Your most common programming mistakes?

Question

I find myself repeating some mistakes over and over again. Some language-dependent, some not. Here are some of mine I could think of right away.

PHP:

• Inside a class method, forgetting that I have to say "$self->some_method()" or "$self->some_attribute" instead of "some_method()" or "$some_attribute" to refer to my instance.

• Having a deservedly global object like $db or $facebook, then trying to call it from inside a function and wondering why that doesn't work. Oh yeah, "global $facebook".

• Trying to return multiple values from a function by saying "return list($a, $b)" instead of "return array($a, $b)".

Python:

• Getting wrong argument count errors because forgetting that "self" has to be first arg.

Actionscript:

• Creating a MovieClip object, wondering why it doesn't appear at all. Oh yeah, forgot to do "addChild(obj)".

Objective-C:

• After switching back after coding a while in some other language, I find myself trying to call methods using dot notation "object.method", instead of "[object method]".

Javascript:

• Manipulating DOM styles but forgetting to say "style". For example "document.getElementById('thing').opacity = 0.5;"

Perl:

• echo "hello world". Huh, what's wrong? ;)

Any language:

• Adding a new argument to a method, but forgetting to add that to the place(s) calling it.

• Creating a method, but forgetting to call it at all.

I could go on and on, but I would rather be reading about what your common little mistakes are :)

Related:

What is your (least) favorite syntax gotcha?

Answer 1

General:

Using unit test framworks which don't have automatic registration of the tests, so when you add a new test you forget to add it to the suite and your test never runs.

PHP:

Stupid scoping rules causing me to modify variables instead of creating new ones

e.g.

$x = 5;

if(...)
{
  foreach($something as $x) // OOPS MODIFIED the $x outside the current context
  {
   ...
  }
}

//$x is probably no longer 5

Answer 2

Pitfalls of C-style Syntax

Although I usually catch myself, here are some common mistakes that can be caused in many languages with C-style syntax:

if (condition = true)
    some_function();

(This can be circumvented by putting the condition first and the variable second, i.e. true == condition, so mistyping the = for the == will cause a compile error rather than a runtime bug.

The Misplaced Semicolon

for (i = 0; i < some_value; i++);
    function_always_performed();

This is probably an example of just having to keep a close eye for that semicolon. I suspect good compilers will catch this one.

As Software Monkey pointed out in the comments, requiring the use of braces for all instances of if and for will reduce the risk of causing these types of errors, and at the very least will allow the compiler to catch unreachable code blocks.

That reminds me of another danger.

Danger of Indentation and Its Intention

It's nice shortcut to be able to leave off braces when writing a if or for statement with one line of execution connected to it:

if (condition == true)
    do_something();

However, when adding a line or two to that statement, it's possible to mistakenly do the following:

if (condition == true)
    do_something();
    do_something_new();

And, there will be a bug where the do_something_new() is executed everytime, regardless of condition. The indentation seems to have the intention to have the do_something() and do_something_new() functions executed as a block, but the code itself says otherwise due to the lack of braces.

The most painful could be:

some_condition = true;

while (some_condition == true);
{
    do_something();
}

Hello infinite loop! (Althought, this one seems to be caught as "unreachable code" by javac.)

Answer 3

Not checking for null would be the most common mistake I see.

The first person who manages to write a static analysis tool to eliminate this type of defect entirely will quickly find themselves in Cyril Sneer territory.

Answer 4

Re Python.

• Use a decent editor that integrates pyChecker and you will not make that mistake.

Here are some of the python quirks I've spent countless hours debugging.

• from my_module import * also imports not only classes/methods that are defined in that file but also ones imported by that module
• It's too easy to hurt yourself when overriding _ getattr _. Also always forget which of _ getattr _ vs _ getattribute _ are OK to use and which is not.
• Why do I have to implement _ _ne __ when I already implemented __ eq __

Answer 5

This is a common mistake that I've seen a couple of times in C#:

private int myVariable;
public int MyVariable
{
    get
    {
        return MyVariable;
    }
    set
    {
        MyVariable = value;
    }
}

The code will compile without problems but you'll get a nice stack overflow at runtime :P

Fortunately, this disappears with C# 3 auto properties!

Answer 6

Cut and paste errors. I often cut and paste a snippet of code to modify elsewhere (often as small as single line), and forget to change some crucial part of it for the new usage.

Answer 7

Accidentally defining a new local variable when I meant to refer to a class variable:

public class Thing {
  private int value;

  public void setValue(int n) {
    int value = n;  // Oops...
  }
}

Answer 8

Not actually overriding a method from a superclass, because the method signature is wrong:

public class Object {
  public boolean equals(Object o) {
    // ...
  }
}

public class Thing extends Object {
  public boolean equals(Thing t) {  // Oops, this doesn't override equals(Object)!
    // ...
  }
}

(Fortunately in Java, the @Override annotation and generics can both help make this problem go away)

Answer 9

Heyall,

Since I program in a few different languages for different projects, the basics spooks me, like the most common: using . instead of + or using + instead of . to append strings.

A not common mistake but happens sometimes is getting an old code in PHP after a few days working with Java and write a few lines of code using the arrays as collections...

<?php
  // old code

  $itemList = array()
  $itemList[0] = "spoon";
  $itemList[1] = "fork";

  ... several lines and days later ...

  // Can't figure why this do not work :p RTFM later
  // $itemList.add("knife");
?>

Answer 10

I can think of two things in C

if( x = 5)  //Whats wrong here :p 

{

  //your code goes here

}



switch(variablename)

{

  case 1:

     //your code here

     break;

  case 2:

      //your code here

      //forget break here

  case 3:

      //your code here

      break;

   default:

      break;

}

Answer 11

In my initial days of programming, I frequently made this mistake. Apparently its not only me !! Java Puzzlers has the following code snippet saying that someone reported this as a Java platform bug :)

public class ApplePie {
    public static void main(String[] args) {
        int count = 0;
        for (int i = 0; i < 100; i++); {
            count++;
        }
        System.out.println(count);
    }
}

Answer 12

Where I work, we call these "Programming Tips of the Day". They include such gems as:

• Changes not taking effect? Try compiling!
• Compile step suspiciously fast? Try saving!
• Program doesn't do anything when you start it up? Did you spell 'main' correctly?
• If your data isn't being processed, perhaps it's because you haven't written the part that actually does the work.
• If the program fails to run the first time, running it a few more times will not "Work the kinks out"
• When you hear the words "in theory", drink!

Answer 13

I use C# on a daily basis and occassionally try to check for a non-null object in a conditional in the C/C++ way:

string foo=GetString();
if(foo)
{
  Console.WriteLine("not null");
}

I kinda miss not being able to do this...!

Answer 14

When writing a varargs function in C, I always forget the order of arguments in the va_start macro. Is it:

va_start( last_arg, list );

or

va_start( list, last_arg );

Answer: the second one, but I have to look it up every single time.

Answer 15

Infinite loops - in classic ASP/JavaScript, I always used to leave out the recordset MoveNext() when looping through results and hang my dev server, even after five years of being an ASP developer...

while(!recordset.EOF)
{
    // Do something here - FOREVER...
}

Answer 16

putting

cout << "wtf?"

in a PHP file, or

echo "wtf?"

in a Perl file, or

for(int i=0;i<100;i++)

in a Verilog file, or....the list goes on :(

Answer 17

Blaming the compiler/linker/IDE for a bug, anything or everything except my code.

Answer 18

Thinking I can make an accurate estimate for any period of time over about 4 hours.

Answer 19

these days, closures in javascript:

var foo = "bar"
return function () { twiddle(foo) }

Doesn't work, usually should be

var foo = "bar"
return function() { function() { twiddle(foo) } }()

GAH! had to fix that about 6 times in as many days.

Answer 20

I like to think that I learn from my mistakes but I often do something like the following:

foreach(MyObject obj in MyObjectCollecion)
{
    remove(obj); // OOPS enumerator no-longer valid now
}

Answer 21

VB6: adding an On Error Goto Label, then forgetting the Exit sub before the error label.

All languages: always trying the wrong one (and getting the parameters in the wrong order) for Instr, indexOf, Contains, charIndex (etc..)

Answer 22

I did this just now... I had to test the sign of one number and set the sign of the other number to the same sign.

if(a > 0.0f){
    if(b < 0.0f)
        b -= b;
}
else if(a < 0.0f){
    if(b > 0.0f)
        b -= b;
}

That should have been:

if(a > 0.0f){
    if(b < 0.0f)
        b = -b;
}
else if(a < 0.0f){
    if(b > 0.0f)
        b = -b;
}

Answer 23

typedef unsigned char UINT8;

void Foo( void )
{
    UINT8 i;

    for( i = 0; i < 256; i++ )
    {
     // Do something
    }
}

Oops! - It's an infinite loop