computation
public static double round(double n) {
    return Math.round(n * 10.0) / 10.0;
}

Chaining

Chaining is bad and you will use points if you do it. It is the practice of calling methods which call other methods which call other methods and they rarely or never return a value back to the caller. Instead you should try and structure your code so that methods return back to their callers.

String

  • Strings in Java are objects
  • Strings can contain the same characters but not be equal because in Java they are stored as different objects (even though they have the same characters)
  • Because of this, you should use .equals() when comparing Strings and not == 
    // word1 and word2 are different objects
String word1 = "hello";
String word2 = "hello";

// do not use!
if (word1 == word2) {
   ...
}

// use this instead!
if (word1.equals(word2)) {
   ...
}

// you could also have use this; it does the same as the one directly above
if (word2.equals(word1)) {
   ...
}

String test methods

Methods Description
str1.equals(str2) tests whether str1 contains the same characters as str2
str1.equalsIgnoreCase(str2) tests whether str1 contains the same characters as str2, ignoring case
str1.startsWith(str2) tesets whether str1 starts with the characters in str2
str1.endsWith(str2) tests whether str1 ends with the characters in str2
str1.contains(str2) tests whether str2 is found inside of str1

Using printf

  • The f in printf stands for formatted
  • Allows you to format what you are printing

General structure

System.out.printf(<format string>, <parameter>, ..., <parameter>);

Example

int x = 38;
int y = 152;
// the below line will output: location: (38, 152)
System.out.printf("location: (%d, %d)\n", x, y);

Common Format Specifiers

Specifier Result
%d Integer
%8d Integer, right-aligned, 8-space-wide field
%6d Integer, left-aligned, 6-space-wide field
%f Floating-point number
%12f Floating-point number, right-aligned, 12-space-wide field
%.2f Floating-point number, rounded to nearest hundredth
%16.3f Floating-point number, rounded to nearest thousandth, 16-space-wide field
%s String
%8s String, right-aligned, 8-space-wide field
%9s String, left-aligned, 9-space-wide field