Printing the same character several times without a loop - dart

I would like to "beautify" the output of one of my Dart scripts, like so:
-----------------------------------------
OpenPGP signing notes from key `CD42FF00`
-----------------------------------------
<Paragraph>
And I wonder if there is a particularly simple and/or optimized way of printing the same character x times in Dart. In Python, print "-" * x would print the "-" character x times.
Learning from this answer, for the purpose of this question, I wrote the following minimal code, which makes use of the core Iterable class:
main() {
// Obtained with '-'.codeUnitAt(0)
const int FILLER_CHAR = 45;
String headerTxt;
Iterable headerBox;
headerTxt = 'OpenPGP signing notes from key `CD42FF00`';
headerBox = new Iterable.generate(headerTxt.length, (e) => FILLER_CHAR);
print(new String.fromCharCodes(headerBox));
print(headerTxt);
print(new String.fromCharCodes(headerBox));
// ...
}
This gives the expected output, but is there a better way in Dart to print a character (or string) x times? In my example, I want to print the "-" character headerTxt.length times.

The original answer is from 2014, so there must have been some updates to the Dart language: a simple string multiplied by an int works.
main() {
String title = 'Dart: Strings can be "multiplied"';
String line = '-' * title.length
print(line);
print(title);
print(line);
}
And this will be printed as:
---------------------------------
Dart: Strings can be "multiplied"
---------------------------------
See Dart String's multiply * operator docs:
Creates a new string by concatenating this string with itself a number of times.
The result of str * n is equivalent to str + str + ...(n times)... + str.
Returns an empty string if times is zero or negative.

I use this way.
void main() {
print(new List.filled(40, "-").join());
}
So, your case.
main() {
const String FILLER = "-";
String headerTxt;
String headerBox;
headerTxt = 'OpenPGP signing notes from key `CD42FF00`';
headerBox = new List.filled(headerTxt.length, FILLER).join();
print(headerBox);
print(headerTxt);
print(headerBox);
// ...
}
Output:
-----------------------------------------
OpenPGP signing notes from key `CD42FF00`
-----------------------------------------

Related

How to calculate this String text ="2+3-5+1" using Split method? [duplicate]

This question already has answers here:
Calculate string value in javascript, not using eval
(12 answers)
Closed 4 months ago.
When the text was '2+3+5+1', the logic was easy
Split('+') so the string is converted to an array.
loop over the array and calculate the sum.
check the code below
void main() {
const text = '2+3+5+1';
final array = text.split('+');
int res =0;
for (var i=0; i<= array.length -1; i++){
res+=int.parse(array[i]);;
}
print(array);
print(res);
}
Now this String "2+3-5+1" contains minus.
how to get the right response using split method?
I am using dart.
note: I don't want to use any library (math expression) to solve this exercice.
Use the .replace() method.
text = text.replace("-", "+-");
When you run through the loop, it will calculate (-).
You can split your string using regex text.split(/\+|\-/).
This of course will fail if any space is added to the string (not to mention *, / or even decimal values).
const text = '20+3-5+10';
const arr = text.split(/\+|\-/)
let tot = 0
for (const num of arr) {
const pos = text.indexOf(num)
if (pos === 0) {
tot = parseInt(num)
} else {
switch (text.substr(text.indexOf(num) - 1, 1)) {
case '+':
tot += parseInt(num)
break
case '-':
tot -= parseInt(num)
break
}
}
}
console.log(tot)
I see 2 maybe 3 options, definitely there are hundreds
You don't use split and you just iterate through the string and just add or subtract on the way. As an example
You have '2+3-5+1'. You iterate until the second operator (+ or -) on your case. When you find it you just do the operation that you have iterated through and then you just keep going. You can do it recursive or not, doesn't matter
"2+3-5+1" -> "5-5+1" -> "0+1" -> 1
You use split on + for instance and you get [ '2', '3-5', '1' ] then you go through them with a loop with 2 conditions like
if(isNaN(x)) res+= x since you know it's been divided with a +
if(!isNaN(x)) res+= x.split('-')[0] - x.split('-')[1]
isNaN -> is not a number
Ofc you can make it look nicer. If you have parenthesis though, none of this will work
You can also use regex like split(/[-+]/) or more complex, but you'll have to find a way to know what operation follows each digit. One easy approach would be to iterate through both arrays. One of numbers and one of operators
"2+3-5+1".split(/[-+]/) -> [ '2', '3', '5', '1' ]
"2+3-5+1".split(/[0-9]*/).filter(x => x) -> [ '+', '-', '+' ]
You could probably find better regex, but you get the idea
You can ofc use a map or a switch for multiple operators

using a string in a math equation in Dart

I store various formulas in Postgres and I want to use those formulas in my code. It would look something like this:
var amount = 100;
var formula = '5/105'; // normally something I would fetch from Postgres
var total = amount * formula; // should return 4.76
Is there a way to evaluate the string in this manner?
As far as I'm aware, there isn't a formula solver package developed for Dart yet. (If one exists or gets created after this post, we can edit it into the answer.)
EDIT: Mattia in the comments points out the math_expressions package, which looks pretty robust and easy to use.
There is a way to execute arbitrary Dart code as a string, but it has several problems. A] It's very roundabout and convoluted; B] it becomes a massive security issue; and C] it only works if the Dart is compiled in JIT mode (so in Flutter this means it will only work in debug builds, not release builds).
So the answer is that unfortunately, you will have to implement it yourself. The good news is that, for simple 4-function arithmetic, this is pretty straight-forward, and you can follow a tutorial on writing a calculator app like this one to see how it's done.
Of course, if all your formulas only contain two terms with an operator between them like in your example snippet, it becomes even easier. You can do the whole thing in just a few lines of code:
void main() {
final amount = 100;
final formula = '5/105';
final pattern = RegExp(r'(\d+)([\/+*-])(\d+)');
final match = pattern.firstMatch(formula);
final value = process(num.parse(match[1]), match[2], num.parse(match[3]));
final total = amount * value;
print(total); // Prints: 4.761904761904762
}
num process(num a, String operator, num b) {
switch (operator) {
case '+': return a + b;
case '-': return a - b;
case '*': return a * b;
case '/': return a / b;
}
throw ArgumentError(operator);
}
There are a few packages that can be used to accomplish this:
pub.dev/packages/function_tree
pub.dev/packages/math_expressions
pub.dev/packages/expressions
I used function_tree as follows:
double amount = 100.55;
String formula = '5/105*.5'; // From Postgres
final tax = amount * formula.interpret();
I haven't tried it, but using math_expressions it should look like this:
double amount = 100.55;
String formula = '5/105*.5'; // From Postgres
Parser p = Parser();
// Context is used to evaluate variables, can be empty in this case.
ContextModel cm = ContextModel();
Expression exp = p.parse(formula) * p.parse(amount.toString());
// or..
//Expression exp = p.parse(formula) * Number(amount);
double result = exp.evaluate(EvaluationType.REAL, cm);
// Result: 2.394047619047619
print('Result: ${result}');
Thanks to fkleon for the math_expressions help.

How does string interpolation / string templates work?

#lf_araujo asked in this question:
var dic = new dict of string, string
dic["z"] = "23"
dic["abc"] = "42"
dic["pi"] = "3.141"
for k in sorted_string_collection (dic.keys)
print (#"$k: $(dic[k])")
What is the function of # in print(# ... ) and lines_add(# ...)?
As this is applicable to both Genie and Vala, I thought it would be better suited as a stand-alone question.
The conceptual question is:
How does string interpolation work in Vala and Genie?
There are two options for string interpolation in Vala and Genie:
printf-style functions:
var name = "Jens Mühlenhoff";
var s = string.printf ("My name is %s, 2 + 2 is %d", name, 2 + 2);
This works using varargs, you have to pass multiple arguments with the correct types to the varargs function (in this case string.printf).
string templates:
var name = "Jens Mühlenhoff";
var s = #"My name is $name, 2 + 2 is $(2 + 2)";
This works using "compiler magic".
A template string starts with #" (rather then " which starts a normal string).
Expressions in the template string start with $ and are enclosed with (). The brackets are unneccesary when the expression doesn't contain white space like $name in the above example.
Expressions are evaluated before they are put into the string that results from the string template. For expressions that aren't of type string the compiler tries to call .to_string (), so you don't have to explicitly call it. In the $(2 + 2) example the expression 2 + 2 is evaluated to 4 and then 4.to_string () is called with will result in "4" which can then be put into the string template.
PS: I'm using Vala syntax here, just remove the ;s to convert to Genie.

How to work with char types in Dart? (Print alphabet)

I am trying to learn the Dart language, by transposing the exercices given by my school for C programming.
The very first exercice in our C pool is to write a function print_alphabet() that prints the alphabet in lowercase; it is forbidden to print the alphabet directly.
In POSIX C, the straightforward solution would be:
#include <unistd.h>
void print_alphabet(void)
{
char c;
c = 'a';
while (c <= 'z')
{
write(STDOUT_FILENO, &c, 1);
c++;
}
}
int main(void)
{
print_alphabet();
return (0);
}
However, as far as I know, the current version of Dart (1.1.1) does not have an easy way of dealing with characters. The farthest I came up with (for my very first version) is this:
void print_alphabet()
{
var c = "a".codeUnits.first;
var i = 0;
while (++i <= 26)
{
print(c.toString());
c++;
}
}
void main() {
print_alphabet();
}
Which prints the ASCII value of each character, one per line, as a string ("97" ... "122"). Not really what I intended…
I am trying to search for a proper way of doing this. But the lack of a char type like the one in C is giving me a bit of a hard time, as a beginner!
Dart does not have character types.
To convert a code point to a string, you use the String constructor String.fromCharCode:
int c = "a".codeUnitAt(0);
int end = "z".codeUnitAt(0);
while (c <= end) {
print(String.fromCharCode(c));
c++;
}
For simple stuff like this, I'd use "print" instead of "stdout", if you don't mind the newlines.
There is also:
int char_a = 'a'.codeUnitAt(0);
print(String.fromCharCodes(new Iterable.generate(26, (x) => char_a + x)));
or, using newer list literal syntax:
int char_a = 'a'.codeUnitAt(0);
int char_z = 'z'.codeUnitAt(0);
print(String.fromCharCodes([for (var i = char_a; i <= char_z; i++) i]));
As I was finalizing my post and rephrasing my question’s title, I am no longer barking up the wrong tree thanks to this question about stdout.
It seems that one proper way of writing characters is to use stdout.writeCharCode from the dart:io library.
import 'dart:io';
void ft_print_alphabet()
{
var c = "a".codeUnits.first;
while (c <= "z".codeUnits.first)
stdout.writeCharCode(c++);
}
void main() {
ft_print_alphabet();
}
I still have no clue about how to manipulate character types, but at least I can print them.

ANTLR best way to include meta-data in lexing/parsing (custom objects, kind of annotation)

I plan to include text metadata (like bold, font-size, etc.) in the process of parsing to achieve better recognition.
For instance, I have a given structure, where a word on its own line word/r/n which is bold and sized 24px, is the title for some article. In order to get better recognition results, I want to take the characters as well as the metadata in account. In terms of ANTRL I'm not sure how this could be done best. I'd like to do something like:
Wrap each character of the original text into a custom object with fields for the metadata and pass that to ANTLR.
Preprocess the text and insert at specific places annotations for the metadata which is considered by the grammer.
I really like to take option 1. but I'm not sure which part from ANTLR I need to subclass etc. Do I have to start at the ANTLRInputStream-Object, in order to get a proper stream for a subclassed Lexer to get custom Tokens for a subclassed Parser etc. Is there a more elegant way, especially in querying the tokens while parsing with actions in a {} block ?
If anyone has some hints and/or experiences this would be great!
EDIT:
Here is a more specific simple example: I have a file wich includes the encoding of metadata which I parse forehand. the actual text including newline look like the following:
entryOne
Here is some content one.
entryTwo
Here is some content two.
Where the titlesentryOneand entryTwo are originally font-size of 24px and the content is font-size of 12px (as exemplary given values). Char by char I create a new instance of a custom object encapsulating the character as String and the font-size.
I initialize respective objects for each of the characters with fields of the font-size, e.g for the first letter of entryOne like
MyChar aTitelChar = new MyChar("e", 24);
For the content, like the second line Here is some content one. I create instances of MyChar like:
MyChar aContentChar= new MyChar("H", 12);
All characters of the texts are wrapped in instances of the below MyChar-Class and added to a List<MyChar> in order to produce a new input for ANTLR.
below is the Java Class for the characters:
public class MyChar {
private int fontSizePx;
private String text;
public MyChar(String text, int fontSizePx) {
this.text = text;
this.fontSizePx = fontSizePx;
}
public int getFontSizePx() {
return fontSizePx;
}
public String getText() {
return text;
}
}
I want that my grammar matches the above two entries (or more formatted this way) which in turn consist each of a title and a content which is terminated with a fullstop. This grammar could look like this:
rule: entry+ NEWLINE
;
entry:
title
content
;
title:
letters NEWLINE
;
content:
(letters)+ '.' NEWLINE
;
letters:
LETTERS
;
LETTERS:
('a'..'z' | 'A'..'Z')+
;
WS:
(' ' | '\t' | 'f' ) + {$channel = HIDDEN;};
NEWLINE:'\r'? '\n';
Now, for instance, what I want to do is to find out if it's really a title of an entry by checking the font-size of all letters encompassing the title-token before titel-rule returns. In case the input conforms to the grammar but is actually some kind of mistake (the original metadata-encoded file starts with something that conforms to the title-rule but its actually the content) the author of the grammar could sort that out if he knows that the original font-size for titles is 24 and check this. If one of the letter-tokens doesn't equal to font-size 24 throw an exception/don't return/do smthg. appropriate.
The thing I'm pondering on is where to plug in the List<MyChar> to provide this functionality (to query kinds of metadata while parsing in context of ANTLR). I'm experimenting with ANTLR's Classes but as I'm new to ANTLR I thought probably some of the experienced users can point me in the right direction, like where would be a good insertion points for custom objects? should I start by implenting CharStream and override some methods? Probably there is something which ANTLR provides which I haven't found yet?
Here's one way to accomplish what I think you're going for, using the parser to manage matching input to metadata. Note that I made whitespace significant because it's part of the content and can't be skipped. I also made periods part of content to simplify the example, rather than using them as a marker.
SysEx.g
grammar SysEx;
#header {
import java.util.List;
}
#parser::members {
private List<MyChar> metadata;
private int curpos;
private boolean isTitleInput(String input) {
return isFontSizeInput(input, 24);
}
private boolean isContentInput(String input){
return isFontSizeInput(input, 12);
}
private boolean isFontSizeInput(String input, int fontSize){
List<MyChar> sublist = metadata.subList(curpos, curpos + input.length());
System.out.println(String.format("Testing metadata for input=\%s, font-size=\%d", input, fontSize));
int start = curpos;
//move our metadata pointer forward.
skipInput(input);
for (int i = 0, count = input.length(); i < count; ++i){
MyChar chardata = sublist.get(i);
char c = input.charAt(i);
if (chardata.getText().charAt(0) != c){
//This character doesn't match the metadata (ERROR!)
System.out.println(String.format("Content mismatch at metadata position \%d: metadata=(\%s,\%d); input=\%c", start + i, chardata.getText(), chardata.getFontSizePx(), c));
return false;
} else if (chardata.getFontSizePx() != fontSize){
//The font is wrong.
System.out.println(String.format("Format mismatch at metadata position \%d: metadata=(\%s,\%d); input=\%c", start + i, chardata.getText(), chardata.getFontSizePx(), c));
return false;
}
}
//All characters check out.
return true;
}
private void skipInput(String str){
curpos += str.length();
System.out.println("\t\tMoving metadata pointer ahead by " + str.length() + " to " + curpos);
}
}
rule[List<MyChar> metadata]
#init {
this.metadata = metadata;
}
: entry+ EOF
;
entry
: title content
{System.out.println("Finished reading entry.");}
;
title
: line {isTitleInput($line.text)}? newline {System.out.println("Finished reading title " + $line.text);}
;
content
: line {isContentInput($line.text)}? newline {System.out.println("Finished reading content " + $line.text);}
;
newline
: (NEWLINE{skipInput($NEWLINE.text);})+
;
line returns [String text]
#init {
StringBuilder builder = new StringBuilder();
}
#after {
$text = builder.toString();
}
: (ANY{builder.append($ANY.text);})+
;
NEWLINE:'\r'? '\n';
ANY: .; //whitespace can't be skipped because it's content.
A title is a line that matches the title metadata (size 24 font) followed by one or more newline characters.
A content is a line that matches the content metadata (size 12 font) followed by one or more newline characters. As mentioned above, I removed the check for a period for simplification.
A line is a sequence of characters that does not include newline characters.
A validating semantic predicate (the {...}? after line) is used to validate that the line matches the metadata.
Here is the code I used to test the grammar (minus imports, for brevity):
SysExGrammar.java
public class SysExGrammar {
public static void main(String[] args) throws Exception {
//Create some metadata that matches our input.
List<MyChar> matchingMetadata = new ArrayList<MyChar>();
appendMetadata(matchingMetadata, "entryOne\r\n", 24);
appendMetadata(matchingMetadata, "Here is some content one.\r\n", 12);
appendMetadata(matchingMetadata, "entryTwo\r\n", 24);
appendMetadata(matchingMetadata, "Here is some content two.\r\n", 12);
parseInput(matchingMetadata);
System.out.println("Finished example #1");
//Create some metadata that doesn't match our input (negative test).
List<MyChar> mismatchingMetadata = new ArrayList<MyChar>();
appendMetadata(mismatchingMetadata, "entryOne\r\n", 24);
appendMetadata(mismatchingMetadata, "Here is some content one.\r\n", 12);
appendMetadata(mismatchingMetadata, "entryTwo\r\n", 12); //content font size!
appendMetadata(mismatchingMetadata, "Here is some content two.\r\n", 12);
parseInput(mismatchingMetadata);
System.out.println("Finished example #2");
}
private static void parseInput(List<MyChar> metadata) throws Exception {
//Test setup
InputStream resource = SysExGrammar.class.getResourceAsStream("SysExTest.txt");
CharStream input = new ANTLRInputStream(resource);
resource.close();
SysExLexer lexer = new SysExLexer(input);
CommonTokenStream tokens = new CommonTokenStream(lexer);
SysExParser parser = new SysExParser(tokens);
parser.rule(metadata);
System.out.println("Parsing encountered " + parser.getNumberOfSyntaxErrors() + " syntax errors");
}
private static void appendMetadata(List<MyChar> metadata, String string,
int fontSize) {
for (int i = 0, count = string.length(); i < count; ++i){
metadata.add(new MyChar(string.charAt(i) + "", fontSize));
}
}
}
SysExTest.txt (note this uses Windows newlines (\r\n)
entryOne
Here is some content one.
entryTwo
Here is some content two.
Test output (trimmed; the second example has deliberately-mismatched metadata):
Parsing encountered 0 syntax errors
Finished example #1
Parsing encountered 2 syntax errors
Finished example #2
This solution requires that each MyChar corresponds to a character in the input (including newline characters, although you can remove that limitation if you like -- I would remove it if I didn't already have this answer written up ;) ).
As you can see, it's possible to tie the metadata to the parser and everything works as expected. I hope this helps.

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