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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.
String to number conversion with error trapping in Ruby

You've read in a string of text in Ruby - perhaps the user's input, or perhaps from a file. You know that string shoould contain a number - so you use the to_i method to do the conversion. But there's a problem - if the string doesn't start with a number, it won't convert correctly - it will just return a zero. And you'll probably want to know there's a problem so that you can deal with it.

The solution is to use the built in Integer function and pass it the string - i.e. replace:
  value = stuff.to_i
by
  value = Integer(stuff)

When you run that code on a string that does correctly contain a string that converts to an integer, there's no difference to see in how you code works. But if there's an error, Integer throws an exception.

Include your Integer call within a begin and end block .. and within that block, also provide a rescue section. Ruby will start running the main part of the block when it arrives at that piece of the code, and if the main part of the block runs correctly it will skip the rescue section. But if something goes in the main section of the block, it will jump straight away to the rescue code.

Here's an example ... if there is a valid integer in the said string, it is added to total_so_far, and the goodvalues variable is incremented. But if there is not a number in said, then a controlled error message is generated:
begin
  total_so_far += Integer(said)
  goodvalues += 1
rescue
  puts "Sorry - not accepted"
end


The complete example, with some sample results, is [here]

(written 2010-02-01)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
R111 - Ruby - Exceptions.
  [1875] What are exceptions - Python based answer - (2008-11-08)
  [2620] Direct access to object variable (attributes) in Ruby - (2010-02-02)
  [2621] Ruby collections and strings - some new examples - (2010-02-03)
  [2622] Handling unusual and error conditions - exceptions - (2010-02-03)
  [3177] Insurance against any errors - Volcanoes and Python - (2011-02-19)
  [3260] Ruby - a training example that puts many language elements together to demonstrate the whole - (2011-04-23)
  [3433] Exceptions - a fail-safe way of trapping things that may go wrong - (2011-09-11)
  [3435] Sorta sorting a hash, and what if an exception is NOT thrown - Ruby - (2011-09-12)
  [4008] Reading and checking user inputs - first lessons - Ruby - (2013-02-17)
  [4675] Exceptions in Ruby - throwing, catching and using - (2016-05-17)


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Passing code to procedures and yield in Ruby
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Comparing floating point numbers - a word of caution and a solution
Defining a static method - Java, Python and Ruby
String to number conversion with error trapping in Ruby
Neatly formatting results into a table
Constants in Ruby
The Model, View, Controller architecture (MVC) - what, why and how.
Sunday Evening, City of London
Cheat Sheet - what do you need for Ruby on Rails?
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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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