Append List Array in for Loop

How to append an array within a for loop that gets new outputs every loop?

array2 = np.array([])
for i in range(len(fps)):
    array1 = np.array([])
    DataStructs.ConvertToNumpyArray((fps[i]), array1)

    # you need to assign the concatenated array to array2
    array2 = np.append(array2,array1)
    
array2



# if you want to have a 2D-array as output, you can do the following:

# initialize the array then assign new elements to it
array2 = np.zeros(shape=(len(fps),1024))
for i in range(len(fps)):
    array1 = np.array([])
    DataStructs.ConvertToNumpyArray((fps[i]), array1)
    array2[i] = array1
    
array2

# or you can use np.concatenate
array2 = np.zeros(shape=(0,1024))
for i in range(len(fps)):
    array1 = np.array([])
    DataStructs.ConvertToNumpyArray((fps[i]), array1)
    array2 = np.concatenate((array2,[array1]))

Append list array in for loop

I'm not an expert on masked arrays but this works:

# create example
>>> a = np.arange(30).reshape(10, 3)
>>> a[[0,7,8]] = 0
>>> a = np.ma.MaskedArray(a, np.isin(a // 3, (2,3)))
>>> 
>>> a
masked_array(
  data=[[0, 0, 0],
        [3, 4, 5],
        [--, --, --],
        [--, --, --],
        [12, 13, 14],
        [15, 16, 17],
        [18, 19, 20],
        [0, 0, 0],
        [0, 0, 0],
        [27, 28, 29]],
  mask=[[False, False, False],
        [False, False, False],
        [ True,  True,  True],
        [ True,  True,  True],
        [False, False, False],
        [False, False, False],
        [False, False, False],
        [False, False, False],
        [False, False, False],
        [False, False, False]],
  fill_value=999999)
>>> 
# cut all rows that have at least one masked or all zero entries
>>> compressed = a.data[~np.any(a.mask, axis=1) & np.any(a.data!=0, axis=1)]
>>> compressed
array([[ 3,  4,  5],
       [12, 13, 14],
       [15, 16, 17],
       [18, 19, 20],
       [27, 28, 29]])

List append() in for loop raises exception: 'NoneType' object has no attribute 'append'

The list.append function does not return any value(but None), it just adds the value to the list you are using to call that method.

In the first loop round you will assign None (because the no-return of append) to a, then in the second round it will try to call a.append, as a is None it will raise the Exception you are seeing

You just need to change it to:

a = []
for i in range(5):
    # change a = a.append(i) to    
    a.append(i)
print(a)
# [0, 1, 2, 3, 4]

list.append is what is called a mutating or destructive method, i.e. it will destroy or mutate the previous object into a new one(or a new state).

If you would like to create a new list based in one list without destroying or mutating it you can do something like this:

a=['a', 'b', 'c']
result = a + ['d']

print result
# ['a', 'b', 'c', 'd']

print a
# ['a', 'b', 'c']

As a corollary only, you can mimic the append method by doing the following:

a = ['a', 'b', 'c']
a = a + ['d']

print a
# ['a', 'b', 'c', 'd']

Python append to array and for loop for it

Can you try this?

{''} means its a set and you only want the element in the set. And pop gives that

for x in sidra:
    link = x.absolute_links.pop()
    links.append(link)

How do you loop an array and append it to a div?

You have to loop the array. Using .forEach() you can use the two arguments that are provided:

item
index.

At each loop iteration, using the item, you can access the objects properties like className, urlSrc and labelName.

Using the index, you can target the right a element with the .eq() method.

You will also notice the use of templating literals to insrt the variables in the string to append.

var arrayTest = [
  {
    className: "class-01",
    urlSrc: "http://via.placeholder.com/100x100?text=Image 1",
    labelName: "01 Label Name"
  },
  {
    className: "class-02",
    urlSrc: "http://via.placeholder.com/100x100?text=Image 2",
    labelName: "02 Label Name"
  },
  { className: "class-03",
   urlSrc: "http://via.placeholder.com/100x100?text=Image 3",
   labelName: "03 Label Name"
  }
];

$(document).ready(function () {
  arrayTest.forEach(function (item, index) {
    $("#menu a").eq(index).append(
`<div class="${item.className}">
  <span class='vertical-align-middle'></span>
  <img src="${item.urlSrc}">
</div>
<div class='filter-label'>${item.labelName}</div>`
    );
  });
});

a{
  text-decoration: none;
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>

<ul id="menu" class="menu">
  <li id="menu-item-1">
    <a href="#">
    </a>
  </li>
  <li id="menu-item-2">
    <a href="#">
    </a>
  </li>
  <li id="menu-item-3">
    <a href="#">
    </a>
  </li>
</ul>

Append array in for loop

List append is always better than np.append. It is faster, and easier to use correctly.

But let's look at your code in more detail:

In [128]: df = pd.DataFrame(np.random.choice([0.0, 0.05], size=(1000,1000)))    
In [129]: l = np.array([])                                                      
In [130]: rand_cols = np.random.permutation(df.columns)[0:5]                    
In [131]: rand_cols                                                             
Out[131]: array([190, 106, 618, 557, 514])
In [132]: df2 = df[rand_cols].copy()                                            
In [133]: df2.shape                                                             
Out[133]: (1000, 5)
In [134]: l1 = np.append(l, df2, axis=0)                                        
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-134-64d82acc3963> in <module>
----> 1 l1 = np.append(l, df2, axis=0)

/usr/local/lib/python3.6/dist-packages/numpy/lib/function_base.py in append(arr, values, axis)
   4692         values = ravel(values)
   4693         axis = arr.ndim-1
-> 4694     return concatenate((arr, values), axis=axis)
   4695 
   4696 

ValueError: all the input arrays must have same number of dimensions

Since you specified the axis, all np.append is doing is:

np.concatenate([l, df2], axis=0)

l is (0,) shape, df2 is (1000,5). 1d and 2d, hence the complaint about dimensions.

Starting with a 2d l array works:

In [144]: l = np.zeros((0,5))                                                   
In [145]: np.concatenate([l, df2], axis=0).shape                                
Out[145]: (1000, 5)
In [146]: np.concatenate([df2, df2], axis=0).shape                              
Out[146]: (2000, 5)

I think np.append should be deprecated. We see too many SO errors. As your case shows, it is hard to create the correct initial array. np.array([]) only works when building a 1d array. Plus repeated concatenates are slow, creating a completely new array each time.

How to append a list to another list in a python for loop when each element of the existing list is being changed inside the for loop?

When you append r1 twice to r2 it essentially makes r2 a list of [r1, r1] not the contents of r1 when it was appended, so when r1 is changed before the second append, the first element in r2 which is a reference to r1 is also changed.

One solution is to not use r1 at all and just append the contents directly:

r2 = []
r_1 = {'G':32,'H':3}
for i in r_1:
    r2.append([i, i+"I"])
print(r2)

A second solution is to append a copy of r1 to avoid the two elements having the same reference:

r2 = []
r_1 = {'G':32,'H':3}
r1 = [None, None]
for i in r_1:
    r1[0] = i
    r1[1] = i + "I"
    r2.append(r1.copy())
print(r2)

Append List Array in for Loop

How to append an array within a for loop that gets new outputs every loop?