What are common mistakes in coding? [closed]

Question

For example, a common error in C/C++ is to use the assignment operator = instead of the comparison operator ==.

Answer 1

Off-by-one errors

Iterating through a loop either once too often or one time less than intended, e.g.

for($i = 1; $i < $number_of_times_to_loop; $i++)

will go through a loop once less than intended. Changing the < to <= would get the above loop to cycle $number_of_times_to_loop as intended.

Another type of this error is the Fencepost error, so-called due to the following illustrative example:

Given a 100m stretch of land, how many fence posts do you need to dividie it in to 10 equal sections? Typical answer is 10 (100/10), forgetting the extra post needed for the outer boundary.

| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | <-- divisions

1---2---3---4---5---6---7---8---9---10---11 <-- posts required

Answer 2

Forgetting to close connections. (HTTP, database, whatever.)

Closing the connections prematurely.

Answer 3

Writing

if (x > 5) return true;
else return false;

instead of

return (x > 5);

Answer 4

In C#, using throw ex instead of throw inside of a catch block. The former will reset the stack trace.

Answer 5

Misplaced semicolon C/C++ and/or alignment:

if(x == 5);  
  doSomething();  

doSomething() is always called even when x is not 5.

Obviously you know not to put a semicolon at the end of the if but sometimes it can be an overlooked typo.

Answer 6

Test using assignment = instead of comparison ==.

if( status = 10 ) {
    doSomething();
} else {
   doSomethingElse();
}

Mhh I wonder we never execute the else part

Answer 7

Forgetting the ; is probably the most common mistake in C-family languages. Failing to check the bounds of an array is probably another very common mistake in languages that allow you to make it...

Answer 8

Doing this in C/C++ :

#define VALUE 5;

It will cause compilation error elsewhere in your code.

Answer 9

Directly using variables without defining it in Javascript. Hence variables will have global scope. And programmer spends whole time debugging the scripts!!

Answer 10

I saw quite a few errors with missing break in switch statements (C++, Java). Fortunately C# forbid this practice.

switch (var) {
    case 1:
        //do something
    case 2:
        //do something else
}

Answer 11

I love the

a =+ 1;

believing that a will be incremented.

Answer 12

Recursive properties in C#:

private string foo;
public string Foo
{
   get { return Foo; }
}

Doesn't happen so often now we have auto-properties, but it's still something I wish the compiler would pick up on.

Answer 13

Character encoding

Working with Unicode: converting to/from/between ASCII/ISO 8859-1/Windows-1252/UTF-8 and other encodings, double encoding, not keeping track across applications, systems, protocols and file formats what encoding textual bytestrings are stored in... the list goes on. Few things are gotten wrong this often.

Very closely related: string escaping and unescaping, the bane of too many websites.

Answer 14

In languages like C#, changing the value of a parameter passed in to a method and then wondering why it's not updated in the calling code.

Answer 15

Forgetting to terminate a worker thread when a GUI app exits, so the process continues to run, out of sight.

Answer 16

Null References.

In OOP an object has control ( or should have control ) over the attributes/data of himself.

By setting them to an usable state and controlling how those attributes are modified, the source code prevents using null references ( Initialize with valid values ).

The common mistake is to allow the modification of these attributes with a potentially null reference.

Ej. in Java:

class Customer {
    private final String name; // null at this point 
    public Customer( String named ) {
        if( named == null ) { throw IllegalArgumentException("name cannot be null"); }
        this.name = named; // no longer null and the object has a valid state 
    }

    public String getName(){
        return this.name;// ok 
    }
    public void setName( String newName ) {
        this.name = newName; // eeerh... common programming error if newName = null 
    }
    public int someCalculation() {
        return this.name.length();// NullPointerException 
    }
}

A possible workaround is:

1.- Remove the setter

2.- Validate the setter

public void setName( String newName ) {
    if( newName == null ) { throw new NullPointerException("newName must be not null"); }
    this.name = newName;
}

That is, reject the invalid value, the let know the caller, they are using the object wrong. If the program is to fail, it is much better to fail fast.

One, mistake to try to mitigate this problem is to validate before each call:

public int someCalculation() {
    if( this.name != null ) {  
        return this.name.length();
    } else {
        return 0;
    }
 }

But, its too late, because at that point the object no longer trust its internal data, and that pattern if( x != null ){} will spread all over the place, just making things worst.

Answer 17

Catching exception that you don't know how to recover from, or catching a large family of exceptions (or even all exceptions) and not re-throwing when you encounter one that you don't know how to handle.

In .NET, not bothering to look up what exceptions can be thrown in the documentation (or even the intellisense tooltip) and allowing exceptions that you should have caught (and either recovered from or wrapped and thrown) be bubbled up and to a caller who neither expects them nor understands them.

Answer 18

One programming error I've seen a ridiculous number of times: Bad list management.
It usually looks like this:

private void TrimList( List<int> values, int garbageValue )
{
    for (int i = 0; i < values.Count; ++i)  
    {
        if (values[i] == garbageValue)
        {
            values.RemoveAt( i );
        }
    }
}

If you ever see this problem, you'll have to review the whole codebase. You're going to see flavours of this problem in multiple places.

Answer 19

• Memory leak in C or C++. Using the RAII idiom in C++ is very helpful.
• Another memory error is double free in C and C++, which can be disastrous as well.

Answer 20

When switching between PHP and C♯/Java, I often forgot and write ' instead of " for string constants and end up having compiling errors.

Like:

PHP

<?php
echo 'hello'; // allowed =)

C♯

Console.Write('hello world'); // <- error!

Answer 21

In managed memory platforms: keeping a reference to an object for more time than it is needed causing memory to grow constantly and thinking that "the garbage collector doesn't work".

I have seen it many times while troubleshooting applications made by programmers new to .NET or Java.

Memory Leaks in Managed code

Answer 22

While taking 101, my teacher and I were combing through a student's program because it wouldn't compile. As it turns out, he used

''

instead of

"

everywhere in his program. Three hours of my life that I will never get back.

Answer 23

Forgetting if string functions are 0-based or 1-based with whatever language I'm using

I'm always getting the wrong index when trying to parse a string using string functions because it's different for different languages. It almost always takes me more then one try to get it right.

For example...... I want a substring of a string starting at the index of X and ending at the index of Y... That's string.SubString(IndexOf('X'), string.IndexOf('Y') - string.IndexOf('X')) and I'm sure there's a +1 or -1 in one of those parameters somewhere....

Answer 24

I always write "pubic" instead of "public" in Java method signatures. Not really a functionality changing mistake, but a funny one all the same

Answer 25

A misplaced semicolon beside a while loop.

    while (i < max); // <-- infinite loop
    {
        // do something
        i++;
    }
    // why isn't the code below executing?

Answer 26

Thinking that just because it runs without an error that it is correct.

Answer 27

Assuming that default arguments in Python are not shared between function calls.

Answer 28

For example, a common error in C/C++ is to use the assignment operator = instead of the comparison operator ==.

And in OCaml language I'm currently using, it's common to use "physical equality operator" == instead of simple "equality operator" =. Ditto for "physical" != and "usual" <>. I'm already used to writing ==, but here I have to wrire =; this is a source of numerous errors.

"Physical equality" means "these two identifiers refer to the same object", contrary to "refer to (possibly different) equal objects". It has synnonyms in other languages as well.

Answer 29

Over reliance on refactoring tools, and code assistance tools (CodeRush / Resharper, etc).

Don't get me wrong, I love CodeRush, but they shouldn't be used solely to write the code.

In my mind a coder should be able to write code, and use them to assist where necessary.

Answer 30

Worst thing I ever saw was a fellow student who wrote something like this:

int main(){

    for(int x = 0; x < 10; x++){
        for(int y = x; y < 10; y++)
            cout << x + y << endl;
            for(int z = 0; z < y; z++){
               cout << x + y * z << endl;
            }
        }
    }

   return 0;
}

and was getting some weird weird errors.

See if you can spot the error...