In Lua what does an if statement with only one argument mean? - if-statement

I've been taught to program in Java. Lua is new to me and I've tried to do my homework but am not sure what an if statement of the following nature means.
The code is as follows:
local function getMinHeight(self)
local minHeight = 0
for i=1, minimizedLines do
local line = select(9+i, self:GetRegions())
**if(line) then
minHeight = minHeight + line:GetHeight() + 2.5
end**
end
if(minHeight == 0) then
minHeight = select(2, self:GetFont()) + 2.5
end
return minHeight
end
The if statement with the ** before and after is the part I'm not sure about. I don't know what the if statement is checking. If the line is not nil? If the line exists? If what?

In Lua, anything that's not nil or false evaluates to true in a conditional.
If the line is not nil? If the line exists?
Yes to both, because they kinda mean the same thing.
The select function returns a specific argument from it's list of arguments. It's used primarily with ..., but in this case it's being used to select the (i+9)th value returned by self:GetRegions. If there is no such value (for instance, if GetRegions only returns 5 values), then select returns nil.
if(line) is checking to see that it got a value back from select.
if(line) is being used as a shortcut for if(line ~= nil), since nil evaluates to false in a conditional.
It's worth pointing out that this shortcut is not always appropriate. For instance, we can iterate all the values in a table like this:
key, val = next(lookup)
while key do
print(key, val)
key, val = next(lookup, key)
end
However, this will fail if one of the table's keys happens be false:
lookup = {
["fred"] = "Fred Flinstone",
[true] = "True",
[false] = "False",
}
So we have to explicitly check for nil:
key, val = next(lookup)
while key ~= nil do
print(key, val)
key, val = next(lookup, key)
end

As Mud says, in lua anything other than nil and false is considered truthy. So the if above will pass as long as line is not nil or false.
That said, it worries me a bit the way you have phrased the question - "an if with only one argument".
First, it's not called "argument" - it's called expression. And in most languages is always one. In java, for example, you could do something like this:
bool found = false
...
if(found) {
...
}
ifs only care about the final value of the expression; they don't care whether it's a single variable or a more complex construction.

Related

Is there an alternative to an if statement in Lua?

I would like to know if there is a alternative to using a normal if statement in lua. For example, in java there are switch statements but I cant seem to find that in Lua
Lua lacks a C-style switch statement.
A simple version of a switch statement can be implemented using a table to map the case value to an action. This is very efficient in Lua since tables are hashed by key value which avoids repetitive if then ... elseif ... end statements.
action = {
[1] = function (x) print(1) end,
[2] = function (x) z = 5 end,
["nop"] = function (x) print(math.random()) end,
["my name"] = function (x) print("fred") end,
}
The frequently used pattern
local var; if condition then var = x else var = y end
can be shortened using an and-or "ternary" substitute if x is truthy:
local var = condition and x or y
if test == nil or test == false then return 0xBADEAFFE else return test end
Can be shorten up to...
return test or 0xBADEAFFEE
This works even where you dont can do: if ... then ... else ... end
Like in a function...
print(test or 0xBADEAFFE)
-- Output: 3135156222
...or fallback to a default if an argument is ommited...
function check(test)
local test = test or 0xBADEAFFE
return test
end
print(check())
-- Returns: 3135156222

nextflow (groovy) check if item in Channel list

I am struggling to use an if/else on a containsAll() statement. It returns the correct true false value when tested with println(), but when put in an if statement it seems to always evaluate to true -- see below.
def examine_phenotype(pheno){
condition_values = \
Channel
.fromPath(pheno)
.splitCsv(header: true, sep: ',')
.map{ row ->
def condition = row.condition
return condition
}
.toList().view()
println(condition_values.containsAll('control'))
if(condition_values.containsAll('control')){
exit 1, "eval true"
}else{
exit 1, "eval false"
}
}
Console output for two different files, one with 'control' and one without 'control' in the column 'condition', which is the point of the function.
[normal, normal, normal, tumor, tumor, tumor]
DataflowInvocationExpression(value=false)
eval true
[control, control, control, tumor, tumor, tumor]
DataflowInvocationExpression(value=true)
eval true
Using collect() instead of toList() where each item within condition_values is enclosed with single quotes did not resolve the issue either. The clue might be in DataflowInvocationExpression but I am not up to speed on Groovy yet and am not sure how to proceed.
Testing the conditional within the function was not working, but applying filter{} and ifEmpty{} was able to produce the desired check:
ch_phenotype = Channel.empty()
if(pheno_path){
pheno_file = file(pheno_path)
ch_phenotype = examine_phenotype(pheno_file)
ch_phenotype.filter{ it =~/control/ }
.ifEmpty{ exit 1, "no control values in condition column"}
}
def examine_phenotype(pheno){
Channel
.fromPath(pheno)
.splitCsv(header: true, sep: ',')
.map{ row ->
def condition = row.condition
return condition
}
.toList()
}

Lua : attempt to index a nil value; avoiding errors in conditionals

Let's say I have a giant table, something like:
test.test[1].testing.test.test_test
The table isn't guaranteed to exist. Neither are the tables containing it. I would like to just be able to do:
if test.test[1].testing.test.test_test then
print("it exits!")
end
But of course, this would give me an "Attempt to index ? (a nil value)" error if any of the indices aren't yet defined. So many times, I'll end up doing something like this:
if test then
if test.test then
if test.test[1] then
if test.test[1].testing then -- and so on
Is there a better, less-tedious way to accomplish this?
You can write a function that takes a list of keys to look up and does whatever action you want if it finds the entry. Here's an example:
function forindices(f, table, indices)
local entry = table
for _,idx in ipairs(indices) do
if type(entry) == 'table' and entry[idx] then
entry = entry[idx]
else
entry = nil
break
end
end
if entry then
f()
end
end
test = {test = {{testing = {test = {test_test = 5}}}}}
-- prints "it exists"
forindices(function () print("it exists") end,
test,
{"test", 1, "testing", "test", "test_test"})
-- doesn't print
forindices(function () print("it exists") end,
test,
{"test", 1, "nope", "test", "test_test"})
As an aside, the functional programming concept that solves this kind of problem is the Maybe monad. You could probably solve this with a Lua implementation of monads, though it wouldn't be very nice since there's no syntactic sugar for it.
You can avoid raising errors by setting an __index metamethod for nil:
debug.setmetatable(nil, { __index=function () end })
print(test.test[1].testing.test.test_test)
test = {test = {{testing = {test = {test_test = 5}}}}}
print(test.test[1].testing.test.test_test)
You also use an empty table:
debug.setmetatable(nil, { __index={} })

Python consecutive if statements [duplicate]

Want to improve this post? Provide detailed answers to this question, including citations and an explanation of why your answer is correct. Answers without enough detail may be edited or deleted.
Is there a ternary conditional operator in Python?
Yes, it was added in version 2.5. The expression syntax is:
a if condition else b
First condition is evaluated, then exactly one of either a or b is evaluated and returned based on the Boolean value of condition. If condition evaluates to True, then a is evaluated and returned but b is ignored, or else when b is evaluated and returned but a is ignored.
This allows short-circuiting because when condition is true only a is evaluated and b is not evaluated at all, but when condition is false only b is evaluated and a is not evaluated at all.
For example:
>>> 'true' if True else 'false'
'true'
>>> 'true' if False else 'false'
'false'
Note that conditionals are an expression, not a statement. This means you can't use statements such as pass, or assignments with = (or "augmented" assignments like +=), within a conditional expression:
>>> pass if False else pass
File "<stdin>", line 1
pass if False else pass
^
SyntaxError: invalid syntax
>>> # Python parses this as `x = (1 if False else y) = 2`
>>> # The `(1 if False else x)` part is actually valid, but
>>> # it can't be on the left-hand side of `=`.
>>> x = 1 if False else y = 2
File "<stdin>", line 1
SyntaxError: cannot assign to conditional expression
>>> # If we parenthesize it instead...
>>> (x = 1) if False else (y = 2)
File "<stdin>", line 1
(x = 1) if False else (y = 2)
^
SyntaxError: invalid syntax
(In 3.8 and above, the := "walrus" operator allows simple assignment of values as an expression, which is then compatible with this syntax. But please don't write code like that; it will quickly become very difficult to understand.)
Similarly, because it is an expression, the else part is mandatory:
# Invalid syntax: we didn't specify what the value should be if the
# condition isn't met. It doesn't matter if we can verify that
# ahead of time.
a if True
You can, however, use conditional expressions to assign a variable like so:
x = a if True else b
Or for example to return a value:
# Of course we should just use the standard library `max`;
# this is just for demonstration purposes.
def my_max(a, b):
return a if a > b else b
Think of the conditional expression as switching between two values. We can use it when we are in a 'one value or another' situation, where we will do the same thing with the result, regardless of whether the condition is met. We use the expression to compute the value, and then do something with it. If you need to do something different depending on the condition, then use a normal if statement instead.
Keep in mind that it's frowned upon by some Pythonistas for several reasons:
The order of the arguments is different from those of the classic condition ? a : b ternary operator from many other languages (such as C, C++, Go, Perl, Ruby, Java, JavaScript, etc.), which may lead to bugs when people unfamiliar with Python's "surprising" behaviour use it (they may reverse the argument order).
Some find it "unwieldy", since it goes contrary to the normal flow of thought (thinking of the condition first and then the effects).
Stylistic reasons. (Although the 'inline if' can be really useful, and make your script more concise, it really does complicate your code)
If you're having trouble remembering the order, then remember that when read aloud, you (almost) say what you mean. For example, x = 4 if b > 8 else 9 is read aloud as x will be 4 if b is greater than 8 otherwise 9.
Official documentation:
Conditional expressions
Is there an equivalent of C’s ”?:” ternary operator?
You can index into a tuple:
(falseValue, trueValue)[test]
test needs to return True or False.
It might be safer to always implement it as:
(falseValue, trueValue)[test == True]
or you can use the built-in bool() to assure a Boolean value:
(falseValue, trueValue)[bool(<expression>)]
For versions prior to 2.5, there's the trick:
[expression] and [on_true] or [on_false]
It can give wrong results when on_true has a false Boolean value.1
Although it does have the benefit of evaluating expressions left to right, which is clearer in my opinion.
1. Is there an equivalent of C’s ”?:” ternary operator?
<expression 1> if <condition> else <expression 2>
a = 1
b = 2
1 if a > b else -1
# Output is -1
1 if a > b else -1 if a < b else 0
# Output is -1
From the documentation:
Conditional expressions (sometimes called a “ternary operator”) have the lowest priority of all Python operations.
The expression x if C else y first evaluates the condition, C (not x); if C is true, x is evaluated and its value is returned; otherwise, y is evaluated and its value is returned.
See PEP 308 for more details about conditional expressions.
New since version 2.5.
An operator for a conditional expression in Python was added in 2006 as part of Python Enhancement Proposal 308. Its form differs from common ?: operator and it looks like this:
<expression1> if <condition> else <expression2>
which is equivalent to:
if <condition>: <expression1> else: <expression2>
Here is an example:
result = x if a > b else y
Another syntax which can be used (compatible with versions before 2.5):
result = (lambda:y, lambda:x)[a > b]()
where operands are lazily evaluated.
Another way is by indexing a tuple (which isn't consistent with the conditional operator of most other languages):
result = (y, x)[a > b]
or explicitly constructed dictionary:
result = {True: x, False: y}[a > b]
Another (less reliable), but simpler method is to use and and or operators:
result = (a > b) and x or y
however this won't work if x would be False.
A possible workaround is to make x and y lists or tuples as in the following:
result = ((a > b) and [x] or [y])[0]
or:
result = ((a > b) and (x,) or (y,))[0]
If you're working with dictionaries, instead of using a ternary conditional, you can take advantage of get(key, default), for example:
shell = os.environ.get('SHELL', "/bin/sh")
Source: ?: in Python at Wikipedia
Unfortunately, the
(falseValue, trueValue)[test]
solution doesn't have short-circuit behaviour; thus both falseValue and trueValue are evaluated regardless of the condition. This could be suboptimal or even buggy (i.e. both trueValue and falseValue could be methods and have side effects).
One solution to this would be
(lambda: falseValue, lambda: trueValue)[test]()
(execution delayed until the winner is known ;)), but it introduces inconsistency between callable and non-callable objects. In addition, it doesn't solve the case when using properties.
And so the story goes - choosing between three mentioned solutions is a trade-off between having the short-circuit feature, using at least Python 2.5 (IMHO, not a problem anymore) and not being prone to "trueValue-evaluates-to-false" errors.
Ternary operator in different programming languages
Here I just try to show some important differences in the ternary operator between a couple of programming languages.
Ternary operator in JavaScript
var a = true ? 1 : 0;
# 1
var b = false ? 1 : 0;
# 0
Ternary operator in Ruby
a = true ? 1 : 0
# 1
b = false ? 1 : 0
# 0
Ternary operator in Scala
val a = true ? 1 | 0
# 1
val b = false ? 1 | 0
# 0
Ternary operator in R programming
a <- if (TRUE) 1 else 0
# 1
b <- if (FALSE) 1 else 0
# 0
Ternary operator in Python
a = 1 if True else 0
# 1
b = 1 if False else 0
# 0
For Python 2.5 and newer there is a specific syntax:
[on_true] if [cond] else [on_false]
In older Pythons, a ternary operator is not implemented but it's possible to simulate it.
cond and on_true or on_false
Though there is a potential problem, which is if cond evaluates to True and on_true evaluates to False then on_false is returned instead of on_true. If you want this behaviour the method is OK, otherwise use this:
{True: on_true, False: on_false}[cond is True] # is True, not == True
which can be wrapped by:
def q(cond, on_true, on_false)
return {True: on_true, False: on_false}[cond is True]
and used this way:
q(cond, on_true, on_false)
It is compatible with all Python versions.
You might often find
cond and on_true or on_false
but this leads to a problem when on_true == 0
>>> x = 0
>>> print x == 0 and 0 or 1
1
>>> x = 1
>>> print x == 0 and 0 or 1
1
Where you would expect this result for a normal ternary operator:
>>> x = 0
>>> print 0 if x == 0 else 1
0
>>> x = 1
>>> print 0 if x == 0 else 1
1
Does Python have a ternary conditional operator?
Yes. From the grammar file:
test: or_test ['if' or_test 'else' test] | lambdef
The part of interest is:
or_test ['if' or_test 'else' test]
So, a ternary conditional operation is of the form:
expression1 if expression2 else expression3
expression3 will be lazily evaluated (that is, evaluated only if expression2 is false in a boolean context). And because of the recursive definition, you can chain them indefinitely (though it may considered bad style.)
expression1 if expression2 else expression3 if expression4 else expression5 # and so on
A note on usage:
Note that every if must be followed with an else. People learning list comprehensions and generator expressions may find this to be a difficult lesson to learn - the following will not work, as Python expects a third expression for an else:
[expression1 if expression2 for element in iterable]
# ^-- need an else here
which raises a SyntaxError: invalid syntax.
So the above is either an incomplete piece of logic (perhaps the user expects a no-op in the false condition) or what may be intended is to use expression2 as a filter - notes that the following is legal Python:
[expression1 for element in iterable if expression2]
expression2 works as a filter for the list comprehension, and is not a ternary conditional operator.
Alternative syntax for a more narrow case:
You may find it somewhat painful to write the following:
expression1 if expression1 else expression2
expression1 will have to be evaluated twice with the above usage. It can limit redundancy if it is simply a local variable. However, a common and performant Pythonic idiom for this use-case is to use or's shortcutting behavior:
expression1 or expression2
which is equivalent in semantics. Note that some style-guides may limit this usage on the grounds of clarity - it does pack a lot of meaning into very little syntax.
One of the alternatives to Python's conditional expression
"yes" if boolean else "no"
is the following:
{True: "yes", False: "no"}[boolean]
which has the following nice extension:
{True: "yes", False: "no", None: "maybe"}[boolean_or_none]
The shortest alternative remains
("no", "yes")[boolean]
which works because issubclass(bool, int).
Careful, though: the alternative to
yes() if boolean else no()
is not
(no(), yes())[boolean] # bad: BOTH no() and yes() are called
but
(no, yes)[boolean]()
This works fine as long as no and yes are to be called with exactly the same parameters. If they are not, like in
yes("ok") if boolean else no() # (1)
or in
yes("ok") if boolean else no("sorry") # (2)
then a similar alternative either does not exist (1) or is hardly viable (2). (In rare cases, depending on the context, something like
msg = ("sorry", "ok")[boolean]
(no, yes)[boolean](msg)
could make sense.)
Thanks to Radek Rojík for his comment
As already answered, yes, there is a ternary operator in Python:
<expression 1> if <condition> else <expression 2>
In many cases <expression 1> is also used as Boolean evaluated <condition>. Then you can use short-circuit evaluation.
a = 0
b = 1
# Instead of this:
x = a if a else b
# Evaluates as 'a if bool(a) else b'
# You could use short-circuit evaluation:
x = a or b
One big pro of short-circuit evaluation is the possibility of chaining more than two expressions:
x = a or b or c or d or e
When working with functions it is more different in detail:
# Evaluating functions:
def foo(x):
print('foo executed')
return x
def bar(y):
print('bar executed')
return y
def blubb(z):
print('blubb executed')
return z
# Ternary Operator expression 1 equals to False
print(foo(0) if foo(0) else bar(1))
''' foo and bar are executed once
foo executed
bar executed
1
'''
# Ternary Operator expression 1 equals to True
print(foo(2) if foo(2) else bar(3))
''' foo is executed twice!
foo executed
foo executed
2
'''
# Short-circuit evaluation second equals to True
print(foo(0) or bar(1) or blubb(2))
''' blubb is not executed
foo executed
bar executed
1
'''
# Short-circuit evaluation third equals to True
print(foo(0) or bar(0) or blubb(2))
'''
foo executed
bar executed
blubb executed
2
'''
# Short-circuit evaluation all equal to False
print(foo(0) or bar(0) or blubb(0))
''' Result is 0 (from blubb(0)) because no value equals to True
foo executed
bar executed
blubb executed
0
'''
PS: Of course, a short-circuit evaluation is not a ternary operator, but often the ternary is used in cases where the short circuit would be enough. It has a better readability and can be chained.
Simulating the Python ternary operator.
For example
a, b, x, y = 1, 2, 'a greather than b', 'b greater than a'
result = (lambda:y, lambda:x)[a > b]()
Output:
'b greater than a'
a if condition else b
Just memorize this pyramid if you have trouble remembering:
condition
if else
a b
The ternary conditional operator simply allows testing a condition in a single line replacing the multiline if-else making the code compact.
Syntax:
[on_true] if [expression] else [on_false]
1- Simple Method to use ternary operator:
# Program to demonstrate conditional operator
a, b = 10, 20
# Copy value of a in min if a < b else copy b
min = a if a < b else b
print(min) # Output: 10
2- Direct Method of using tuples, Dictionary, and lambda:
# Python program to demonstrate ternary operator
a, b = 10, 20
# Use tuple for selecting an item
print( (b, a) [a < b] )
# Use Dictionary for selecting an item
print({True: a, False: b} [a < b])
# lambda is more efficient than above two methods
# because in lambda we are assure that
# only one expression will be evaluated unlike in
# tuple and Dictionary
print((lambda: b, lambda: a)[a < b]()) # in output you should see three 10
3- Ternary operator can be written as nested if-else:
# Python program to demonstrate nested ternary operator
a, b = 10, 20
print ("Both a and b are equal" if a == b else "a is greater than b"
if a > b else "b is greater than a")
Above approach can be written as:
# Python program to demonstrate nested ternary operator
a, b = 10, 20
if a != b:
if a > b:
print("a is greater than b")
else:
print("b is greater than a")
else:
print("Both a and b are equal")
# Output: b is greater than a
Vinko Vrsalovic's answer is good enough. There is only one more thing:
Note that conditionals are an expression, not a statement. This means you can't use assignment statements or pass or other statements within a conditional expression
Walrus operator in Python 3.8
After the walrus operator was introduced in Python 3.8, something changed.
(a := 3) if True else (b := 5)
gives a = 3 and b is not defined,
(a := 3) if False else (b := 5)
gives a is not defined and b = 5, and
c = (a := 3) if False else (b := 5)
gives c = 5, a is not defined and b = 5.
Even if this may be ugly, assignments can be done inside conditional expressions after Python 3.8. Anyway, it is still better to use normal if statement instead in this case.
More a tip than an answer (I don't need to repeat the obvious for the hundredth time), but I sometimes use it as a one-liner shortcut in such constructs:
if conditionX:
print('yes')
else:
print('nah')
, becomes:
print('yes') if conditionX else print('nah')
Some (many :) may frown upon it as unpythonic (even, Ruby-ish :), but I personally find it more natural - i.e., how you'd express it normally, plus a bit more visually appealing in large blocks of code.
You can do this:
[condition] and [expression_1] or [expression_2];
Example:
print(number%2 and "odd" or "even")
This would print "odd" if the number is odd or "even" if the number is even.
The result: If condition is true, exp_1 is executed, else exp_2 is executed.
Note: 0, None, False, emptylist, and emptyString evaluates as False.
And any data other than 0 evaluates to True.
Here's how it works:
If the condition [condition] becomes "True", then expression_1 will be evaluated, but not expression_2.
If we "and" something with 0 (zero), the result will always to be false. So in the below statement,
0 and exp
The expression exp won't be evaluated at all since "and" with 0 will always evaluate to zero and there is no need to evaluate the expression. This is how the compiler itself works, in all languages.
In
1 or exp
the expression exp won't be evaluated at all since "or" with 1 will always be 1. So it won't bother to evaluate the expression exp since the result will be 1 anyway (compiler optimization methods).
But in case of
True and exp1 or exp2
The second expression exp2 won't be evaluated since True and exp1 would be True when exp1 isn't false.
Similarly in
False and exp1 or exp2
The expression exp1 won't be evaluated since False is equivalent to writing 0 and doing "and" with 0 would be 0 itself, but after exp1 since "or" is used, it will evaluate the expression exp2 after "or".
Note:- This kind of branching using "or" and "and" can only be used when the expression_1 doesn't have a Truth value of False (or 0 or None or emptylist [ ] or emptystring ' '.) since if expression_1 becomes False, then the expression_2 will be evaluated because of the presence "or" between exp_1 and exp_2.
In case you still want to make it work for all the cases regardless of what exp_1 and exp_2 truth values are, do this:
[condition] and ([expression_1] or 1) or [expression_2];
Many programming languages derived from C usually have the following syntax of the ternary conditional operator:
<condition> ? <expression1> : <expression2>
At first, the Python's benevolent dictator for life (I mean Guido van Rossum, of course) rejected it (as non-Pythonic style), since it's quite hard to understand for people not used to C language. Also, the colon sign : already has many uses in Python. After PEP 308 was approved, Python finally received its own shortcut conditional expression (what we use now):
<expression1> if <condition> else <expression2>
So, firstly it evaluates the condition. If it returns True, expression1 will be evaluated to give the result, otherwise expression2 will be evaluated. Due to lazy evaluation mechanics – only one expression will be executed.
Here are some examples (conditions will be evaluated from left to right):
pressure = 10
print('High' if pressure < 20 else 'Critical')
# Result is 'High'
Ternary operators can be chained in series:
pressure = 5
print('Normal' if pressure < 10 else 'High' if pressure < 20 else 'Critical')
# Result is 'Normal'
The following one is the same as previous one:
pressure = 5
if pressure < 20:
if pressure < 10:
print('Normal')
else:
print('High')
else:
print('Critical')
# Result is 'Normal'
Yes, Python have a ternary operator, here is the syntax and an example code to demonstrate the same :)
#[On true] if [expression] else[On false]
# if the expression evaluates to true then it will pass On true otherwise On false
a = input("Enter the First Number ")
b = input("Enter the Second Number ")
print("A is Bigger") if a>b else print("B is Bigger")
Other answers correctly talk about the Python ternary operator. I would like to complement by mentioning a scenario for which the ternary operator is often used, but for which there is a better idiom. This is the scenario of using a default value.
Suppose we want to use option_value with a default value if it is not set:
run_algorithm(option_value if option_value is not None else 10)
or, if option_value is never set to a falsy value (0, "", etc.), simply
run_algorithm(option_value if option_value else 10)
However, in this case an ever better solution is simply to write
run_algorithm(option_value or 10)
The syntax for the ternary operator in Python is:
[on_true] if [expression] else [on_false]
Using that syntax, here is how we would rewrite the code above using Python’s ternary operator:
game_type = 'home'
shirt = 'white' if game_type == 'home' else 'green'
It's still pretty clear, but much shorter. Note that the expression could be any type of expression, including a function call, that returns a value that evaluates to True or False.
Python has a ternary form for assignments; however there may be even a shorter form that people should be aware of.
It's very common to need to assign to a variable one value or another depending on a condition.
>>> li1 = None
>>> li2 = [1, 2, 3]
>>>
>>> if li1:
... a = li1
... else:
... a = li2
...
>>> a
[1, 2, 3]
^ This is the long form for doing such assignments.
Below is the ternary form. But this isn't the most succinct way - see the last example.
>>> a = li1 if li1 else li2
>>>
>>> a
[1, 2, 3]
>>>
With Python, you can simply use or for alternative assignments.
>>> a = li1 or li2
>>>
>>> a
[1, 2, 3]
>>>
The above works since li1 is None and the interpreter treats that as False in logic expressions. The interpreter then moves on and evaluates the second expression, which is not None and it's not an empty list - so it gets assigned to a.
This also works with empty lists. For instance, if you want to assign a whichever list has items.
>>> li1 = []
>>> li2 = [1, 2, 3]
>>>
>>> a = li1 or li2
>>>
>>> a
[1, 2, 3]
>>>
Knowing this, you can simply such assignments whenever you encounter them. This also works with strings and other iterables. You could assign a whichever string isn't empty.
>>> s1 = ''
>>> s2 = 'hello world'
>>>
>>> a = s1 or s2
>>>
>>> a
'hello world'
>>>
I always liked the C ternary syntax, but Python takes it a step further!
I understand that some may say this isn't a good stylistic choice, because it relies on mechanics that aren't immediately apparent to all developers. I personally disagree with that viewpoint. Python is a syntax-rich language with lots of idiomatic tricks that aren't immediately apparent to the dabbler. But the more you learn and understand the mechanics of the underlying system, the more you appreciate it.
Pythonic way of doing the things:
"true" if var else "false"
But there always exists a different way of doing a ternary condition too:
"true" and var or "false"
There are multiple ways. The simplest one is to use the condition inside the "print" method.
You can use
print("Twenty" if number == 20 else "Not twenty")
Which is equivalent to:
if number == 20:
print("Twenty")
else:
print("Not twenty")
In this way, more than two statements are also possible to print. For example:
if number == 20:
print("Twenty")
elif number < 20:
print("Lesser")
elif 30 > number > 20:
print("Between")
else:
print("Greater")
can be written as:
print("Twenty" if number == 20 else "Lesser" if number < 20 else "Between" if 30 > number > 20 else "Greater")
The if else-if version can be written as:
sample_set="train" if "Train" in full_path else ("test" if "Test" in full_path else "validation")
Yes, it has, but it's different from C-syntax-like programming languages (which is condition ? value_if_true : value_if_false
In Python, it goes like this: value_if_true if condition else value_if_false
Example: even_or_odd = "even" if x % 2 == 0 else "odd"
A neat way to chain multiple operators:
f = lambda x,y: 'greater' if x > y else 'less' if y > x else 'equal'
array = [(0,0),(0,1),(1,0),(1,1)]
for a in array:
x, y = a[0], a[1]
print(f(x,y))
# Output is:
# equal,
# less,
# greater,
# equal
I find the default Python syntax val = a if cond else b cumbersome, so sometimes I do this:
iif = lambda (cond, a, b): a if cond else b
# So I can then use it like:
val = iif(cond, a, b)
Of course, it has the downside of always evaluating both sides (a and b), but the syntax is way clearer to me.

Search for an item in a Lua list

If I have a list of items like this:
local items = { "apple", "orange", "pear", "banana" }
how do I check if "orange" is in this list?
In Python I could do:
if "orange" in items:
# do something
Is there an equivalent in Lua?
You could use something like a set from Programming in Lua:
function Set (list)
local set = {}
for _, l in ipairs(list) do set[l] = true end
return set
end
Then you could put your list in the Set and test for membership:
local items = Set { "apple", "orange", "pear", "banana" }
if items["orange"] then
-- do something
end
Or you could iterate over the list directly:
local items = { "apple", "orange", "pear", "banana" }
for _,v in pairs(items) do
if v == "orange" then
-- do something
break
end
end
Use the following representation instead:
local items = { apple=true, orange=true, pear=true, banana=true }
if items.apple then
...
end
You're seeing firsthand one of the cons of Lua having only one data structure---you have to roll your own. If you stick with Lua you will gradually accumulate a library of functions that manipulate tables in the way you like to do things. My library includes a list-to-set conversion and a higher-order list-searching function:
function table.set(t) -- set of list
local u = { }
for _, v in ipairs(t) do u[v] = true end
return u
end
function table.find(f, l) -- find element v of l satisfying f(v)
for _, v in ipairs(l) do
if f(v) then
return v
end
end
return nil
end
Write it however you want, but it's faster to iterate directly over the list, than to generate pairs() or ipairs()
#! /usr/bin/env lua
local items = { 'apple', 'orange', 'pear', 'banana' }
local function locate( table, value )
for i = 1, #table do
if table[i] == value then print( value ..' found' ) return true end
end
print( value ..' not found' ) return false
end
locate( items, 'orange' )
locate( items, 'car' )
orange found
car not found
Lua tables are more closely analogs of Python dictionaries rather than lists. The table you have create is essentially a 1-based indexed array of strings. Use any standard search algorithm to find out if a value is in the array. Another approach would be to store the values as table keys instead as shown in the set implementation of Jon Ericson's post.
This is a swiss-armyknife function you can use:
function table.find(t, val, recursive, metatables, keys, returnBool)
if (type(t) ~= "table") then
return nil
end
local checked = {}
local _findInTable
local _checkValue
_checkValue = function(v)
if (not checked[v]) then
if (v == val) then
return v
end
if (recursive and type(v) == "table") then
local r = _findInTable(v)
if (r ~= nil) then
return r
end
end
if (metatables) then
local r = _checkValue(getmetatable(v))
if (r ~= nil) then
return r
end
end
checked[v] = true
end
return nil
end
_findInTable = function(t)
for k,v in pairs(t) do
local r = _checkValue(t, v)
if (r ~= nil) then
return r
end
if (keys) then
r = _checkValue(t, k)
if (r ~= nil) then
return r
end
end
end
return nil
end
local r = _findInTable(t)
if (returnBool) then
return r ~= nil
end
return r
end
You can use it to check if a value exists:
local myFruit = "apple"
if (table.find({"apple", "pear", "berry"}, myFruit)) then
print(table.find({"apple", "pear", "berry"}, myFruit)) -- 1
You can use it to find the key:
local fruits = {
apple = {color="red"},
pear = {color="green"},
}
local myFruit = fruits.apple
local fruitName = table.find(fruits, myFruit)
print(fruitName) -- "apple"
I hope the recursive parameter speaks for itself.
The metatables parameter allows you to search metatables as well.
The keys parameter makes the function look for keys in the list. Of course that would be useless in Lua (you can just do fruits[key]) but together with recursive and metatables, it becomes handy.
The returnBool parameter is a safe-guard for when you have tables that have false as a key in a table (Yes that's possible: fruits = {false="apple"})
function valid(data, array)
local valid = {}
for i = 1, #array do
valid[array[i]] = true
end
if valid[data] then
return false
else
return true
end
end
Here's the function I use for checking if data is in an array.
Sort of solution using metatable...
local function preparetable(t)
setmetatable(t,{__newindex=function(self,k,v) rawset(self,v,true) end})
end
local workingtable={}
preparetable(workingtable)
table.insert(workingtable,123)
table.insert(workingtable,456)
if workingtable[456] then
...
end
The following representation can be used:
local items = {
["apple"]=true, ["orange"]=true, ["pear"]=true, ["banana"]=true
}
if items["apple"] then print("apple is a true value.") end
if not items["red"] then print("red is a false value.") end
Related output:
apple is a true value.
red is a false value.
You can also use the following code to check boolean validity:
local items = {
["apple"]=true, ["orange"]=true, ["pear"]=true, ["banana"]=true,
["red"]=false, ["blue"]=false, ["green"]=false
}
if items["yellow"] == nil then print("yellow is an inappropriate value.") end
if items["apple"] then print("apple is a true value.") end
if not items["red"] then print("red is a false value.") end
The output is:
yellow is an inappropriate value.
apple is a true value.
red is a false value.
Check Tables Tutorial for additional information.
function table.find(t,value)
if t and type(t)=="table" and value then
for _, v in ipairs (t) do
if v == value then
return true;
end
end
return false;
end
return false;
end
you can use this solution:
items = { 'a', 'b' }
for k,v in pairs(items) do
if v == 'a' then
--do something
else
--do something
end
end
or
items = {'a', 'b'}
for k,v in pairs(items) do
while v do
if v == 'a' then
return found
else
break
end
end
end
return nothing
A simple function can be used that :
returns nil, if the item is not found in table
returns index of item, if item is found in table
local items = { "apple", "orange", "pear", "banana" }
local function search_value (tbl, val)
for i = 1, #tbl do
if tbl[i] == val then
return i
end
end
return nil
end
print(search_value(items, "pear"))
print(search_value(items, "cherry"))
output of above code would be
3
nil