DCS/100: Procedural Programming

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Lucy Marshall
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1 DCS/100: wk 5 p.1/33 DCS/100: Procedural Programming Week 5: While Loops Queen Mary, University of London

2 DCS/100: wk 5 p.2/33 Last week You should now be able to: write programs that follow instructions a known number of times explain what is meant by a for loop trace the execution of programs containing for loops

3 DCS/100: wk 5 p.3/33 A program from last week int sum = 0; for (int i=1; i<=5; i++) { sum = sum + i; } out.writeln("total is "+ sum);

4 DCS/100: wk 5 p.4/33 This week At the end of this week you will be able to: write programs that follow instructions repeatedly where the number of times is not known at the outset explain what is meant by for loops and while loops, their similarities and differences trace the execution of programs containing loops

5 DCS/100: wk 5 p.5/33 While loops Loops generally have two parts: a test to tell you whether to do it again a thing to be done repeatedly There is also always something that is changed by the action of the loop. When writing loops always work out these parts first.

6 DCS/100: wk 5 p.6/33 While loops You do not always know how many times to repeat... A pure for loop is then little help. Let us invent a new loop construct... We need to tell the computer what is to be repeated, do we stop yet? while (<a test about continuing is true>) { <do these instructions over and over> }

7 DCS/100: wk 5 p.7/33 While Loops char ans= y ; while (ans== y ) { out.writeln("i did it!"); out.write("shall I go on? (y/n) "); ans = in.read(); in.readln(); }

8 DCS/100: wk 5 p.8/33 While loops Format: while (<condition>) <body> Check <condition>; if true do <body>, go back, check <condition>; if true do <body>, go back, check <condition>; if true do <body>,... until <condition> is false If <condition> is false initially, <body> is never executed.

9 DCS/100: wk 5 p.9/33 While loops Can go on for ever: while (true){} Stop by typing Ctrl-C (Control-C).

10 DCS/100: wk 5 p.10/33 While loops and accumulators Often use accumulator variables with while loops too: int count=0; char ans = y ; while (ans== y ) { count = count+1; out.write("shall I go on? (y/n) "); ans = in.read(); in.readln(); } out.writeln("that was "+count+" times");

11 DCS/100: wk 5 p.11/33 Another Accumulator int ans=0; int count=0,sum=0; out.write("enter a number (eof to terminate): "); while (in.digit()) { ans = in.readint(); count = count+1; sum = sum+ans; out.write("enter a number (eof to terminate): "); in.readblanks(); } out.writeln("that was "+count+" numbers"); out.writeln("with a sum of "+sum); out.writeln("and an average of "+((double) sum/ (double) count));

12 DCS/100: wk 5 p.12/33 While loops Of course you can wrap a while loop round lots of the previous exercises, so that you can do something a number of times: char ans= y ; while (ans== y ) { <program goes here> } out.write("another? (y/n) "); ans = in.read(); in.readln();

13 DCS/100: wk 5 p.13/33 Beware You have to make sure that the program you drop in reads all of any input it calls for, including any newlines! This is typical in Computer Science: some operation only works if the thing you apply it to is well-behaved in some way: it obeys some protocol.

14 DCS/100: wk 5 p.14/33 hile loops and structured programming This illustrates the structured approach. Be clear about what the body of a loop is supposed to do. Develop it as an entity in its own right. Brinch Hansen s approach: Program is a sequence of (named) subtasks. Develop the subtasks be refining them towards code. As you do this put in if statements and loops. Branches of if s and bodies of loops are subtasks.

15 DCS/100: wk 5 p.15/33 While loops and for loops Any standard for loop can be programmed up using a while loop: for (int i=<start>; i <= <finish>; i++) { <body> } is the same as {int i=<start>; while (i <= <finish>) { <body> i++; } }

16 DCS/100: wk 5 p.16/33 While loops and for loops The reverse is not true. While loops cannot be programmed up as standard for loops. Evidence: It s easy to write non-terminating programs using while loops. You can t do this if you only use standard for loops.

17 While loops and for loops The C/Java for loop is more general. It is equivalent to while loops. A general Java for loop: for (<init>;<test>;<reinit>) <body> means: first do <init>, then if <test> is true do <body> then do <reinit> and return to the start of the loop if <test> is true do <body> then do <reinit> and return to the start of the loop... until test is false. DCS/100: wk 5 p.17/33

18 DCS/100: wk 5 p.18/33 While loops and for loops for (<init>;<test>;<reinit>) <body> means: 1. do <init>, 2. evaluate <test> 3. if <test> is true (a) do <body> (b) do <reinit> (c) return to stage 2 otherwise end the loop and carry on with the rest of the program.

19 DCS/100: wk 5 p.19/33 While loops and for loops So for (<init>;<test>;<reinit>) <body> is equivalent to { <init>; while (<test>) { <body> <reinit> } }

20 DCS/100: wk 5 p.20/33 While loops and for loops But a general while loop: while (<test>) <body> is equivalent to for ( ; <test>; ) <body> So in Java (and C/C++) anything you can write with while you can write with for and vice versa.

21 DCS/100: wk 5 p.21/33 The basic control constructs Sequence: do this, then do that. In Java this is done just by writing one command after another. Branching: choose between different things. In Java this is if (<test>) <case> else <case> (and the switch construct).

22 DCS/100: wk 5 p.22/33 The basic control constructs Looping: keep repeating something until some exit condition is met. There s a difference between for loops (do something a fixed number of times) and while loops In Java: for (int i=<start>; i<=<finish>; i++) <body> while (<test>) <body> for (<init>;<test>;<reinit>) <body>

23 DCS/100: wk 5 p.23/33 Basic Control Constructs THAT S IT. THERE ARE NO MORE BASIC CONTROL CONSTRUCTS!!!! A language that has these control constructs is Turing complete. You can write any computable operation in it.

24 DCS/100: wk 5 p.24/33 Basic Control Constructs Everything else is there for other reasons to make the language clearer to allow you to break up code better to make code easier to reuse to make programs easier to develop to make programs more secure in their running...

25 DCS/100: wk 5 p.25/33 Debugging Most of you are getting code that compiles. But what do you do when it doesn t work?

26 DCS/100: wk 5 p.26/33 Debugging First Step INDENT YOUR CODE PROPERLY Execution follows the grammatical structure of code. Indentation makes that structure visible. It lets you see where choice points are. It lets you break the program up into phases. It lets you see what bits you can comment out. It often makes it obvious if you ve put something simple (a read(), a writeln()) in the wrong place (inside or outside a loop).

27 DCS/100: wk 5 p.27/33 Debugging First Step INDENT YOUR CODE PROPERLY You should indent your code properly as you write it. This stops some of the mistakes ever happening.

28 DCS/100: wk 5 p.28/33 Proper indentation See Brinch Hansen See Jon Rowson s layout notes The two crucial points: indent blocks a fixed amount make ends line up with beginnings

29 DCS/100: wk 5 p.29/33 Proper indentation The two crucial points: indent blocks a fixed amount make ends line up with beginnings Brinch Hansen: end bracket of block lines up with beginning bracket. Rowson: end bracket of block lines up with enlosing if, while, for, switch.

30 DCS/100: wk 5 p.30/33 Proper indentation Advanced editors (eg emacs) give machine support for indentation.

31 DCS/100: wk 5 p.31/33 Debugging Second step LOCALISE THE PROBLEM Put in print statements to tell you how you are going through the code. out.writeln("we got to here"); Print out values of variables, so that you can compare them with what you expect. Use these to find out where in the program the problem is.

32 DCS/100: wk 5 p.32/33 Debugging Third step FIX THE PROBLEM Once you have the problem localised, put in more print statements systematically so that you can trace the code. Use your knowledge of what the program is actually doing versus what it should be doing to fix it.

33 This Week By the end of the week you should be able to: write programs that follow instructions repeatedly where the number of times is not known at the outset explain what is meant by for loops and while loops, their similarities and differences trace the execution of programs containing loops Reading Brinch Hansen chapters 4 and 5. Computing Without Computers chapter 6 (ignoring sections on recursion for now) DCS/100: wk 5 p.33/33

DCS/100: Procedural Programming

DCS/100: wk 3 p.1/50 DCS/100: Procedural Programming Week 3: Making Decisions Queen Mary, University of London DCS/100: wk 3 p.2/50 Last Week From last week you should be able to explain and write programs