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For 2021 - online Python 3 training - see ((here)).

Our plans were to retire in summer 2020 and see the world, but Coronavirus has lead us into a lot of lockdown programming in Python 3 and PHP 7.
We can now offer tailored online training - small groups, real tutors - works really well for groups of 4 to 14 delegates. Anywhere in the world; course language English.

Please ask about private 'maintenance' training for Python 2, Tcl, Perl, PHP, Lua, etc.
Learning to program - Loop statements such as while

If your program always ran each statement just once (indeed skipping over statements which were in blocks in false conditions) it would run very quickly and would have little use. You couldn't (for example) run a program which went through a whole series of results from a database query and displayed particular data from each of them (plus perhaps a summary on the end).

while

So all programming languages have the ability to repeat a block of code, and the most fundamentel of these repeats ("loops") looks like this:
  while {some sort of condition is true} then {run a group of statements}

You'll note that this is exactly the same format as the if statement in the previous section, apart from the replacement of the word if by the word while. Operationally, it differs in that once the group of statements in the block has been performed, the program rechecks the condition and if it's still true it runs the block again, keeping doing so until the condition becomes false.

Note

* If the condition is false when first checked, the block isn't performed at all - a loop runs zero or more times
* If the condition never goes false you potentially have an infinite loop that goes on for ever
* Conditions are just the same as the conditions mentioned for the if statement in the language you're learning.

Newcomers to programming sometimes take a few minutes to grasp their first program with a loop statement, as for the first time the code jups backwards as well as forwards as it runs. And they sometimes have trouble working out which statements go where.

* If something may need to be run multiple times, it goes within the block (or within the condition to the while)
* If something only runs once and that's BEFORE the code in the block that may repeat, it goes before the while ("initialisation")
* If something runs once after any repeated code, that goes AFTER the block that may repeat (e.g. printing a total

Your tutor will show you a while loop in the language you're learning at this point, and have you write one too.

breaking out of loops

At times, you'll want to jump out of the middle of a loop and continue running code below - if (for eaxmple) you've identified inoming record that you were looking for from a stream of data, or if you have reached a threshold. So languages provide you with a statement that you can put witin a loop to get out of that loop even though the condition at the top hasn't been checked and gone false. The keyword used is usually break but sometimes (in some languages) it's last.

Putting a break into a loop to be run unconditionally would be rather pointless, so you'll find that any break statements will be within a conditional statement such as i(but not limited to) an if within the loop.

Most languages also support a continue statement (sometimes next) which allows you to skip the rest of the block code and go back up to test the condition straight away. Very useful if you're filtering a stream of data and you've identified a record such as a comment that you want to skip over without further processing.

Some languages have other flow controls in loops to - you may come across redo and retry. Early programming languages supported goto statements (and indeed some still do), but except in exceptional circumstances, their use is discouraged - they make for code that it very difficult to debug or to follow, and often impractical to upgrade when specifications change ... and there are now far better ways.

Other loops

while is the most fundamental loop statement, and it's the natural one for us to choose to introduce on this first day of our "learning to program in ..." course. But there are many other forms of loops that you'll come across later in the course, and in practise you'll find such other loops used more that while. Keyowords are for, foreach, loop, until and repeat.

If you are writing web applications (Java Servlets or JSPs, PHP scripts, or CGI or otherwise server based code in other languages such as Ruby, Python, Tcl, Lua, C or C++) Your whole application will potentially be run in a loop. Each time a user submits a form to your code, (or an automated client calls up your URL), the code is run, in effect in a loop that's controlled by the web server with your code be the innards or the loop. This means that there are times wher code towards the top of your web application may be run after code than appears later in your web application (due to condtionals) even though it doesn't look like it's in a loop at first glance.
(written 2014-11-22)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
J704 - Java - Loops and Conditional Statements
  [353] Wimbledon Neck - (2005-06-20)
  [1220] for loop - how it works (Perl, PHP, Java, C, etc) - (2007-06-06)
  [1582] Ruby, C, Java and more - getting out of loops - (2008-03-19)
  [1696] Saying NOT in Perl, PHP, Python, Lua ... - (2008-07-04)
  [2734] for and foreach in Java - (2010-04-22)
  [3038] Setting up individual variables, and arrays, in Java - some commented examples - (2010-11-09)
  [3200] How a for loop works Java, Perl and other languages - (2011-03-12)
  [3397] Does a for loop evaluate its end condition once, or on every iteration? - (2011-08-18)
  [4322] Learning to Program - the conditional statement (if) - (2014-11-21)
  [4346] A behaviour driven example of writing a Java program - (2014-12-09)
  [4427] Java example - for loop and conditionals from course exercise - (2015-02-16)

R104 - Ruby - Control Structures
  [960] 1st, 2nd, 3rd revisited in Ruby - (2006-12-02)
  [962] Breaking a loop - Ruby and other languages - (2006-12-03)
  [985] Equality in Ruby - == eql? and equal? - (2006-12-14)
  [995] Ruby's case - no break - (2006-12-17)
  [1163] A better alternative to cutting and pasting code - (2007-04-26)
  [1587] Some Ruby programming examples from our course - (2008-03-21)
  [1738] Clean code, jump free (Example in Lua) - (2008-08-06)
  [1870] What to do with a huge crop of apples - (2008-11-04)
  [1887] Ruby Programming Course - Saturday and Sunday - (2008-11-16)
  [1891] Ruby to access web services - (2008-11-16)
  [1904] Ruby, Perl, Linux, MySQL - some training notes - (2008-11-23)
  [2287] Learning to program in Ruby - examples of the programming basics - (2009-07-15)
  [2471] A short form of if ... then ... else - (2009-10-23)
  [2619] Passing code to procedures and yield in Ruby - (2010-02-02)
  [2711] For loop - checked once, or evety time? Ruby v Perl comparison and contrast - (2010-04-07)
  [2892] Alternative loops and conditionals in Ruby and Perl - (2010-07-28)
  [2975] Why do I need brackets in Ruby ... or Perl, Python, C or Java - (2010-09-29)
  [3156] Splitting data reading code from data processing code - Ruby - (2011-02-04)
  [3158] Ruby training - some fresh examples for string handling applications - (2011-02-05)
  [3159] Returning multiple values from a function call in various languages - a comparison - (2011-02-06)
  [3253] Is this number between? Does this list include? - Ruby - (2011-04-18)
  [3254] Multiple inputs, multiple out, ruby functions - (2011-04-19)
  [3422] Assigning values to variables within other statements - Ruby - (2011-09-07)
  [3619] Ruby v Perl - a comparison example - (2012-02-21)
  [3620] Finding the total, average, minimum and maximum in a program - (2012-02-22)
  [3769] Muttable v immutable and implications - Ruby - (2012-06-20)
  [4370] Conditionals, loops and methods in Ruby - a primer with simple examples - (2014-12-29)
  [4402] Finding sum, minimum, maximum and average in Python (and Ruby) - (2015-01-19)
  [4503] Separating your code for easier testing, understanding and re-use; example in Ruby - (2015-06-02)
  [4504] Where does Ruby load modules from, and how to load from current directory - (2015-06-03)
  [4674] Alternating valuses / flip-flop / toggle - example in Ruby - (2016-05-17)

C203 - C and C based languages - Conditionals and Loops
  [1191] Smart English Output - via PHP and Perl ? : operator - (2007-05-18)
  [2002] New C Examples - pointers, realloc, structs and more - (2009-01-20)
  [2570] Function Prototypes in C - (2010-01-11)
  [3004] Increment operators for counting - Perl, PHP, C and others - (2010-10-18)
  [3121] New year, new C Course - (2011-01-05)
  [3243] Breaking the running sequence - an introduction to conditional statements and loops - (2011-04-11)
  [3384] Loops - a comparison of goto, while and for - (2011-08-10)
  [4337] Learning to program sample program - past its prime, but still useful - (2014-12-02)

T203 - Tcl/Tk - Conditionals and Loops
  [210] Joining lists in Tcl. Indirect variables in Tcl. - (2005-02-12)
  [1401] Tcl - using [] or {} for conditions in an if (and while) - (2007-10-23)
  [1477] Decisions - small ones, or big ones? - (2007-12-18)
  [2261] Tcl - nice and nasty - (2009-06-29)
  [2681] Tcl - a great engineering language - (2010-03-17)
  [3189] Tcl - the danger of square brackets in a while command - (2011-03-02)
  [3570] Trapping errors in Tcl - the safety net that catch provides - (2012-01-06)
  [3571] Comparing loop commands in Tcl - (2012-01-06)
  [4455] Working out distance between places, using OS grid references and a program in Tcl - (2015-03-11)

P204 - Perl - Conditionals and Loops
  [930] -> , >= and => in Perl - (2006-11-18)
  [1468] Lexical v Arithemetic testing, Bash and Perl - (2007-12-11)
  [1607] Learning to program in Perl - (2008-04-11)
  [1727] Equality and looks like tests - Perl - (2008-07-29)
  [2351] Ternary operators alternatives - Perl and Lua lazy operators - (2009-08-12)
  [2550] Do not copy and paste code - there are much better ways - (2009-12-26)
  [2832] Are you learning Perl? Some more examples for you! - (2010-06-27)
  [3895] Flowchart to program - learning to program with Well House - (2012-10-14)
  [4031] Showing what programming errors look like - web site pitfall - (2013-03-06)

U103 - Lua - Conditionals and loops
  [1685] Short circuit evaluation (lazy operator) in Lua - (2008-06-22)
  [2454] For loops in Lua - (2009-10-14)
  [2455] Lua examples - coroutines, error handling, objects, etc - (2009-10-15)
  [3558] Python or Lua - which should I use / learn? - (2011-12-21)
  [3686] The goto statement in Lua - (2012-04-06)
  [4272] Lazy operators in Lua - what they mean, and examples - (2014-05-05)
  [4574] repeat until in Lua - a one or more rather than a zero or more loop - (2015-11-05)

Y103 - Python - Conditionals and Loops
  [299] What - no switch or case statement? - (2005-05-03)
  [657] The ternary operator in Python - (2006-03-25)
  [668] Python - block insets help with documentation - (2006-04-04)
  [788] New - Conditional expressions in Python 2.5 - (2006-07-01)
  [835] Python - when to use the in operator - (2006-08-16)
  [909] Python is like a narrowboat - (2006-10-30)
  [1201] No switch in Python - (2007-05-23)
  [1661] Equality, sameness and identity - Python - (2008-05-31)
  [2778] Learning to program in Python 2 ... and / or in Python 3 - (2010-05-24)
  [2899] Groupsave tickets - 3 or 4 train tickets for the price of 2 - (2010-08-02)
  [3083] Python - fresh examples from recent courses - (2010-12-11)
  [3439] Python for loops - applying a temporary second name to the same object - (2011-09-14)
  [3762] Learning to program - the if statement. Python. - (2012-06-12)
  [4092] Identity in Python - (2013-05-17)
  [4210] If elif elif elif - multiway selection in Python - (2013-11-16)
  [4541] Setting up and tearing down with the Python with keyword - (2015-10-16)
  [4545] Method, Class, Module, Package - how to they relate in Python? - (2015-10-17)
  [4713] Equality (in Python) - (2016-10-30)
  [4723] Conditional operators in Python - (2016-11-05)

H104 - PHP - Control Statements
  [340] Code and code maintainance efficiency - (2005-06-08)
  [406] Assignment, equality and identity in PHP - (2005-08-08)
  [421] Don't repeat code - use loops or functions - (2005-08-21)
  [863] Double and Triple equals operator in PHP - (2006-09-12)
  [1199] Testing for one of a list of values. - (2007-05-22)
  [1825] Question Mark - Colon operator (Perl and PHP) - (2008-10-08)
  [2304] Extracting real data from an exported file in PHP or Perl - (2009-07-25)
  [2912] Predictions for the seagull population - (2010-08-09)
  [3914] While, for, foreach or something else to loop. - (2012-11-06)

Q100 - Object Orientation and General technical topics - Learning to Progam
  [116] The next generation of programmer - (2004-11-13)
  [1605] Learning and understanding scripting programming techniques - (2008-04-08)
  [1963] Best source to learn Java (or Perl or PHP or Python) - (2008-12-28)
  [1985] Learning to program as a part of your job - (2009-01-10)
  [2001] I have not programmed before, and need to learn - (2009-01-19)
  [2048] Learning to program in PHP, Python, Java or Lua ... - (2009-02-19)
  [2092] Tracking difficult bugs, the programmer / customer relationship - (2009-03-20)
  [2286] New to programming? It is natural (but needless) for you to be nervous - (2009-07-14)
  [2294] Can you learn to program in 4 days? - (2009-07-16)
  [2326] Learn a new programming language this summer. - (2009-08-06)
  [2504] Learning to program in ... - (2009-11-15)
  [2505] I almost put the bins out this morning - (2009-11-16)
  [2898] Programming Standards from the start! - (2010-08-02)
  [2973] Learning to program - where to start if you have never programmed before - (2010-09-28)
  [3120] Learning to write good programs in C and C++ - separating out repeated code - (2011-01-04)
  [3551] Some terms used in programming (Biased towards Python) - (2011-12-12)
  [4008] Reading and checking user inputs - first lessons - Ruby - (2013-02-17)
  [4318] Learning to Program - how we start to teach you at Well House Consultants - (2014-11-16)
  [4324] Learning to program - variables and constants - (2014-11-22)
  [4325] Learning to program - what are algorithms and design patterns? - (2014-11-22)
  [4326] Learning to program - comments, documentation and test code - (2014-11-22)
  [4575] Learning not just what a program does, but how to design it in the first place. - (2015-11-06)


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