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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
C203 - C and C based languages - Conditionals and Loops
  [4337] Learning to program sample program - past its prime, but still useful - (2014-12-02)
  [4323] Learning to program - Loop statements such as while - (2014-11-22)
  [4322] Learning to Program - the conditional statement (if) - (2014-11-21)
  [3384] Loops - a comparison of goto, while and for - (2011-08-10)
  [3243] Breaking the running sequence - an introduction to conditional statements and loops - (2011-04-11)
  [3200] How a for loop works Java, Perl and other languages - (2011-03-12)
  [3121] New year, new C Course - (2011-01-05)
  [3004] Increment operators for counting - Perl, PHP, C and others - (2010-10-18)
  [2570] Function Prototypes in C - (2010-01-11)
  [2002] New C Examples - pointers, realloc, structs and more - (2009-01-20)
  [1696] Saying NOT in Perl, PHP, Python, Lua ... - (2008-07-04)
  [1582] Ruby, C, Java and more - getting out of loops - (2008-03-19)
  [1220] for loop - how it works (Perl, PHP, Java, C, etc) - (2007-06-06)
  [1191] Smart English Output - via PHP and Perl ? : operator - (2007-05-18)
  [962] Breaking a loop - Ruby and other languages - (2006-12-03)
  [353] Wimbledon Neck - (2005-06-20)

H104 - PHP - Control Statements
  [3914] While, for, foreach or something else to loop. - (2012-11-06)
  [3895] Flowchart to program - learning to program with Well House - (2012-10-14)
  [2912] Predictions for the seagull population - (2010-08-09)
  [2304] Extracting real data from an exported file in PHP or Perl - (2009-07-25)
  [1825] Question Mark - Colon operator (Perl and PHP) - (2008-10-08)
  [1477] Decisions - small ones, or big ones? - (2007-12-18)
  [1199] Testing for one of a list of values. - (2007-05-22)
  [863] Double and Triple equals operator in PHP - (2006-09-12)
  [657] The ternary operator in Python - (2006-03-25)
  [421] Don't repeat code - use loops or functions - (2005-08-21)
  [406] Assignment, equality and identity in PHP - (2005-08-08)
  [340] Code and code maintainance efficiency - (2005-06-08)

J704 - Java - Loops and Conditional Statements
  [4427] Java example - for loop and conditionals from course exercise - (2015-02-16)
  [4346] A behaviour driven example of writing a Java program - (2014-12-09)
  [3038] Setting up individual variables, and arrays, in Java - some commented examples - (2010-11-09)
  [2734] for and foreach in Java - (2010-04-22)

P204 - Perl - Conditionals and Loops
  [4031] Showing what programming errors look like - web site pitfall - (2013-03-06)
  [2832] Are you learning Perl? Some more examples for you! - (2010-06-27)
  [2711] For loop - checked once, or evety time? Ruby v Perl comparison and contrast - (2010-04-07)
  [2550] Do not copy and paste code - there are much better ways - (2009-12-26)
  [2351] Ternary operators alternatives - Perl and Lua lazy operators - (2009-08-12)
  [1727] Equality and looks like tests - Perl - (2008-07-29)
  [1607] Learning to program in Perl - (2008-04-11)
  [1468] Lexical v Arithemetic testing, Bash and Perl - (2007-12-11)
  [930] -> , >= and => in Perl - (2006-11-18)

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

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

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

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


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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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