Programming for Beginners

Programming for Beginners Lecture 3: Flow Control I: For Loops Martin Nelson Elizabeth FitzGerald

Revision of Session 2 • Declaration of variables and assigning values to them. • The main Java variable types: • char • byte • boolean • Performing arithmetic. • Decision making • double • String • integer class myprog { public static void main(String[] args) { //Declare an integer int x=1; //Do some arithmetic x=x+10; //Print the result System.out.println(“The value of x is”+x); } }

Session 3 - aims & objectives • Find out more about decision making using logical operators • Learn how to use ‘for’ loops to automate repetitive processes in your program

Conditional branching • Comparison operators > >= == < <= != • Logical operators && AND | | OR ! NOT

Comparison of data types • You will need to use different methods for comparing Strings than for comparing doubles, integers or chars • We will discuss how to do this in Session 8.

Looping • A block of code is repeated • A pre-defined number of times(determinate) • Until a certain (boolean) condition is met(indeterminate) • The contents of variables often change in each cycle • Beware of perpetual or infinite loops!

‘For’ loops • These are probably the most common type of loop. • They require a starting condition, something to test to decide whether to continue, and a statement of what changes on each iteration. • In Java, the syntax is: for(start condition; continue condition; update instruction) { Stuff to do on each iteration of the loop. This will be a list of statements, often a few lines long. }

For loop example • Some code to print the numbers 1 to 100 on the screen. • ‘for’ is a reserved word which tells java that this is a loop. • Here x is declared when the for loop starts, but you could declare x before the loop. • x++ is shorthand for ‘x=x+1’. for(int x=1; x<=100; x++) { System.out.println(x); }

Exercises • Look at the example code in for this session: • Simple determinate loop • Loop a block of code: going up/going down • N green bottles! • N men went to mow! • Then attempt today’s exercises. A different kind of loop…

Coming up in Session 4... • More on flow control...

Programming for Beginners