I have been learning mysqli object oriented method, although I am confused sometimes now I need to blind some variable for insertion and I search on how to do so but I got some code with something like:
$stmt->bind_param("sss", $email, $handle, $mobile);
and other like
$stmt->bind_param('sssd', $code, $language, $official, $percent);
Now I am totally confused. I learn on a website that sss means number of variable and object type, can someone explain better I have many variable to blind and they are of different type.
The characters represent the data types passed to the query:
i - integer
d - double
s - string
b - BLOB
in this query:
$stmt->bind_param("sss", $email, $handle, $mobile);
the variables are three strings ($email, $handle, and $mobile).
This query has three strings, and the final variable is a "double":
$stmt->bind_param('sssd', $code, $language, $official, $percent);
More information about MySQL data types and numeric types.
Related
I have taken a principles of programming class and have been given a Perl expression that concatenates a string number to an int number and then adds another number to it and it evaluates fine. i.e. ("4" . 3) + 7 == 50.
I'm trying to understand why Perl does this and what concerns it may bring up. I'm having a hard time grasping many of the concepts of the class and am trying to get explanations from different sources apart from my horrible teacher and equally horrible notes.
Can the concept behind this kind of expression be explained to me as well as concerns they might bring up? Thanks in advance for the help.
Edit: For Clarity
Perl is built around the central concept of 'do what I mean'.
A scalar is a multi purpose variable type, and is intended to implicitly cast values to a data type that's appropriate to what you're doing.
The reason this works is because perl is context sensitive - it knows the difference between different expected return values.
At a basic level, you can see this with the wantarray function. (Which as noted below - is probably badly named, because we're talking about a LIST context)
sub context_test {
if ( not defined wantarray() ) {
print "Void context\n";
}
if ( wantarray() ) {
return ( "List", "Context" );
}
else {
return "scalar context";
}
}
context_test();
my $scalar = context_test();
my #list = context_test();
print "Scalar context gave me $scalar\n";
print "List context gave me #list\n";
This principle occurs throughout perl. If you want, you can use something like Contextual::Return to extend this further - testing the difference between numeric, string and boolean subsets of scalar contexts.
The reason I mention this is because a scalar is a special sort of data type - if you look at Scalar::Util you will see a capability of creating a dualvar - a scalar that has different values in different contexts.
my $dualvar = Scalar::Util::dualvar ( 666, "the beast" );
print "Numeric:",$dualvar + 0,"\n";
print "String:",$dualvar . '',"\n";
Now, messing around with dualvars is a good way to create some really annoying and hard to trace bugs, but the point is - a scalar is a magic datatype, and perl is always aware of what you're doing with the result.
If you perform a string operation, perl treats it as a string. If you perform a numeric operation, perl tries to treat it as a number.
my $value = '4'; #string;
my $newvalue = $value . 3; #because we concat, perl treats _both_ as strings.
print $newvalue,"\n";
my $sum = $newvalue + 7; #perl turns strings back to numbers, because we're adding;
print $sum,"\n";
if ( Scalar::Util::isdual ( $newvalue ) ) { print "newvalue Is a dual var\n" };
if ( not Scalar::Util::isdual ( $sum ) ) { print "sum is NOT a dual var\n"; };
Mostly 'context' is something that happens behind the scenes in perl, and you don't have to worry about it. If you've come from a programming background, the idea of implicit casting between int and string may seem a little bit dirty. But it mostly works fine.
You may occasionally get errors like:
Argument "4a3" isn't numeric in addition (+)
One of the downsides of this approach is these are runtime errors, because you're not doing strong type checking at 'compile' time.
So in terms of specific concerns:
You're runtime type checking, not compile time. If you have strict types, you can detect an attempt to add a string to an int before you start to run anything.
You're not always operating in the context that you assume you are, which can lead to some unpredictable behaviour. One of the best examples is that print operates in a list context - so to take the example above:
print context_test();
You'll get List Context.
If you monkey around with context sensitive return types, you can create some really annoying bugs that are immensely irritating to back trace and troubleshoot.
So I'm new to Ada, and I'm attempting to write a kernel in it, but I cannot seem to find any good information on how to do this properly. In C, I would write:
unsigned char* videoram = (char*) 0xB8000;
videoram[0] = 65;
to access the video ram directly and write 'a' to it. I've heard I need to use an Ada array and other pragma's to do this in a typesafe manner in Ada. Is there any good resources on this kind of Ada programming?
You can use the 'Address attribute:
Videoram : String (1 .. Videoram_Size);
for Videoram'Address use 16#B8000#;
-- ...
Videoram (1) := 'a';
If you don't want to use String and Characters, you can define your own data types.. like:
type Byte is mod 2**8; -- unsigned char
type Byte_Array is array (Natural range <>) of Byte;
Videoram : Byte_Array (0 .. Videoram_Size - 1);
for Videoram'Address use 16#B8000#;
-- ...
Videoram (0) := 65;
Btw, you even get range checking for the index, so you can't write outside of the Videoram range.
If you use an address attribute (i.e. for Object'Address use ... ), you should use the To_Address() function found in System.Storage_Elements because the Address type doesn't have to be an integer. The Ada Reference Manual only states:
"Address is a definite, nonlimited type with preelaborable initialization"
Whereas for the Integer_Address type in System.Storage_Elements it states:
"Integer_Address is a (signed or modular) integer subtype. To_Address and To_Integer convert back and forth between this type and Address."
So, you should really use:
for Object'Address use To_Address( 16#B8000# );
One last thing to point out from T.E.D's answer is that if you are concerned about object initialization using this method, simply add a pragma Import( Ada, your_object ) after the declaration so that default initialization is suppressed.
There are actually two ways.
One is to set a pointer to the address you want to use, and access the object via the pointer.
type Video_RAM_Pointer is access all My_Video_Ram_Struct;
package Convert is new System.Address_To_Access_Conversions (Video_RAM_Pointer);
Video_RAM : constant Video_RAM_Pointer := Convert.To_Access (16#B8000#);
The other is to overlay your data right on top of the location.
Video_RAM : My_Video_RAM_Struct;
for Video_RAM'address use at 16#B8000#;
Generally, I prefer using the former. Among other issues, the latter counts as a declaration, which means that any fields in My_Video_RAM_Struct that have initialization code will get reinitialized every time you declare your overlay. Additionally, it is tempting to folks to overuse (abuse) that feature to alias objects all over the place, which is both hard on the optimizer and hard on the maintanence programmer.
The pointer method just tells the compiler to assume the given address holds the structure you told it, which IMHO is exactly what you want to happen.
i have designed a function which can return 2 different types based on the input parameters
ex: &Foo(12,"count") -> returns record count from DB for value 12
&Foo(12,"details") -> returns resultset from DB for value 12 in hash format
My question is is this a good approach? in C# i can do it with function overload.
Please think what part of your code gets easier by saying
Foo(12, "count")
instead of
Foo_count(12)
The only case I can think of is when the function name ("count") itself is input data. And even then do you probably want to perform some validation on that, maybe by means of a function table lookup.
Unless this is for an intermediate layer that just takes a command name and passes it on, I'd go with two separate functions.
Also, the implementation of the Foo function would look at the command name and then just split into a private function for every command anyway, right?
additionally you might consider the want to make foo return the details if you wanted a list.
return wantarray ? ($num, #details) : $num;
I am mulling over a best practice for passing hash references for return data to/from functions.
On the one hand, it seems intuitive to pass only input values to a function and have only return output variables. However, passing hashes in Perl can only be done by reference, so it is a bit messy and would seem more of an opportunity to make a mistake.
The other way is to pass a reference in the input variables, but then it has to be dealt with in the function, and it may not be clear what is an input and what is a return variable.
What is a best practice regarding this?
Return references to an array and a hash, and then dereference it.
($ref_array,$ref_hash) = $this->getData('input');
#array = #{$ref_array};
%hash = %{$ref_hash};
Pass in references (#array, %hash) to the function that will hold the output data.
$this->getData('input', \#array, \%hash);
Just return the reference. There is no need to dereference the whole
hash like you are doing in your examples:
my $result = some_function_that_returns_a_hashref;
say "Foo is ", $result->{foo};
say $_, " => ", $result->{$_} for keys %$result;
etc.
I have never seen anyone pass in empty references to hold the result. This is Perl, not C.
Trying to create copies by saying
my %hash = %{$ref_hash};
is even more dangerous than using the hashref. This is because it only creates a shallow copy. This will lead you to thinking it is okay to modify the hash, but if it contains references they will modify the original data structure. I find it better to just pass references and be careful, but if you really want to make sure you have a copy of the reference passed in you can say:
use Storable qw/dclone/;
my %hash = %{dclone $ref_hash};
The first one is better:
my ($ref_array,$ref_hash) = $this->getData('input');
The reasons are:
in the second case, getData() needs to
check the data structures to make
sure they are empty
you have freedom to return undef as a special value
it looks more Perl-idiomatic.
Note: the lines
#array = #{$ref_array};
%hash = %{$ref_hash};
are questionable, since you shallow-copy the whole data structures here. You can use references everywhere where you need array/hash, using -> operator for convenience.
If it's getting complicated enough that both the callsite and the called function are paying for it (because you have to think/write more every time you use it), why not just use an object?
my $results = $this->getData('input');
$results->key_value_thingies;
$results->listy_thingies;
If making an object is "too complicated" then start using Moose so that it no longer is.
My personal preference for sub interfaces:
If the routine has 0-3 arguments, they may be passed in list form: foo( 'a', 12, [1,2,3] );
Otherwise pass a list of name value pairs. foo( one => 'a', two => 12, three => [1,2,3] );
If the routine has or may have more than one argument seriously consider using name/value pairs.
Passing in references increases the risk of inadvertent data modification.
On returns I generally prefer to return a list of results rather than an array or hash reference.
I return hash or array refs when it will make a noticeable improvement in speed or memory consumption (ie BIG structures), or when a complex data structure is involved.
Returning references when not needed deprives one of the ability to take advantage of Perl's nice list handling features and exposes one to the dangers of inadvertent modification of data.
In particular, I find it useful to assign a list of results into an array and return the array, which provides the contextual return behaviors of an array to my subs.
For the case of passing in two hashes I would do something like:
my $foo = foo( hash1 => \%hash1, hash2 => \%hash2 ); # gets number of items returned
my #foo = foo( hash1 => \%hash1, hash2 => \%hash2 ); # gets items returned
sub foo {
my %arg = #_;
# do stuff
return #results;
}
I originally posted this to another question, and then someone pointed to this as a "related post", so I'll post it here to for my take on the subject, assuming people will encounter it in the future.
I'm going to contradict the Accepted Answer and say that I prefer to have my data returned as a plain hash (well, as an even-sized list which is likely to be interpreted as a hash). I work in an environment where we tend to do things like the following code snippet, and it's much easier to combine and sort and slice and dice when you don't have to dereference every other line. (It's also nice to know that someone can't damage your hashref because you passed the entire thing by value -- though someone pointed out that if your hash contains more than simple scalars it's not so simple.)
my %filtered_config_slice =
hashgrep { $a !~ /^apparent_/ && defined $b } (
map { $_->build_config_slice(%some_params, some_other => 'param') }
($self->partial_config_strategies, $other_config_strategy)
);
This approximates something that my code might do: building a configuration for an object based on various configuration strategy objects (some of which the object knows about inherently, plus some extra guy) and then filters out some of them as irrelevant.
(Yes, we have nice tools like hashgrep and hashmap and lkeys that do useful things to hashes. $a and $b get set to the key and the value of each item in the list, respectively). (Yes, we have people who can program at this level. Hiring is obnoxious, but we have a quality product.)
If you don't intend to do anything resembling functional programming like this, or if you need more performance (have you profiled?) then sure, use hashrefs.
Uh... "passing hashes can only be done by reference"?
sub foo(%) {
my %hash = #_;
do_stuff_with(%hash);
}
my %hash = (a => 1, b => 2);
foo(%hash);
What am I missing?
I would say that if the issue is that you need to have multiple outputs from a function, it's better as a general practice to output a data structure, probably a hash, that holds everything you need to send out rather than taking modifiable references as arguments.
Can someone suggest a good module in perl which can be used to store collection of objects?
Or is ARRAY a good enough substitute for most of the needs?
Update:
I am looking for a collections class because I want to be able to do an operation like compute collection level property from each element.
Since I need to perform many such operations, I might as well write a class which can be extended by individual objects. This class will obviously work with arrays (or may be hashes).
There are collection modules for more complex structures, but it is common style in Perl to use Arrays for arrays, stacks and lists. Perl has built in functions for using the array as a stack or list : push/pop, shift/unshift, splice (inserting or removing in the middle) and the foreach form for iteration.
Perl also has a map, called a hashmap which is the equivalent to a Dictionary in Python - allowing you to have an association between a single key and a single value.
Perl developers often compose these two data-structures to build what they need - need multiple values? Store array-references in the value part of the hashtable (Map). Trees can be built in a similar manner - if you need unique keys, use multiple-levels of hashmaps, or if you don't use nested array references.
These two primitive collection types in Perl don't have an Object Oriented api, but they still are collections.
If you look on CPAN you'll likely find modules that provide other Object Oriented data structures, it really depends on your need. Is there a particular data structure you need besides a List, Stack or Map? You might get a more precise answer (eg a specific module) if you're asking about a particular data structure.
Forgot to mention, if you're looking for small code examples across a variety of languages, PLEAC (Programming Language Examples Alike Cookbook) is a decent resource.
I would second Michael Carman's comment: please do not use the term "Hashmap" or "map" when you mean a hash or associative array. Especially when Perl has a map function; that just confuses things.
Having said that, Kyle Burton's response is fundamentally sound: either a hash or an array, or a complex structure composed of a mixture of the two, is usually enough. Perl groks OO, but doesn't enforce it; chances are that a loosely-defined data structure may be good enough for what you need.
Failing that, please define more exactly what you mean by "compute collection level property from each element". And bear in mind that Perl has keywords like map and grep that let you do functional programming things like e.g.
my $record = get_complex_structure();
# $record = {
# 'widgets' => {
# name => 'ACME Widgets',
# skus => [ 'WIDG01', 'WIDG02', 'WIDG03' ],
# sales => {
# WIDG01 => { num => 25, value => 105.24 },
# WIDG02 => { num => 10, value => 80.02 },
# WIDG03 => { num => 8, value => 205.80 },
# },
# },
# ### and so on for 'grommets', 'nuts', 'bolts' etc.
# }
my #standouts =
map { $_->[0] }
sort {
$b->[2] <=> $a->[2]
|| $b->[1] <=> $a->[1]
|| $record->{$a->[0]}->{name} cmp $record->{$b->[0]}->{name}
}
map {
my ($num, $value);
for my $sku (#{$record->{$_}{skus}}) {
$num += $record->{$_}{sales}{$sku}{num};
$value += $record->{$_}{sales}{$sku}{value};
}
[ $_, $num, $value ];
}
keys %$record;
Reading from back to front, this particular Schwarztian transform does three things:
3) It takes a key to $record, goes through the SKUs defined in this arbitrary structure, and works out the aggregate number and total value of transactions. It returns an anonymous array containing the key, the number of transactions and the total value.
2) The next block takes in a number of arrayrefs and sorts them a) first of all by comparing the total value, numerically, in descending orders; b) if the values are equal, by comparing the number of transactions, numerically in descending order; and c) if that fails, by sorting asciibetically on the name associated with this order.
1) Finally, we take the key to $record from the sorted data structure, and return that.
It may well be that you don't need to set up a separate class to do what you want.
I would normally use an #array or a %hash.
What features are you looking for that aren't provided by those?
Base your decision on how you need to access the objects. If pushing them onto an array, indexing into, popping/shifting them off works, then use an array. Otherwise hash them by some key or organize them into a tree of objects that meets your needs. A hash of objects is a very simple, powerful, and highly-optimized way of doing things in Perl.
Since Perl arrays can easily be appended to, resized, sorted, etc., they are good enough for most "collection" needs. In cases where you need something more advanced, a hash will generally do. I wouldn't recommend that you go looking for a collection module until you actually need it.
Either an array or a hash can store a collection of objects. A class might be better if you want to work with the class in certain ways but you'd have to tell us what those ways are before we could make any good recommendations.
i would stick with an ARRAY or a HASH.
#names = ('Paul','Michael','Jessica','Megan');
and
my %petsounds = ("cat" => "meow",
"dog" => "woof",
"snake" => "hiss");
source
It depends a lot; there's Sparse Matrix modules, some forms of persistence, a new style of OO etc
Most people just man perldata, perllol, perldsc to answer their specific issue with a data structure.