calculator: the chain gang

Let me be blunt.  The calculator is not a difficult project once you’re aware of the eval() function that takes a string and treats it as an arithmetic expression and evaluates it. If I knew the name of the person who mentioned it, I’d share.  The trouble is, it was a toss-off comment in a reply to someone else and I happened to be paying attention.  For the record, attention is good.

I hang out in the gitter FreeCodeCamp room quite often and this sort of thing is there, going on all the time.  Someone posts a problem, whether it’s with a challenge (“how do I nest my image element in an anchor tag?”), an algorithm (“can someone check my Roman number code?”) or a project (“my Twitch viewer shows the same channel eight times instead of showing the eight channels from my array”) and others offer help. Usually, the problem is spotted and resolved (often the person posting sees the issue as soon as they shared it). Someone else’s calculator issue was solved by eval() and I was watching.  This was back when I was doing random quotes or something.

The difficult part of the calculator for me was how do I make something that looks like a calculator in HTML and CSS. The answer is by looking at someone else’s calculator.  Google Images to the rescue.  I found the OSX calculator icon. It has buttons on it for Clear and ± and memory management stuff in the top row that I don’t need. So if I delete the top row, I have a basic idea of what a calculator should have. If you look at it side-by-side with my calculator, you can see how it was inspired by this one. My phone calculator looks like someone was too lazy to bother making it look like it wasn’t an app on a phone. So using that as a guide would have given me something really ugly and not like the project was looking for.
OSX calculator icon phone calcmy calc

The colors for my calculator are courtesy of Computer Hopes HTML Color Codes & Names page.  I found the red for that button as a trinary color for the blue of the other buttons. You can find trinary colors by typing or pasting the hex code for a color in the box at the top. I got the display box color by sampling the color from the OSX calculator and averaging it. It took a bit of trial-and-error to get good button colors with my faceplate color.

But that’s only part of it, there are also fonts. For the display, I wanted a font with something in the zero.  A slash or dot or something. But not for the buttons. Zero on a button should look empty and the C should look reasonable.  So I hit up Google Fonts.  I specifically typed in the digits and the key signs (so I could see what the keys would look like) and told it to make that be the sample for all of the fonts.  Then I said I only wanted monospace fonts. And alphabetical because I was looking for specific things, not popularity. The fonts I chose were Abel for the the keys and Allerta for the display. I liked the look of the C in Abel and the fact the 4 was closed.

So, on to making it function.  I’m sure there’s a better way—there always is—but what I did was made a click event for each button and then handled it. For the digits, it was easy—check if the the entry was zero—if it was, replace it with the number, otherwise, add the digit to the string for the current number.  Decimal point was tough—you can only have one per number.  And signs meant I had to peel off the current number and make a new one.  And then the equals meant, run eval().  Okay, that all worked so I took it to gitter.

And someone said it didn’t “chain”.  Now, I could type 2+5*4  and press the equal sign and get 22 so I knew it worked.  But that wasn’t what they meant.  They meant, if I pressed equals again, it should repeat the last operation so I should get 88 (22 * 4) the next time, and so on.  While I was fixing it, we got into a side conversation about how I don’t generally use a calculator so I was unaware of that expected behavior.  Then, right when I had typed “try it now” they typed, “don’t worry about it, mine doesn’t do that, either,” and I had to laugh because, well, mine does it.

Here’s how.  I’m already keeping the current number separate from the rest of the current expression.  I have to so I can use the ± on it.  So what I need to do is grab the last operator from the string (it will be the last character) and combine them and put that in the chain:

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$("#eqButton").on("click", function(){
if(curNum != ""){
// if the expression isn't empty, evaluate it
curResult = eval(curExp + curNum);
if(curResult === 3/0){
curResult = "0 Divide Error";
}
chain = curExp.substr(curExp.length-1) + curNum;
curExp += curNum;
oldExp = curExp;
}
else{
// they pressed = again
curResult = eval(oldExp += chain);
}
curNum = "";
dispResult();
curExp = "";
dispCurExp();
});

It’s still on CodePen but will be moved to GitHub before I turn it in.  The links on the right will always be current.

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