Keyboard combinations and keyboard mistakes. I'm a longtime Mathematica user but am surprised by the recent inadvertent results when trying to add comments to input. The second line below, the red square, is particularly problematic. It appears at the beginning of the input line and will change the properties of what is usually a variable and assignment operator (set). If it occurs with a variable that is subscripted, I have to reboot Mathematica; undo, Clear[], ClearAll[] will not fix whatever that red square does to my subscripted variables. The freeform input symbol shown is easy to search for (orange box with equals sign), but not so with the red box and other mistakes shown....

Don't perform operations on whole arrays by indexing individual elements....

Horrible:
output = ConstantArray[0, Length[data]]; 
For[j = 1, j <= Length[data], j++, output[[j]] = Sin[data[[j]]]];
output

Bad:
Table[Sin[data[[j]]], {j, 1, Length[data]}]

Nice:
Map[Sin, data]

And if the operation is Listable... Best: Sin[data]

Subscripted variables are a pain...use [i] indexing instead.

Import and "CurrencyTokens"

Perhaps not 'common' but I was bitten by it twice until I made it 'my' default for data import. I assume for some legacy reason the stripping of currency symbols is the default behavior of the Import[] function. Seems an odd default behavior to me and worse the default behavior is not pointed out as boldly as I think it should be in the documentation.

Import[   , "CurrencyTokens" -> None]

Case sensitivity and typos

I am far from a novice user, but I make a lot of typing mistakes, so case sensitivity catches me often...especially on properties and names in WDF, etc.

This command gets an error

ResourceData["On the origin of Species"]

This command gets the data

ResourceData["On the Origin of Species"]

This command gets an error...also I find the error message to be somewhat opaque

DateObject[{2016, 8, 4}, "week"]

This command evaluates the object

DateObject[{2016, 8, 4}, "Week"]

Alternatives to Reap/Sow

1. I would like to mention Table[] in addition. In complicated situations, Reap and Sow maybe be a better alternative, but I've seen quite a bit of beginner's code that looks just like the Do-AppendTo loop above. In that particular case, Table[] is superior both in speed and legibility. It's about 20 times faster than the Reap version:

data = Table[RandomReal[x], {x, 0, 40000}];

(Asides: In some use cases, Map may be an even better choice. Also, many functions are "vectorized" or have special cases for efficiently constructing arrays; for instance, RandomReal[1, 40001] * Range[0., 40000.] produces nearly the same data 150 times faster than Reap, the maximum relative error being less than 10^-12 -- learning to think computationally in terms of arrays can speed up code significantly.)

2. Let me also add linked lists as an alternative (see also this answer by Leonid Shifrin). It is 5-10% faster than Reap & Sow.

data = {};
Do[data = {data, RandomReal[x]}, {x, 0, 40000}];
data = Flatten@data;

If the data is an multidimensional array, then we can structure the flattening operation by using a special head.

Block[{ll},    (* not necessary if  ll  is undefined *)
 data = ll[];
 Do[data = ll[data, RandomReal[x, 2]], {x, 0, 40000}];
 data = List @@ Flatten[data, Infinity, ll];
 ]

Example timing:

SeedRandom[0];    (* for reproducible randomness *)
data = Reap[Do[Sow[RandomReal[x, 2]], {x, 0, 40000}]][[2, 1]]; // AbsoluteTiming
(*  {0.097571, Null}  *)

SeedRandom[0];    (* for reproducible randomness *)
dataLL = ll[];
(Do[dataLL = ll[dataLL, RandomReal[x, 2]], {x, 0, 40000}]; 
   dataLL = List @@ Flatten[dataLL, Infinity, ll]); // AbsoluteTiming
(*  {0.08676, Null}  *)

data == dataLL
(*  True  *)

New in 12.2, ApplyTo x//=f

It's included on the Input Operator Forms webpage.

see "x|->f — new syntax for Function with named variables"

btw your post is the only bookmark I am keeping from this community site.

For some reason the following code is not working for me: ?5.5?=Missing["UnknownSymbol", "5.5?"] Has the floor function syntax been updated?

The forum screwed up some of the symbols and replaced them with ?. Now fixed.

Learn how to use the Documentation Center effectively

Mathematica comes with the most comprehensive documentation I have ever seen in a software product. This documentation contains

• reference pages for every Mathematica function
• tutorials for various topics, which show you step by step how to achieve something
• guide pages to give you an overview of functions about a specific topic
• a categorised function navigator, to help you find appropriate guide pages and reference pages.
• finally, the complete interactive Mathematica book

You can always open the Documentation Center by pressing F1. When the cursor (the I-beam) is anywhere near a function, then the help page of this function is opened. E.g. when your cursor is anywhere at the position where the dots are in .I.n.t.e.g.r.a.t.e., you will be directed to the help page of Integrate.

Reference pages:

A reference page is a help page which is dedicated to exactly one Mathematica function (or symbol). In the image below you see the reference page for the Sin function. Usually, some of the sections are open, but here I closed them so you see all parts at once.

• In blue, you see the usage. It gives you instantly information about how many arguments the function expects. Often there is more than one usage. Additionally, a short description is given.
• The Details section gives you further information about Options, behavioural details and things which are important to note. In general, this section is only important in a more advanced state.
• In some cases, extra information is provided on the mathematical Background of the function explaining the depths of the method, its relation to other functions and its limitations (for example FindHamiltonianCycle).
• The Examples section is the most important, because there you have a lot of examples, showing everything starting from simple use cases to very advanced things. Study this section carefully!
• See Also gives you a list of functions which are related. Very helpful, when a function does not exactly what you want, because most probably you find help in the referenced pages.
• Tutorials shows you tutorials which are related to the function. In the case of Sin it is e.g. the Elementary Transcendental Functions tutorial.
• Related Guides gives you a list of related guide pages.
• Related Links references to material in the web: Demonstrations, MathWorld pages, etc.

In general, my recommendation for viewing a help page is the following:

1. Study the usage carefully
2. Look up basic examples. If you don't find what you need, look up all examples
3. Read the Details

And of course if you like the how-to style, you should read the referenced tutorials.

Guide pages:

Guide pages collect all functions which belong to a certain topic and they are an excellent resource when you try to find a function you do not know yet.

The guide page itself is often divided into several subsections collecting similar functions. In the image above, for instance, the Trigonometric Functions. Furthermore, you can find links to tutorials, etc. when you open the Learning Resources tab. At the end of each guide page, you will find references to related guide pages.

Final notes:

• Since the complete documentation consists of usual Mathematica notebooks, all calculations and examples can be tested inside the help pages. Of course, you cannot destroy the documentation, because everything is reset when you close a help page.

• You can always search the documentation by typing into the search bar on top of the Documentation Center:

  

• When coming from a different programming language, and you are not sure that a certain Mathematica function is equivalent to what you are used to, be sure to check the Properties & Relations section in the reference page to get ideas on what other functions could be relevant for your case.

Please find the original discussion here.