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Why does i + 2 result in an infinite loop where i += 2 doesn't? [duplicate]

I am at a loss how best to approach for loops in JavaScript. Hopefully an understanding of for loops will help shed light on the other types of loops.
Sample code
for (var i=0; i < 10; i=i+1) {
document.write("This is number " + i);
}
My understanding is that when i has been initialized, it starts with the value of 0 which then evaluated against the condition < 10. If it is less than 10, it the executes the statement document.write("This is number + i); Once it has executed the preceding statement, only then does it increment the next value by 1.
Guides I have consulted:
http://www.functionx.com/javascript/Lesson11.htm
http://www.cs.brown.edu/courses/bridge/1998/res/javascript/javascript-tutorial.html#10.1
http://www.tizag.com/javascriptT/javascriptfor.php
Now the guide at http://www.functionx.com/javascript/Lesson11.htm seems to indicate otherwise i.e.
To execute this loop, the Start condition is checked. This is usually
the initial value where the counting should start. Next, the Condition
is tested; this test determines whether the loop should continue. If
the test renders a true result, then the Expression is used to modify
the loop and the Statement is executed. After the Statement has been
executed, the loop restarts.
The line that throws me is "If the test renders a true result, then the Expression is used to modify the loop and the Statement is executed". It seems to imply that because 0 is less than 10, increment expression is modified which would be 0 + 1 and THEN the statement, e.g. document.write is executed.
My problem
What is the correct way to interpret for loops? Is my own comprehension correct? Is the same comprehension applicable to other programming languages e.g. PHP, Perl, Python, etc?

Think of a for loop as the following
for(initializers; condition; postexec) {
execution
}
When the loop is first started the code var i = 0 is run. This initializes the variable that you will be testing for inside the loop
Next the loop evaluates the i < 10 expression. This returns a boolean value which will be true for the first 10 times it is run. While this expression keeps evaluating to true the code inside the loop is run.
document.write("This is number " + i);
Each time after this code is run the last part of the loop i++ is executed. This code in this example adds 1 to i after each execution.
After that code is executed the condition of the loop is check and steps 2 and 3 repeat until finally the condition is false in which case the loop is exited.
This the way loops work in the languages you mentioned.

Lets have a look at the corresponding section in the ECMAScript specification:
The production
IterationStatement : for ( var VariableDeclarationListNoIn ; Expressionopt ; Expressionopt) Statement
is evaluated as follows:
1. Evaluate VariableDeclarationListNoIn.
2. Let V = empty.
3. Repeat
a. If the first Expression is present, then
i. Let testExprRef be the result of evaluating the first Expression.
ii. If ToBoolean(GetValue(testExprRef)) is false,
return (normal, V, empty).
b. Let stmt be the result of evaluating Statement.
...
f. If the second Expression is present, then
i. Let incExprRef be the result of evaluating the second Expression.
ii. Call GetValue(incExprRef). (This value is not used.)
As you can see, in step 1, the variable assignment is evaluated. In step 3a, the condition is tested. In step 3b, the loop body is evaluated, and after that the third expression is evaluated in step 3f.
Therefore your understanding of the for loop is correct.
It is to assume that it works the same way in other languages, since the for loop is such a common statement in programming languages (note that Python does not have such a statement). But if you want to be absolutely certain, you better consult their specification as well.

Your quoted source is wrong, and we can prove it...
The basis of the for loop has four separate blocks which may be executed:
for(initialise; condition; finishediteration) { iteration }
Fortunately we can execute a function in each of these blocks. Therefore we can create four functions which log to the console when they execute like so:
var initialise = function () { console.log("initialising"); i=0; }
var condition = function () { console.log("conditioning"); return i<5; }
var finishediteration = function () { console.log("finished an iteration"); i++; }
var doingiteration = function () { console.log("doing iteration when `i` is equal", i); }
Then we can run the following, which places the above functions into each block:
for (initialise(); condition(); finishediteration()) {
doingiteration();
}
Kaboom. Works.
If you viewing this page using Safari on the Mac then you can AppleAlt + I and copy the above two snippets, in order, into the console and see the result.
EDIT, extra info....
Also... the finished iteration block is optional. For example:
for (var i=0; i<10;) {
console.log(i); i++;
};
does work.

The second reference is wrong. Your explanation is correct.

Another way to think about it, if this helps you:
var i = 0;
while (i < 10) {
document.write("This is number " + i);
i++;
}

This is for statement syntax:
for(initalize, condition, increment) {
Do_some_things();
}
initalize Will executed only one time when for begin then it execute Do_some_things(); statement, and while condition still true it will execute increment and then Do_some_things();. if co condition false, for would exit.
for (var i=0; i < 10; i=i+1) {
document.write("This is number " + i);
}
var i=0 will execute one time (initalize).
i < 10 condition was always checked after a loop.
i=i+1 will execute after check i < 10 and result is true.
Value of i is: 0, 1, 3, 4, 5, 6, 7, 8, 9 (10 times loop)

JavaScrpit crashes when incrementing by 2 in a for loop

before you call me out for using wrong terms or sth, I am a newbie programmer.
So, this would be the correct form of the code I'm experimenting with:
for (var i = 0; i <= 10; i += 2) {
nums.push(i);
}
console.log(nums)
However, I don't get why I need to put the = sign after the +. I tried to research this but I couldn't find anything. When I removed the =, JS (Scrimba) crashed. Can anyone explain this to me please?

The third expression in the for loop is to indicate how the i variable increases. This expression is run after every loop iteration.
The second i<=10 is the condition that has to be true for the loop to continue.
Now in your loop starts with i=0, then i=2 is run after 1st iteration. And after every iteration i=2. So i<=10 is true, which is the condition. So this is an infinite loop, leading to the crash.
i+=2 is shorthand for i = i +2, so i is increasing everytime and at some point will exceed 10.
This assumes you have defined nums at top like:
var nums = []. Otherwise you get a different error.

i += 2 mean i = i + 2. In this context, for(A；B; C)，C part will run every time in every loop. If you only use "+", the i value will never changed, and loop will be non-stop considering your judgement to stop the loop i < 10 equal to 0 < 10, which is true forever.

Let's take a look at for loop before go to your problem:
for (statement 1; statement 2; statement 3) {
// code block to be executed
}
Statement 1 is executed (one time) before the execution of the code block.
Statement 2 defines the condition for executing the code block.
Statement 3 is executed (every time) after the code block has been executed.
If you want to end the loop, statement 2 should return a false value.
In your case:
statement 1 is i=1 -> Assign 1 to i variable.
statement 2 is i<=10 -> Execute the code inside brackets if and only if i is less than or equal to 10
statement 3 is i+=2 -> in crease i by 2 after every run.
the point is if you change statement 3 to i+2, it is not an assignment statement anymore, but an operator. So the value of i won't be changed after statement 3 execution.
=> The value of i will always be 1
=> the statement 2 always returns true
=> infinite loop
So your code crashes because of an infinite loop

i += 2 means your are incrementing the value of i by 2.
So, in your case, the loop will start from 0, and it will push in your array. It will keep increasing your i value by 2 and pushing them in the array till it (i) becomes more than 10

while VS do while surprising results [duplicate]

This is a highly subjective question, so I'll be more specific. Is there any time that a do-while loop would be a better style of coding than a normal while-loop?
e.g.
int count = 0;
do {
System.out.println("Welcome to Java");
count++;
} while (count < 10);`
It doesn't seem to make sense to me to check the while condition after evaluating the do-statement (aka forcing the do statement to run at least once).
For something simple like my above example, I would imagine that:
int count = 0;
while(count < 10) {
System.out.println("Welcome to Java"); count++;
}
would be generally considered to have been written in a better writing style.
Can anyone provide me a working example of when a do-while loop would be considered the only/best option? Do you have a do-while loop in your code? What role does it play and why did you opt for the do-while loop?
(I've got an inkling feeling that the do-while loop may be of use in coding games. Correct me, game developers, if I am wrong!)

If you want to read data from a network socket until a character sequence is found, you first need to read the data and then check the data for the escape sequence.
do
{
// read data
} while ( /* data is not escape sequence */ );

The while statement continually executes a block of statements while a particular condition is true
while (expression) {
statement(s)
}
do-while evaluates its expression at the bottom of the loop, and therefore, the statements within the do block are always executed at least once.
do {
statement(s)
} while (expression);
Now will talk about functional difference,
while-loops consist of a conditional branch instructions such as if_icmpge or if_icmplt and a goto statement. The conditional instruction branches the execution to the instruction immediately after the loop and therefore terminates the loop if the condition is not met. The final instruction in the loop is a goto that branches the byte code back to the beginning of the loop ensuring the byte code keeps looping until the conditional branch is met.
A Do-while-loops are also very similar to for-loops and while-loops except that they do not require the goto instruction as the conditional branch is the last instruction and is be used to loop back to the beginning
A do-while loop always runs the loop body at least once - it skips the initial condition check. Since it skips first check, one branch will be less and one less condition to be evaluated.
By using do-while you may gain performance if the expression/condition is complex, since it is ensured to loop atleast once. In that casedo-while could call for performance gain
Very Impressive findings here,
http://blog.jamesdbloom.com/JavaCodeToByteCode_PartOne.html#while_loop

The do-while loop is basically an inverted version of the while-loop.
It executes the loop statements unconditionally the first time.
It then evaluates the conditional expression specified before executing the statements again.
int sum = 0;
int i = 0;
do
{
sum += ids[i];
i++;
} while (i < 4);
Reference material

Simply, when you want to check condition before and then perform operation while is better option, and if you want to perform operation at least once and then check the condition do-while is better.
As per your question a working example,
1. when I needed to find the field which could be declared in the same class or the super class or the super class of that super class and so on i.e. finding the field located in deep class hierarchy. (A extends B B extends C and so on)
public Field SearchFieldInHierarchy(Object classObj, String fieldName )
{
Field type = null;
Class clz = classObj.getClass();
do
{
try
{
type = clz.getDeclaredField(fieldName);
break;
} catch (NoSuchFieldException e)
{
clz = clz.getSuperclass();
}
} while(clz != null || clz != Object.class);
return type;
}
2. When reading input stream from Http response
do
{
bytesRead = inputStream.read(buffer, totalBytesRead, buffer.length - totalBytesRead);
totalBytesRead += bytesRead;
} while (totalBytesRead < buffer.length && bytesRead != 0);

You kind of answer the question yourself-when it needs to run at least once, and it makes sense to read it that way.

do - while loop allows you to ensure that the piece of code is executed at least once before it goes into the iteration.

In a while loop, the condition is tested before it executes code in the loop. In a do while loop, the code is executed before the condition is tested, resulting in the code always being executed at least once. Example:
$value = 5;
while($value > 10){
echo "Value is greater than 10";
}
The above would never output anything. If we do the same again like this:
$value = 5;
do{
echo "Value is greater than 10";
}while($value > 10)
It would output Value is greater than 10 because the condition is tested after the loop is executed. After this it would not output anything further.

The difference between do-while and while is that do-while evaluates its expression at the bottom of the loop instead of the top. Therefore, the statements within the do block are always executed at least once.
For example do check this link: http://docs.oracle.com/javase/tutorial/java/nutsandbolts/while.html

If the looping condition can only be known after a first step of the loop (when you do not want a condition before you enter the loop).
Typically:
do {
expr = ...;
while (expr);

Use the while Statement when you have to check a condition repeatedly and only when the condition is satisfied execute the loop
while(condition) //eg. a>5
{
Body of Loop
}
If you see the flow of control here you can see that the condition is checked before the execution of the loop, if the condition is not met the loop will not execute at all
In the Do-While statement the program will execute the body of the loop once and then it will check if the statement is true or not
do
{
Body of Loop
}
while(condition); //eg. a>5
If you notice the flow of control here you will see that the body is executed once, then the condition is checked. If the condition is False the Program will break out of the loop, if True it will continue executing till the condition is not satisfied
It is to be noted that while and do-while give the same output only the flow of control is different

/*
while loop
5 bucks
1 chocolate = 1 bucks
while my money is greater than 1 bucks
select chocolate
pay 1 bucks to the shopkeeper
money = money - 1
end
come to home and cant go to while shop because my money = 0 bucks
*/
#include<stdio.h>
int main(){
int money = 5;
while( money >= 1){
printf("inside the shopk and selecting chocolate\n");
printf("after selecting chocolate paying 1 bucks\n");
money = money - 1 ;
printf("my remaining moeny = %d\n", money);
printf("\n\n");
}
printf("dont have money cant go inside the shop, money = %d", money);
return 0;
}
infinite money
while( codition ){ // condition will always true ....infinite loop
statement(s)
}
please visit this video for better understanding
https://www.youtube.com/watch?v=eqDv2wxDMJ8&t=25s

It is very simple to distinguish between the two. Let's take While loop first.
The syntax of while loop is as follows:
// expression value is available, and its value "matter".
// if true, while block will never be executed.
while(expression) {
// When inside while block, statements are executed, and
// expression is again evaluated to check the condition.
// If the condition is true, the while block is again iterated
// else it exists the while block.
}
Now, let's take the do-while loop.
The syntax of do-while is different:
// expression value is available but "doesn't matter" before this loop, & the
// control starts executing the while block.
do {
// statements are executed, and the
// statements is evaluated and to check the condition. If true
// the while block is iterated, else it exits.
} while(expression);
A sample program is given below to make this concept clear:
public class WhileAndDoWhile {
public static void main(String args[]) {
int i = 10;
System.out.println("While");
while (i >= 1) {
System.out.println(i);
i--;
}
// Here i is already 0, not >= 1.
System.out.println("do-while");
do {
System.out.println(i);
i--;
} while (i >= 1);
}
}
Compile and run this program, and the difference becomes apparent.

JavaScript For Loop - execution termination condition

I was looking at source code of Ext object in ExtJs docs here and I noticed this for loop:
for (j = enumerables.length; j--;) {
k = enumerables[j];
if (config.hasOwnProperty(k)) {
object[k] = config[k];
}
}
So, normally in a for loop, we have do initialization, specify condition and then increment/decrement the counter. I do see an initial condition and j-- which decrements the counter after each iteration. However, what I can't figure out is how is the loop going to be terminated? I neither see any condition nor a break keyword that will terminate the loop.
What am I missing here?

the j-- is the loop condition, that's how it will terminate. the trailing ;) is the missing increment/decrement.
The j-- construct both decrements j and tests the old un-decremented value for non-zero to decide whether to continue looping. The loop stops when j is zero; below the loop, the value of j will be -1.
The key parts of the for loop to look for are the semicolons -- for ( ; ; )
Any valid expression (or comma-separated expression list) can go in any of the three slots, before, between, and after the semicolons.
The loop initializers go after the ( and before the first ;. The loop condition is the value of the expression between the two ; ;. The post-loop update goes after the last ; and before the ).

JavaScript: What will break, and what will break it? [closed]

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Potentially dangerous code below, line 2
Repeat: POTENTIALLY DANGEROUS CODE BELOW
var history = "";
for (start = 3; start = 1000; start += 1){
if(start % 5 == 0 || start % 3 == 0)
history += start + " "; }
Okay, this is the tenth time I've put JavaScript code in that's frozen my browser. It's putting my computer in shock. Are these panic attacks going to destroy her heart? Where can I learn about all the crap that might break my computer as I continue to learn and practice JavaScript? I'm looking for an exhaustive list, only.

Your loop: for (start = 3; start = 1000; start += 1){
The second part of a for( ; ; ) loop is the condition test. The loop will continue until the second part evaluates to false. To not create an infinite loop, change your code to:
for (var start = 3; start < 1000; start += 1){
Note: start+=1 is equal to start++. If you want a compact code, you can replace +=1 by ++.
An overview of the three-part for-loop, for(initialise; condition; increment):
initialise - Create variables (allowed to be empty)
condition - The loop will stop once this expression evaluates to false
increment - This expression is executed at the end of the loop
Always check against infinite loops: Make sure that the condition is able to evaluate to false.
Commonly made mistakes:
A negative incremental expression in conjunction with a is-lower-than comparison:
i-- decreases the counter, so i<100 will always be true (unless the variable i is initialized at 100)
A positive incremental expression in conjunction with a is-higher-than comparison.
A non-incrementing expression: for(var i=0,j=0; i<100; j++) (i doesn't increase)
A condition which is always true (such as in your case)

You just have to learn and read about it properly. Your loop condition start = 1000 will always evaluate to true, that's why the loop never terminates (an assignment returns the value which was assigned and any other number than 0 is evaluates to true).
The MDN JavaScript Guide is a great resource for learning JavaScript. Particular for this situation:
A for loop repeats until a specified condition evaluates to false. The JavaScript for loop is similar to the Java and C for loop. A for statement looks as follows:
for ([initialExpression]; [condition]; [incrementExpression])
statement
When a for loop executes, the following occurs:
The initializing expression initialExpression, if any, is executed. This expression usually initializes one or more loop counters, but the syntax allows an expression of any degree of complexity. This expression can also declare variables.
The condition expression is evaluated. If the value of condition is true, the loop statements execute. If the value of condition is false, the for loop terminates. If the condition expression is omitted entirely, the condition is assumed to be true.
The statement executes. To execute multiple statements, use a block statement ({ ... }) to group those statements.
The update expression incrementExpression, if there is one, executes, and control returns to step 2.
As the others said, it mostly comes down to try and error.... that is a good way of learning anyway.

your conditional statement start=1000 will always return true. You cant found such fool proof's list for this, you have to learn from these mistakes on your own.

Uh - what did you wanted here ?
for (start = 3; start = 1000; start += 1)
do you wanted this ? ( from 3 to 1000 )
for (start = 3; start <= 1000; start += 1)
in first case you will stuck on 1000