I was reading through Chris Pine's Learn to Program and I encountered this weird code snippet in Chapter 10: Blocks and Procs:
def doUntilFalse firstInput, someProc
input = firstInput
output = firstInput
while output
input = output
output = someProc.call input
end
input
end
buildArrayOfSquares = Proc.new do |array|
lastNumber = array.last
if lastNumber <= 0
false
else
array.pop # Take off the last number...
array.push lastNumber*lastNumber # ...and replace it with its square...
array.push lastNumber-1 # ...followed by the next smaller number.
end
end
What is the condition being checked for, in the above while loop? It doesn't seem to be a shorthand for while output == true.
while output means run the loop till while has any values other than false or nil. As in Ruby everything is a truthy value except these two.
Related
Can anyone tell me why this program is not producing an output? The output it should be producing is: Line read: 0
Line read: 1 Line read: 2 Line read: 3 and so on.
So far, I am not getting an output even though I have fixed a number of bugs. Any help or suggestions would be much appreciated.
# takes a number and writes that number to a file then on each line
# increments from zero to the number passed
def write(aFile, number)
# You might need to fix this next line:
aFile.puts(number)
index = 0
while (index < number)
aFile.puts(index.to_s)
index += 1
end
end
# Read the data from the file and print out each line
def read(aFile)
# Defensive programming:
count = aFile.gets
if (is_numeric?(count))
count = count.to_i
index = 0
while (index < count)
line = aFile.gets
puts "line read: " + line
index+=1
end
end
end
# Write data to a file then read it in and print it out
def main
aFile = File.new("mydata.txt", "w") # open for writing
write(aFile, 10)
aFile.close
aFile = File.new("mydata.txt", "r")
read(aFile)
aFile.close
end
# returns true if a string contains only digits
def is_numeric?(obj)
if /[^0-9]/.match(obj) == nil
true
end
false
end
main
Your code isn't written in the Ruby way.
This is how I'd write it if I wanted to closely mimic your code's logic:
# takes a number and writes that number to a file then on each line
# increments from zero to the number passed
def write_data(fname, counter)
File.open(fname, 'w') do |fo|
fo.puts(counter)
counter.times do |n|
fo.puts n
end
end
end
# returns true if a string contains only digits
def is_numeric?(obj)
obj[/^\d+$/]
end
# Read the data from the file and print out each line
def read_data(fname)
File.open(fname) do |fi|
counter = fi.gets.chomp
if is_numeric?(counter)
counter.to_i.times do |n|
line_in = fi.gets
puts 'Line read: %s' % line_in
end
end
end
end
# Write data to a file then read it in and print it out
DATA_FILE = 'mydata.txt'
write_data(DATA_FILE, 10)
read_data(DATA_FILE)
Which outputs:
Line read: 0
Line read: 1
Line read: 2
Line read: 3
Line read: 4
Line read: 5
Line read: 6
Line read: 7
Line read: 8
Line read: 9
Notice these things:
Method (or variable) names are not in camelCase in Ruby, they're snake_case. ItsAReadabiltyThing.
Ruby encourages us to use a block when opening files for reading or writing, to automatically close the file when we're finished with it. Leaving danging file handles opened then not closed, in a loop, in a long-running program, is a great way for your program to crash in a way that's hard to figure out. SO has many questions that resulted from doing that. This is from the IO#open documentation:
With no associated block, ::open is a synonym for ::new. If the optional code block is given, it will be passed io as an argument, and the IO object will automatically be closed when the block terminates. In this instance, ::open returns the value of the block.
Usually you'll see code use File.open instead of IO.open, mostly out of habit in Ruby coders. File inherits from IO and adds some additional file-oriented methods to the class, so it's a little more full-featured.
Ruby has many methods that help us avoid using while loops. Getting the counters wrong or missing a condition that should terminate the loop, is all too common in programming, so Ruby makes it easy to loop "n times" or to iterate over all the elements in an array. The times method accomplishes that nicely.
String's [] method is really powerful and makes it easy to look at the contents of a string and apply a pattern or a slice. Using /^\d+$/ checks the entire string to make sure all characters are digits, so some_string[/^\d+$/] is a shorter version than what you're doing and accomplishes the same thing, returns a "truthy" value.
We don't use a main method. That's old-school Pascal, C or Java and is artificially structured. Ruby's a little more friendly than that.
Instead of using
3.times do |n|
puts n
end
# >> 0
# >> 1
# >> 2
I'd probably use
puts (0..(3 - 1)).to_a * "\n"
# >> 0
# >> 1
# >> 2
just because I tend to think in Perl terms. It's another old habit.
I found 2 errors. Fixing those errors gives you desired output.
Error #1.
Your method is_numeric? always returns false. Even if your condition is true. The last line of the method is false and therefore the whole method ALWAYS returns false.
You can fix it in 2 steps.
Step #1:
if /[^0-9]/.match(obj) == nil
true
else
false
end
It's not a good practice to return booleans within conditional. You can simplify it this way:
def is_numeric?(obj)
/[^0-9]/.match(obj) == nil
end
or even better
def is_numeric?(obj)
/[^0-9]/.match(obj).nil?
end
Error #2 is inside your read method. If you try to output the value of count after you read it from the file it gives you "10\n". That \n at the end messes you up.
To get rid of \n when you read from the file you could possibly use chomp. So then your reading line would be:
count = aFile.gets.chomp
and the rest works like magic
I'm learning Ruby with 'Learn to Program' by Chris Pine. On chapter 10 I should write a program where the user types as many words as he like and when he's done, he can just press Enter on an empty line and exit.
I came up with this:
puts "Type whatever you want!"
index = 0
word = ''
array = []
while word != nil
word << gets.chomp
array[index] = word
index = index + 1
end
puts ''
puts array.sort
But that doesn't work. What did I miss? Is there another way I could define word without having to repeat it?
The word will not have nil value. It will be an empty string. So you need to check for that:
while word != ""
# or even better
while !word.empty?
Also, you are adding everything to your word. You probably want to assign to it instead:
word = gets.chomp
Per author's comment:
begin
# your code here
end while !word.empty?
# OR more readable
begin
# your code here
end until word.empty?
It seems like there's a simpler solution, if I'm reading the question correctly.
You could do something like this:
user_input = gets.chomp.split(" ").sort
ex)
input: bananas clementine zebra tree house plane mine
output: ["bananas", "clementine", "house", "mine", "plane", "tree", "zebra"]
Here's a simple loop that you could do just for kicks:
arr = []
arr << $_.strip until gets =~ /^\s*$/
puts arr.sort
$_ is a special variable that evaluates to the last input read from STDIN. So basically this reads "Call gets and check if the input is just spaces. If it is then break out of the loop, otherwise append the last input with whitespace removed value onto the array and continue looping."
Or even more fun, a one liner:
puts [].tap {|arr| arr << $_.strip until gets =~ /^\s*$/}.sort
Basically same thing as above except using tap to initialize the variable.
To answer your questions:
Is there another way I could define word without having to repeat it?
Use side effects of assignment. In ruby when you assign a variable the return value of that assignment is the assigned variable, as in:
irb(main):001:0> (variable = 2) == 2
=> true
The idea would be to put the assignment in the your conditional. If I were to write something like this in a comprehensible loop, as opposed to those above, I'd write something like this:
arr = []
while !(word = gets.strip).empty?
arr << word
end
puts arr.sort
Using loop might simplify the code:
a = []
loop do
input = gets.chomp
if input.empty?
break
else
a << input
end
end
a.sort!
puts a
Let's say that I have this simple if-elsif-else block of code.
def
# some code...
input = get.chomp
if input == 1
puts "foo"
elsif input == 2
puts "bar"
else
# exit
end
# some more code...
end
How do I tell program to go back and ask for input again for cases 1 and 2 and continue with code within this method if else is triggered? I do not want to go back to start of the method, instead I just want to back to input variable declaration.
def
# some code...
loop do
input = get.chomp
if input == 1
puts "foo"
break
elsif input == 2
puts "bar"
break
end
end
# some more code...
end
Note: Your two if/elsif conditions will never be satisfied.
# main procedure
# defined here so other functions could be declared after
# the main procedure is called at the bottom
def main
loop do
puts "Insert a number"
input = gets.chomp.to_i
if isValidInput input
puts case input
when 1
"foo"
when 2
"bar"
end
break
end
end #loop
puts "Other code would execute here"
end
# Validity Checker
# makes sure your input meets your condition
def isValidInput(input)
if [1,2].include? input
return true
end
return false
end
main
Is there a way to make this code shorter and simpler?
loop do
if possibleSet.split(" ").map(&:to_i).any? {|e| (e<0 || e>12)}
print "Please enter valid numbers (between 1 and 12): "
possibleSet = gets
errorinput = false
else
errorinput = true
end
break if errorinput
end
Refactored a bit :)
loop do
print "Please enter valid numbers (between 1 and 12): "
possibleSet = gets.chomp
break unless possibleSet.split(" ").map(&:to_i).any? {|e| (e<0 || e>12)}
end
The code below will check input for correctness:
input = loop do
print "Please enter valid numbers (between 1 and 12): "
# ⇓⇓⇓ as many spaces as user wants
input = gets.chomp.split(/\s+/).map(&:to_i) rescue []
break input unless input.empty? || input.any? { |i| !(0..12).include? i }
end
This parses the user input in an array (not exactly the same behavior, but I hope it is cleaner and you can work from there)
set = []
until set.all? {|i| (1..11).include?(i) } && !set.empty? do
set = gets.split(' ').map(&:to_i)
end
I'm messing around in Ruby some more. I have a file containing a class with two methods and the following code:
if __FILE__ == $0
seq = NumericSequence.new
puts "\n1. Fibonacci Sequence"
puts "\n2. Pascal\'s Triangle"
puts "\nEnter your selection: "
choice = gets
puts "\nExcellent choice."
choice = case
when 1
puts "\n\nHow many fibonacci numbers would you like? "
limit = gets.to_i
seq.fibo(limit) { |x| puts "Fibonacci number: #{x}\n" }
when 2
puts "\n\nHow many rows of Pascal's Triangle would you like?"
n = gets.to_i
(0..n).each {|num| seq.pascal_triangle_row(num) \
{|row| puts "#{row} "}; puts "\n"}
end
end
How come if I run the code and supply option 2, it still runs the first case?
Your case syntax is wrong. Should be like this:
case choice
when '1'
some code
when '2'
some other code
end
Take a look here.
You also need to compare your variable against strings, as gets reads and returns user input as a string.
Your bug is this: choice = case should be case choice.
You're providing a case statement with no "default" object, so the first clause, when 1, always returns true.
Effectively, you've written: choice = if 1 then ... elsif 2 then ... end
And, as Mladen mentioned, compare strings to strings or convert to int: choice = gets.to_i