Getting a Rogue Iteration from My .Each Loop

Trying to iterate my function many times using i

i think you are looking for this

sample_gamma <- function(alpha, beta, n, iter) {
mean_s = c()
mean_sd = c()
for(i in 1:iter){
a = rgamma(n, shape = alpha, scale = 1/beta)
print(c(mean(a), sd(a)))

Modifying size of iterable during for loop - how is looping determined?

The loop executes till iterable says it has no more elements. After two cycles, the iterable has gone through two elements, and has lost two elements, which means it is at its end, and the loop terminates.

Your code is equivalent to this:

y = [1, 2, 3, 4]
i = iter(y)
while True:
except StopIteration:

The list iterator holds the index that is up to be read next. In the third cycle, the list is [3, 4], and next(i) would be needing to read y[2], which is not possible, so next raises StopIteration, which ends the loop.

EDIT As to your other questions:

How do iter and StopIteration, and __getitem__(i) and IndexError factor in?

The first two are as described above: it is what defines a for loop. Or, if you will, it is the contract of iter: it will yield stuff till it stops with StopIteration.

The latter two, I don't think participate at all, since the list iterator is implemented in C; for example, the check for whether the iterator is exhausted directly compares the current index with PyList_GET_SIZE, which directly looks at ->ob_size field; it doesn't pass through Python any more. Obviously, you could make a list iterator that would be fully in pure Python, and you'd likely be either using len to perform the check, or catching IndexError and again letting the underlying C code perform the check against ->ob_size.

What about iterators that aren't lists?

You can define any object to be iterable. When you call iter(obj), it is the same as calling obj.__iter__(). This is expected to return an iterator, which knows what to do with i.__next__() (which is what next(i) translates to). I believe dicts iterate (I think, haven't checked) by having an index into the list of its keys. You can make an iterator that will do anything you want, if you code it. For example:

class AlwaysEmpty:
def __iter__(self):
return self
def __next__(self):
raise StopIteration

for x in AlwaysEmpty():
print("there was something")

will, predictably, print nothing.

And most importantly, is this / where is this in the docs?

Iterator Types

array.forEach running faster than native iteration? How?

There are many iteration optimizations that your for loop is missing such as:

  • cache the array length
  • iterate backwards
  • use ++counter instead of counter++

These are the ones that I have heard of and used, I am sure there are more. If memory serves me correct, the backwards iterating while loop is the fastest of all looping structures (in most browsers).

See this jsperf for some examples.

links for postfix vs prefix perf test and iterating backwards.
I was not able to find my reference for using +=1 instead of ++, so I have removed it from the list.

GoTo Next Iteration in For Loop in java


continue; key word would start the next iteration upon invocation

For Example

for(int i= 0 ; i < 5; i++){

This will print



  • Document

For loop with an else and one iteration

As user2357112 mentions, and as stated in the Python docs here

a loop’s else clause runs when no break occurs

You could try something along the lines of:

def searchContact(self, search_name):
contact_found = False

print("Your search matched the following:")
for contact in self.contacts:
if search_name in
contact_found = True

if not contact_found:
print("Sorry that contact does not exist!!")

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