Lecture 4 CS140 Dick Steflik

Reading Keyboard Input Import java.util.Scanner – A simple text scanner which can parse primitive types and strings using regular expressions. Can be used to extract tokens from strings, System.in, files, Streams…. Import java.util.Scanner; // System.in is the keyboard; create a Scanner for the keyboard Scanner in = new Scanner(System.in); // print a prompt System.out.print("Enter an integer"); int myInteger = in.nextInt();

Output System.out.print( ) System.out.println( ) System.out.printf("formatting string", v0,v1…)

Decisions This should be review Since C, C++ and Java are all syntactically very similar the if statement should be familiar. The general format is: // in its simplest form if (some predicate to evaluate to true or false) // a statement (simple or complex) { … }; -or- // slightly more complex if (some predicate to evaluate to true or false) // a statement (simple or complex) { … }; else // a statement (simple or complex) { … };

Statements A simple statement ends with a ";" character can stand on their own Compound statements (sequences of simple statements) must be enclosed between "{ }" characters. As a matter of coding style if the then (assumed) part of an if or the else part of an if is a simple statement surround the statement with "{ }"; its safer in the long run if (a == b) { c = 5; } else { c = 6;}

Decisions (more) Note: the ";" belongs as the terminator of the simple statement, not the compound statement if (a == b) { c = 5; d = d * c + a; } else { c = 6; d = d * c + b; }

Decisions (more) We can also use if to make ladders/filters/demultoplexers: if (expression) { statement } else if (expression) { statement } else if ( expression ) { statement } else if (expression) { statement } else // optional { statement }

Decisions (more) if is used to make a 1 of 2 decision; if true do this else do that another way to accomplish what an if ladder does is to use the switch statement: switch (value) { case 1: statement ; break; case 2: statement ; break; case 3: statement ; break; default: statement; break; } is equivalent to: if (value ==1) statement ; else if (value == 2) statement ; else if (value == 3) statement; else statement;

Strings Something to note about using Strings in the predicate of an if statement – == can be used to test primitives for equality but not objects – Strings are immutable objects, objects must have their own methods for doing comparisons The String class has a method called equals that takes a string as its argument and compares the value of the String to that argument and returns true or false: Scanner in = new Scanner(System.in); String name = in.next(); String dick = "Dick Steflik"; if ( name.equals(dick) // it must be Dick else // it isn't Dick

Loops Definite iteration; we know how many times we want to execute the body – for Indefinite iteration, we don't know how many times we want to execute the body – while – may never execute the body of the loop – do – will always execute the body of the loop at least once

for general form is: for ( initialization ; termination ; increment) { … } ex: for (i=0 ; i < 10 ; i++) System.out.print( i + " "); // prints : for ( i=0 ; i < 10 ; i = i + 2) System.out.print(I + " "); // prints:

while something in the body of the loop or something happening before entering the loop must terminate the loop : while (predicate) { body of loop} ex int v = 10; while (v != 0) { System.out.print(v + " "); v--; } // print: In this case the variable v is called a sentinal value, it is being watched to know when to terminate the loop note that if the loop were entered with v set to 0 then the body would never be executed

do in some languages this construct is called a do/until loop do {body of loop} while (some predicate) ex: int v = 0 do { System.out.print( v + " ") v=v+1; } while ( v != 10) // prints : note : if v was > 10 at the beginning of the loop we would have a never terminating loop