Writing a function that "solves" an equation - javascript

I want to write a function which will allow me to "solve" an equation in js.
what I want (not in a programming language):
function f(x) { 1 + x * x }
var z = 2
var y = f(z) //y will be 5 as a number
what I have written in JS:
function P(cfg) { ....
this.equation = "1 + x";
....};
P.prototype.eqn = function(x) {
var tmp = eval(this.equation);
return tmp;
};
....
P.prototype.draw = function() {....
for(var i = 0; i < z; i++)
ctx.lineTo(i, this.eqn(i));
....};
also I've read that using eval in a loop is probably not a good idea, but I have not figured out another way (yet) (JS beginner)...
The problem with this code is, that at least in FF the var tmp will STILL contain the string from this.equation instead of the calculated value.
I would appreciate any further insight very much!
Thank you for your time :)
EDIT: because my question was not formulated very well:
after the execution of line
var tmp = eval(this.equation);
the var tmp will hold a STRING which equals the string this.equation, instead of the desired solution y value.
Also I do not mean solve but evaluate, thanks for that tip :)

Based on your example, I'd say that you want to "evaluate an expression", rather than "solve an equation". For evaluating an expression, you can probably find many tutorials. I'll break it down in brief though. You need to do a few steps.
Starting with your string "1 + x * x", you need to break it into tokens. Specifically, break it down into: "1", "+", "x", "*", "x". At this point, you can substitute your variables ("x") for their literal values ("2"), giving you "1", "+", "2", "*", "2"
Now you need to parse the expression. Based on order of operations PEMDAS you need to create a tree data structure, where parenthetical clauses (stuff surrounded by parenthesis) are executed first, multiplication and division next, and then additions and subtraction last. Parsing is often not an easy task, and you may want to put together a simpler BNF grammar (though you can probably find a grammar for simple math expressions with some googling).
Next, walk the tree, depth first, evaluating the operations as you go up the tree. Once you get to the top of the tree, you have your solution.
If instead you want to "solve an equation", you're going to need something much more sophisticated, like Sage

I have used this expression evaluator before. It seemed to work very well. It allows you to pass expressions into a Parser that returns a function object that can then evaluate inputs.
var expr = Parser.parse("2 ^ x");
expr.evaluate({ x: 3 }); // 8
It supports trig functions (sin, cos, ect...) and other handy built in functions such as abs & ciel.
var expr = Parser.parse("sin(x/2) + cos(x/2)")
expr.evaluate({x: Math.PI / 2}); // 1
Examples: http://silentmatt.com/javascript-expression-evaluator/
Code: https://github.com/silentmatt/js-expression-eval
Note that this lib does not use eval().

Not sure I entirely understand your question but how about:
var makeFunctionOfX = function(src) {
return Function('x', 'return ' + src);
};
Then you can say things like:
var g = makeFunctionOfX('2 * x')
var y = g(3); // y contains 6
The great advantage of this over eval is that the Function we create has no magic ability to see variables in the scope (hence the need to explicitly pass it x as a parameter name).

Using eval is safe if you trust the input from the user, and works just fine. (I have no idea what you mean by "the var tmp will still have the string this.equation".)
function FuncCreator(eqn){ this.eqn = eqn }
FuncCreator.prototype.run = function(x,y,z){ return eval(this.eqn) }
var add1 = new FuncCreator('1+x');
var result = add1.run(41); // 42
var complex = new FuncCreator('Math.sin(x*y) / Math.cos(z)');
var result = complex.run(3,4,5); // -1.891591285331882
If you don't trust the user input, you'll need to actually parse the input and process it yourself. This is non-trivial.

You can use the expression parser from the math.js library and do something like this:
var parser = math.parser();
var f = parser.eval('function f(x) = 1 + x * x');
// use the created function f in expressions:
parser.eval('z = 2'); // 2
parser.eval('y = f(z)'); // 5
// or use the created function f in JavaScript:
var z = 2; // 2
var y = f(z); // 5
Creating functions in math.js is quite currently limited, loops and blocks needed to define more extensive functions are not yet supported.

This is an old thread, but I wrote this equation calculator, this doesn't solve algebraic equations though. There is however a function that will allow you to provide an array containing assigned variables. But this doesn't solve for variables that don't have an assigned value.
I probably haven't permuted every test case scenario, but it seems to work pretty decent.
Edit: This would have to be modified to handle negative numbers. Other than that... works fine.
Here is a fiddle
<!doctype html>
<html>
<head>
<title>Javascript Equation Calculator</title>
</head>
<body>
<input type="button" onclick="main()" value="calculate"><br>
<input type="text" id="userinput"><br>
<span id="result">Ready.</span><br>
<script>
function Calculator(){}
String.prototype.replaceLast = function (what, replacement)
{
var pcs = this.split(what);
var lastPc = pcs.pop();
return pcs.join(what) + replacement + lastPc;
};
function inS(substr, str){return (str.indexOf(substr) > -1);}
function arrayValueOrToken(arr, key, token)
{
if(key in arr)
{
return arr[key];
}
return token;
}
function reduceEquation(inputStr)
{
console.log("reduceEquation Executed-----");
while(hasNest(inputStr))
{
if(hasNest(inputStr))
{
inputStr = inputStr.replace(")(",')*(');
for(var i=0;i<=9;i++)
{
inputStr = inputStr.replace(i+"(",i+'*(');
inputStr = inputStr.replace(")"+i,')*'+i);
}
var s = inputStr.lastIndexOf("(");
var e = 0;
for(i=s;i,inputStr.length;i++){if(inputStr[i]==")"){e=i+1;break;}}
var eq = inputStr.substring(s,e);
var replace = eq;
eq = eq.replace(/[()]/g, '');
var substitution = solveEquation(eq);
inputStr = inputStr.replaceLast(replace,substitution);
}
}
return inputStr;
}
function solveEquation(eq)
{
console.log("solveEquation Executed-----");
eq = doFirstOrder(eq);
eq = doLastOrder(eq);
return eq;
}
function doFirstOrder(eq)
{
console.log("doFirstOrder Executed-----");
for(var i=0;i<eq.length;i++)
{
if(eq[i]=="*"){eq = solve(eq,"*");return doFirstOrder(eq);}
if(eq[i]=='/'){eq = solve(eq,'/');return doFirstOrder(eq);}
}
return eq;
}
function doLastOrder(eq)
{
console.log("doLastOrder Executed-----");
for(var i=0;i<eq.length;i++)
{
if(eq[i]=="+"){eq = solve(eq,"+");return doLastOrder(eq);}
if(eq[i]=="-"){eq = solve(eq,"-");return doLastOrder(eq);}
}
return eq;
}
function solve(eq, operator)
{
var setOp = operator;
console.log("solve Executed-----");
var buildEq = "",var1 = true,done = false,char="";
var operators = "+-/*";
var ops = operators.replace(operator, '').split('');
var a=ops[0];
var b=ops[1];
var c=ops[2];
for(var i=0;i<eq.length;i++)
{
char = eq[i];
switch(true)
{
case(char==operator):if(var1===true){var1 = false;}else{done = true;}break;
case(char==a):
case(char==b):
case(char==c):if(var1){char = ""; buildEq = "";}else{done = true;}
}
if(done){break;}
buildEq = buildEq + char;
}
var parts = parts = buildEq.split(operator);
var solution = null;
if(operator=="+"){solution = parseFloat(parts[0]) + parseFloat(parts[1]);}
if(operator=="-"){solution = parseFloat(parts[0]) - parseFloat(parts[1]);}
if(operator=="*"){solution = parseFloat(parts[0]) * parseFloat(parts[1]);}
if(operator=="/"){solution = parseFloat(parts[0]) / parseFloat(parts[1]);}
return eq.replace(buildEq, solution);
}
function hasNest(inputStr){return inS("(",inputStr);}
function allNestsComplete(inputStr)
{
var oC = 0, cC = 0,char="";
for(var i=0;i<inputStr.length;i++){char = inputStr[i];if(char=="("){oC+=1;}if(char==")"){cC+=1;}}
return (oC==cC);
}
Calculator.prototype.calc = function(inputStr)
{
console.log("Calc Executed-----");
inputStr = inputStr.replace(/ /g, "");
inputStr = inputStr.replace(/\\/g, '/');
inputStr = inputStr.replace(/x/g, "*")
inputStr = inputStr.replace(/X/g, "*")
if(!allNestsComplete(inputStr)){return "Nested operations not opened/closed properly.";}
inputStr=reduceEquation(inputStr);
inputStr = solveEquation(inputStr);
return inputStr;
};
Calculator.prototype.calcWithVars = function(inputList)
{
if(inputList.length < 2){return "One or more missing arguments!";}
var vars = [];
var assocVars = [];
var lastVarIndex = inputList.length - 2;
var i = 0;
var inputStr = inputList[inputList.length-1];
for(i=0;i<=lastVarIndex;i++)
{
vars.push(inputList[i].replace(/ /g, ""));
}
for(i=0;i<=vars.length-1;i++)
{
var vParts = vars[i].split("=");
var vName = vParts[0];
var vValue = vParts[1];
assocVars[vName] = vValue;
}
inputStr = inputStr.replace(/ /g, "");
var eqVars = inputStr.replace(/\s+/g, ' ').replace(/[^a-zA-Z-]/g, ' ').replace(/\s\s+/g, ' ');
if(inS(" ", eqVars))
{
eqVars = eqVars.split(" ");
}
else{eqVars = [eqVars];}
eqVars.sort(function(a, b){return a.length - a.length;});
var tempTokens = [];
var tempCount = 1;
for(i=0;i<eqVars.length;i++)
{
var eqVname = eqVars[i];
var substitution = arrayValueOrToken(assocVars, eqVname, "<unknown>");
if(substitution != "<unknown>")
{
inputStr = inputStr.replace(eqVname,substitution);
}
else
{
var tempToken = "#______#"+tempCount+"#______#";
tempCount++;
tempTokens.push(tempToken + "?" + eqVname);
inputStr = inputStr.replace(eqVname,tempToken);
}
}
for(i=0;i<tempTokens.length;i++)
{
var tokenSet = tempTokens[i];
var tokenParts = tokenSet.split("?");
var token = tokenParts[0];
var variableName = tokenParts[1];
inputStr = inputStr.replace(token,variableName);
}
var answerName = "<unknown>";
var eq = inputStr;
if(inS("=", inputStr))
{
var eqParts = inputStr.split("=");
answerName = eqParts[0];
eq = eqParts[1];
}
eq = this.calc(eq);
var result = [];
for(i=0;i<eqVars.length;i++)
{
var v = arrayValueOrToken(assocVars, eqVars[i], "<unknown>");
if(v != "<unknown>")
{
result.push(assocVars[eqVars[i]]);
}
}
result.push(eq);
return result;
};
function main()
{
var calculator = new Calculator();
elUserInput = document.getElementById('userinput');
console.log("input: "+ elUserInput.value);
elResult = document.getElementById('result');
equation = elUserInput.value;
result = calculator.calc(equation);
console.log("result: "+ result);
elResult.innerHTML = result;
}
</script>
</body>
</html>

Related

Return multiple strings from function based on variable number

I have a string
var str = 'string'
I have a multiplier
var mult = 3
I want to return stringstringstring
The mult will change. Basically mult is kind of like a power but this is a string not a number. And I need to return multiple strings. I'm thinking of looping where mult = the number of times to loop and each would conceptually 'push' but I don't want an array or something like =+ but not a number. I'm thinking I could have the output push to an array the number of times = to mult, and then join the array - but I don't know if join is possible without a delimiter. I'm new at javascript and the below doesn't work but it's what I'm thinking. There's also no ability to input a function in the place I'm running javascript and also no libraries.
var loop = {
var str = 'string'
var arr = [];
var mult = 3;
var i = 0
for (i = 0, mult-1, i++) {
arr.push('string'[i]);
}
}
var finalString = arr.join(''); // I don't know how to get it out of an object first before joining
Not sure if what I want is ridiculous or if it's at all possible
You mean something like below,
var str = 'string'
var mult = 3
var str2 = ''
for(i = 0; i < mult; i++) {
str2 += str
}
console.log(str2)
var str = 'string'
var mult = 3;
var sol =""
while(mult--) {
sol +=str;
}
console.log(sol)
Using resusable function:
const concatStr= (str, mult)=>{
var sol =""
while(mult--) {
sol +=str;
}
console.log(sol)
}
concatStr("string",3)
Using the inbuilt Array.from method:
var str = "string"
var mult = 3
var sol = Array.from({length: mult}, ()=> str).join("")
console.log(sol)
function concatString(str, mult) {
var result = ''
for(i = 0; i < mult; i++) {
result = result.concat(str);
}
return result;
}
const value = concatString('string', 3);
console.log(value);
Also you can use array inbuilt methods,
const mult = 3, displayVal = 'str';
Array(mult).fill(displayVal).join('');
// the string object has a repeat method
console.log('string'.repeat(3));

Getting 6.33 from an equation in Javascript when it should output 8.99

I'm doing some simple math in Javascript, but my equation's result is drastically different than what it should be. The math is:
3.05+(((0.32*0)+3.28)+(1+(0.19*0))*(2.66*1^2))*1;
When I did it out by hand, and then used Wolfram Alpha (https://www.wolframalpha.com/) I get the correct result of 8.99. However, when I use the equation in Javascript I mysteriously get 6.33
The actual equation looks like
VO2move = VO2rest+(((C1*g2)+VO2walkmin)+(1+(C2*g2))*(C3*s2^2))*t2;
but I removed all the variables in an attempt to debug (I thought it might be some error where I needed parseInt)
Here are the whole functions for reference
function calc(){
var temp = 0;
var total = 0;
for(i = 0; i<sArr.length; i++){
total = total + calc2(i);
}
var o = document.getElementById("output");
o.value = total;
}
function calc2(i){
var s = document.getElementById("s"+i);
var g = document.getElementById("g"+i);
var t = document.getElementById("t"+i);
var VO2walkmin = 3.28;
var VO2rest = 3.05;
var C1 = 0.32;
var C2 = 0.19;
var C3 = 2.66;
var Cdecline = 0.73;
var s2 = s.value;
var g2 = g.value;
var t2 = t.value;
var negGrade = g.value;
if(g2 < 0){g2 = 0};
//VO2move = ((C1 * g2)+VO2walkmin)+((1+(C2*g2))*(C3*s2^2)); //ORIGINAL TRANSCRIPTION
//VO2move = VO2rest+(((C1*g2)+VO2walkmin)+(1+(C2*g2))*(C3*s2^2))*t2; // TRANSLATED FROM COPY PASTE
VO2move = 3.05+(((0.32*0)+3.28)+(1+(0.19*0))*(2.66*1^2))*1; // COPY - PASTED FROM EXCEL
return VO2move;
}
Even naked numbers I still get the output of 6.33. I'm totally puzzled, and any help is appreciated.
You need to take the power (exponentiation) operator ** instead of the bitwise XOR operator ^.
console.log(3.05+(((0.32*0)+3.28)+(1+(0.19*0))*(2.66*1**2))*1);

RegExpr and variable

I have just one question maybe stupid (like every day)
var word = []; (an array with 100 words for example)
var tab = []; // resultat
var root = "test";
var debut = "Anti";
var reg1=new RegExp("^"+debut + "+." + root,"g")
for(var i = 0;i<word.length; i++){
// a word begin with Anti and contain test pls
if (word[i].match(reg1)){ยด
tab.push(word[i])
}
}
console.log(tab.join(', ');
but it is dont work, i dont know how to use variable with regexpr, thanks, sorry for my english
var r = new RegExp('anti.*esis', 'ig')
document.write('antithesis'.match(r), '<br/>') // ["antithesis"]
document.write('antihero'.match(r), '<br/>') // null
Here is the code, but is used the test() instead of match()
var word=["yea","antiboyahtest","antigssjshbztest"];
var debut="anti";
var root="test";
var reg=new RegExp("^"+debut+".*"+root,"g");
var tabs=[];
for(i in word){
if(reg.test(word[i])){
tabs.push(word[i]);
}
}
alert(tabs);
The solution using RegExp.test and Array.filter functions:
var word = ['Antitest', 'Antidot', 'Anti-next-test', 'testAnti'],
root = "test", debut = "Anti",
reg1 = new RegExp("^"+debut + ".*?" + root, "g");
var result = word.filter(function (w) {
return reg1.test(w);
});
console.log(result); // ["Antitest", "Anti-next-test"]
Also, there's an additional approach using Array.indexOf function without any regex which will give the same result:
...
var result = word.filter(function (w) {
return w.indexOf(debut) === 0 && w.indexOf(root) !== -1;
});

javascript - Get a portion of the cookie value with substr method

I have a cookie called "login" that contains a structure like "username|hashcode|salt".
Here's my code:
function readTheCookie(the_info)
{
var the_cookie = document.cookie;
var the_cookie = unescape(the_cookie);
var broken_cookie2 = the_cookie.substr(6);
alert(broken_cookie2);
}
readTheCookie('login');
I'ts giving me
pickup22|d47f45d141bf4ecc999ec4c083e28cf7|4ece9bce292e1
Now I just want the first part (everything before the first pipe , in that case, I want pickup22)
How can I do that? Cause the username will never be the same, so I cant put a "fixed" lenght.
Any help appreciated!
var readTheCookie = function (the_info) {
var the_cookie = document.cookie.split(";"), a = the_cookie.length, b;
for (b = 0; b < a; b += 1) {
if (the_cookie[b].substr(0, the_info.length) === the_info) {
return the_cookie.split("=")[1].split("|")[0];
}
}
if (b === a) {
return "";
}
},
username = readTheCookie('login');
That is nice and compact, plus easy to read, and finally it is JSLint compliant. Enjoy!
best way is to use split() method.
var parts = new Array();
parts = broken_cookie2.split("|");
var username = parts[0];
var hashcode = parts[1];
var salt = parts[2];

Javascript token replace/append

I have a string that looks something like the following 'test:1;hello:five;just:23'. With this string I need to be able to do the following.
....
var test = MergeTokens('test:1;hello:five;just:23', 'yes:23;test:567');
...
The end result should be 'test:567;hello:five;just:23;yes:23' (note the exact order of the tokens is not that important).
Just wondering if anyone has any smart ideas of how to go about this. I was thinking a regex replace on each of the tokens on right and if a replace didn't occur because there was not match just append it. But maybe there is better way.
Cheers
Anthony
Edit: The right side should override the left. The left being what was originally there and the right side being the new content. Another way of looking at it, is that you only keep the tokens on the left if they don't exist on the right and you keep all the tokens on the right.
#Ferdinand
Thanks for the reply. The problem is the efficiency with which the solution you proposed. I was initially thinking down similar lines but discounted it due to the O(n*z) complexity of the merge (where n and z is the number tokens on the left and right respectively) let alone the splitting and joining.
Hence why I was trying to look down the path of a regex. Maybe behind the scenes, regex is just as bad or worse, but having a regex which removes any token from the left string that exists on the right (O(n) for the total amount of token on the right) and then just add the 2 string together (i.e. vat test = test1 + test2) seems more efficient. thanks
I would use join() and split() to create some utility functions to pack and unpack your token data to an object:
// Unpacks a token string into an object.
function splitTokens(str) {
var data = {}, pairs = str.split(';');
for (var i = 0; i < pairs.length; ++i) {
var pair = pairs[i].split(':');
data[pair[0]] = pair[1];
}
return data;
}
// Packs an object into a token string.
function joinTokens(data) {
var pairs = [];
for (var key in data) {
pairs.push(key + ":" + data[key]);
}
return pairs.join(';');
}
Using these, merging is easy:
// Merges all token strings (supports a variable number of arguments).
function mergeTokens() {
var data = {};
for (var i = 0; i < arguments.length; ++i) {
var d = splitTokens(arguments[i]);
for (var key in d) {
data[key] = d[key];
}
}
return joinTokens(data);
}
The utility functions are also useful if you want to extract some keys (say,"test") and/or check for existence:
var data = splitTokens(str);
if (data["test"] === undefined) {
// Does not exist
} else {
alert("Value of 'test': " + data["test"]);
}
The following is what I ended thiking about. What do you guys recon?
Thanks
Anthony
function Tokenizer(input, tokenSpacer, tokenValueSpacer) {
this.Tokenizer = {};
this.TokenSpacer = tokenSpacer;
this.TokenValueSpacer = tokenValueSpacer;
if (input) {
var TokenizerParts = input.split(this.TokenSpacer);
var i, nv;
for (i = 0; i < TokenizerParts.length; i++) {
nv = TokenizerParts[i].split(this.TokenValueSpacer);
this.Tokenizer[nv[0]] = nv[1];
}
}
}
Tokenizer.prototype.add = function(name, value) {
if (arguments.length == 1 && arguments[0].constructor == Object) {
this.addMany(arguments[0]);
return;
}
this.Tokenizer[name] = value;
}
Tokenizer.prototype.addMany = function(newValues) {
for (nv in newValues) {
this.Tokenizer[nv] = newValues[nv];
}
}
Tokenizer.prototype.remove = function(name) {
if (arguments.length == 1 && arguments[0].constructor == Array) {
this.removeMany(arguments[0]);
return;
}
delete this.Tokenizer[name];
}
Tokenizer.prototype.removeMany = function(deleteNames) {
var i;
for (i = 0; i < deleteNames.length; i++) {
delete this.Tokenizer[deleteNames[i]];
}
}
Tokenizer.prototype.MergeTokenizers = function(newTokenizer) {
this.addMany(newTokenizer.Tokenizer);
}
Tokenizer.prototype.getTokenString = function() {
var nv, q = [];
for (nv in this.Tokenizer) {
q[q.length] = nv + this.TokenValueSpacer + this.Tokenizer[nv];
}
return q.join(this.TokenSpacer);
}
Tokenizer.prototype.toString = Tokenizer.prototype.getTokenString;
i am a few years late, but i think this is what you are looking for:
function MergeTokens(input, replace){
var replaceTokens = replace.split(";");
for(i=0; i<replaceTokens.length; i++){
var pair = replaceTokens[i].split(":");
var result = input;
regString = "\\b" + pair[0] + ":[\\w]*\\b";
var reg = new RegExp(regString);
if(reg.test(result)){
result = result.replace(reg, replaceTokens[i]);
}
else{
result = result + replaceTokens[i];
}
}
return result;
}

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