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For 2023 (and 2024 ...) - we are now fully retired from IT training.
We have made many, many friends over 25 years of teaching about Python, Tcl, Perl, PHP, Lua, Java, C and C++ - and MySQL, Linux and Solaris/SunOS too. Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own risk.

Lisa and I (Graham) now live in what was our training centre in Melksham - happy to meet with former delegates here - but do check ahead before coming round. We are far from inactive - rather, enjoying the times that we are retired but still healthy enough in mind and body to be active!

I am also active in many other area and still look after a lot of web sites - you can find an index ((here))
Does a for loop evaluate its end condition once, or on every iteration?

All the languages that we teach have a for loop or the equivalent, which is a clean way of repeating a block of code with a rising or falling index number. It's used in many circumstances - for example in iterating through the months of the year (for m goes from 1 to 12) of in stepping through all the members of an ordered [list / array / table] *

In many cases, the end condition in this type of structure is evaluated before every iteration of the loop so that a changing end condition will be reflected immediatley. Look at this (Perl)

  $j = 8;
  for ($k=0; $k<$j; $k++) {
    print "Value $k\n";
    $j--;
    }


which runs like this:

  munchkin:laug11 grahamellis$ perl ph
  Value 0
  Value 1
  Value 2
  Value 3
  munchkin:laug11 grahamellis$


There's a similar structure / setup in PHP, Java, C and C++. See previous article [here] for a diagram and further example. And it applies to a Tcl for loop too - there's an example [here].

However, in some languages the loop terminator is evaluated once before the loop is entered, and no matter how the termination condition / value is changed while the loop is running, the for statement knows the iteration count from the start %. Here's a Ruby example:

  j = 8
  for i in 0..j
    puts "Value of loop variable is #{i}"
    j -= 1
  end


which runs like this:

  munchkin:laug11 grahamellis$ ruby rx
  Value of loop variable is 0
  Value of loop variable is 1
  Value of loop variable is 2
  Value of loop variable is 3
  Value of loop variable is 4
  Value of loop variable is 5
  Value of loop variable is 6
  Value of loop variable is 7
  Value of loop variable is 8
  munchkin:laug11 grahamellis$


In Lua, too, the loop count is evaluated once before the loop is entered, and that's the iteration count used. From this week's Lua course there's an example showing that - [here] and another [here].

In Python, too, the for and range / xrange construct is a single evaluation at the start. See an example of that [here].

* - Use "list", "array" or "table" depending on the language you're programming in - they are different words for a sequentially numerically indexed (ordere) collection.

% - In all languages, there is at least one mechanism such as break or last to exit prematurley. Rather more dramatically, return and exit will get you out of a loop and more besides.
(written 2011-08-18, updated 2012-04-15)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
Y103 - Python - Conditionals and Loops
  [299] What - no switch or case statement? - (2005-05-03)
  [353] Wimbledon Neck - (2005-06-20)
  [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)
  [1477] Decisions - small ones, or big ones? - (2007-12-18)
  [1661] Equality, sameness and identity - Python - (2008-05-31)
  [1696] Saying NOT in Perl, PHP, Python, Lua ... - (2008-07-04)
  [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)
  [3200] How a for loop works Java, Perl and other languages - (2011-03-12)
  [3439] Python for loops - applying a temporary second name to the same object - (2011-09-14)
  [3558] Python or Lua - which should I use / learn? - (2011-12-21)
  [3620] Finding the total, average, minimum and maximum in a program - (2012-02-22)
  [3762] Learning to program - the if statement. Python. - (2012-06-12)
  [3895] Flowchart to program - learning to program with Well House - (2012-10-14)
  [4092] Identity in Python - (2013-05-17)
  [4210] If elif elif elif - multiway selection in Python - (2013-11-16)
  [4322] Learning to Program - the conditional statement (if) - (2014-11-21)
  [4323] Learning to program - Loop statements such as while - (2014-11-22)
  [4402] Finding sum, minimum, maximum and average in Python (and Ruby) - (2015-01-19)
  [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)

U103 - Lua - Conditionals and loops
  [1685] Short circuit evaluation (lazy operator) in Lua - (2008-06-22)
  [1738] Clean code, jump free (Example in Lua) - (2008-08-06)
  [2351] Ternary operators alternatives - Perl and Lua lazy operators - (2009-08-12)
  [2454] For loops in Lua - (2009-10-14)
  [2455] Lua examples - coroutines, error handling, objects, etc - (2009-10-15)
  [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)

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)
  [2261] Tcl - nice and nasty - (2009-06-29)
  [2471] A short form of if ... then ... else - (2009-10-23)
  [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)

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)
  [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)
  [1587] Some Ruby programming examples from our course - (2008-03-21)
  [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)
  [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)
  [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)
  [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)

P204 - Perl - Conditionals and Loops
  [930] -> , >= and => in Perl - (2006-11-18)
  [1191] Smart English Output - via PHP and Perl ? : operator - (2007-05-18)
  [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)
  [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)
  [3004] Increment operators for counting - Perl, PHP, C and others - (2010-10-18)
  [4031] Showing what programming errors look like - web site pitfall - (2013-03-06)

J704 - Java - Loops and Conditional Statements
  [2734] for and foreach in Java - (2010-04-22)
  [3038] Setting up individual variables, and arrays, in Java - some commented examples - (2010-11-09)
  [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)

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)

C203 - C and C based languages - Conditionals and Loops
  [2002] New C Examples - pointers, realloc, structs and more - (2009-01-20)
  [2570] Function Prototypes in C - (2010-01-11)
  [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)


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Horse's mouth home
Forward to
Perl - making best use of the flexibility, but also using good coding standards
Some other Articles
Open Source Training Schedule - learn a programming language - in Autumn 2011 or 2012
$ is atomic and % and @ are molecular - Perl
From fish, loaves and apples to money, plastic cards and BACS (Perl references explained)
Perl - making best use of the flexibility, but also using good coding standards
Does a for loop evaluate its end condition once, or on every iteration?
Tables as Objects in Lua - a gentle introduction to data driven programming
Parallel but not really parallel. Moving game characters. Coroutines in Lua.
The difference between lists and strings - Tcl
Rodwell Trail, Weymouth
What costs 8.20 from Melksham, or 22.30 via Chippenham?
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This is a page archived from The Horse's Mouth at http://www.wellho.net/horse/ - the diary and writings of Graham Ellis. Every attempt was made to provide current information at the time the page was written, but things do move forward in our business - new software releases, price changes, new techniques. Please check back via our main site for current courses, prices, versions, etc - any mention of a price in "The Horse's Mouth" cannot be taken as an offer to supply at that price.

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