How to Select Every 2Nd Element of an Array in a for Loop

 var log = function(val){document.body.innerHTML+='<div></pre>'+val+'</pre></div>'} 
var oldArr = [0,1,2,3,4,5,6,7,8,9,10]var arr = [];
var maxVal = 5;
var delta = Math.floor( oldArr.length / maxVal );
// avoid filter because you don't want// to loop over 10000 elements !// just access them directly with a for loop !//                                 |//                                 Vfor (i = 0; i < oldArr.length; i=i+delta) {  arr.push(oldArr[i]);}

log('delta : ' + delta + ' length = ' + oldArr.length) ;log(arr);

Selecting every nth item from an array

A foreach loop provides the fastest iteration over your large array based on comparison testing. I'd stick with something similar to what you have unless somebody wishes to solve the problem with loop unrolling.

This answer should run quicker.

$result = array();
$i = 0;
foreach($source as $value) {
    if ($i++ % 205 == 0) {
        $result[] = $value;
    }
}

I don't have time to test, but you might be able to use a variation of @haim's solution if you first numerically index the array. It's worth trying to see if you can receive any gains over my previous solution:

$result = array();
$source = array_values($source);
$count = count($source);
for($i = 0; $i < $count; $i += 205) {
    $result[] = $source[$i];
}

This would largely depend on how optimized the function array_values is. It could very well perform horribly.

Iterate over every nth element in string in loop - python

If you want to do something every nth step, and something else for other cases, you could use enumerate to get the index, and use modulus:

sample = "This is a string"
n = 3 # I want to iterate over every third element
for i,x in enumerate(sample):
    if i % n == 0:
        print("do something with x "+x)
    else:
        print("do something else with x "+x)

Note that it doesn't start at 1 but 0. Add an offset to i if you want something else.

To iterate on every nth element only, the best way is to use itertools.islice to avoid creating a "hard" string just to iterate on it:

import itertools
for s in itertools.islice(sample,None,None,n):
    print(s)

result:

T
s
s

r
g

How to start from second index for for-loop

First thing is to remember that python uses zero indexing.

You can iterate throught the list except using the range function to get the indexes of the items you want or slices to get the elements.

What I think is becoming confusing here is that in your example, the values and the indexes are the same so to clarify I'll use this list as example:

I = ['a', 'b', 'c', 'd', 'e']
nI = len(I) # 5

The range function will allow you to iterate through the indexes:

for i in range(1, nI):
    print(i)
# Prints:
# 1
# 2
# 3
# 4

If you want to access the values using the range function you should do it like this:

for index in range(1, nI):
    i = I[index]
    print(i)
# Prints:
# b
# c
# d
# e

You can also use array slicing to do that and you don't even need nI. Array slicing returns a new array with your slice.

The slice is done with the_list_reference[start:end:steps] where all three parameters are optional and:

start is the index of the first to be included in the slice

end is the index of the first element to be excluded from the slice

steps is how many steps for each next index starting from (as expected) the start (if steps is 2 and start with 1 it gets every odd index).

Example:

for i in I[1:]:
    print(i)
# Prints:
# b
# c
# d
# e

How do you select every nth item in an array?

You could also use step:

n = 2
a = ["cat", "dog", "mouse", "tiger"]
b = (n - 1).step(a.size - 1, n).map { |i| a[i] }

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