Lecture 4 CS 140 Dick Steflik Reading Keyboard

Lecture 4 CS 140 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…. • http: //docs. oracle. com/javase/1. 5. 0/docs/api/java/util/Scanner. html 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 my. Integer = in. next. Int();

Output • System. out. print( ) • System. out. println( ) • System. out. printf("formatting string", v 0, v 1…) http: //docs. oracle. com/javase/tutorial/java/data/numberformat. html

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 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 // 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 : 0 1 2 3 4 5 6 7 8 9 for ( i=0 ; i < 10 ; i = i + 2) System. out. print(I + " "); // prints: 0 2 4 6 8

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: 10 9 8 7 6 5 4 3 2 1 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 : 0 1 2 3 4 5 6 7 8 9 10 • note : if v was > 10 at the beginning of the loop we would have a never terminating loop