Storing items in a list for python

Question

I am new to python and would like to ask a simple question.

I am trying to get this program to loop and every time it loops, it should store a variable in a list.

Please help

This is my code so far:

import random
for x in range(1,100):
    q=random.randint(0,99)
    w=random.randint(0,99)
    e=q*w
    q=[]#This is the list
    print '%s * %s = %s'%(q,w,e)

You have a variable named `q`. You create an empty list called `q` every time you loop. You don't use the `x` in your `for` loop. To add a variable to your list, you can use `listname.append(variable)`. — Jose Magana, Sep 09 '14 at 01:24

aknuds1 · Accepted Answer · 2014-09-09T01:33:40.783

3

You need to define the list outside of the loop, and append to it within the loop.

import random

# Define your list
l = []
for x in range(1,100):
    q=random.randint(0,99)
    w=random.randint(0,99)
    e=q*w
    # Append the result to your list
    l.append(e)
    print '%s * %s = %s'%(q,w,e)

print 'The list becomes: %s' % l

edited Sep 09 '14 at 01:33

answered Sep 09 '14 at 01:28

aknuds1

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@user3612541 Glad to hear it. – aknuds1 Sep 09 '14 at 01:38

score 0 · Answer 2 · answered Sep 09 '14 at 01:34

Let's go through your code line-for-line

import random

for x in range(1,100):        # this will go 1, 2, ... , 99
    q = random.randint(0, 99) # q is a random number 0-99
    w = random.randint(0, 99) # w is a random number 0-99
    e = q * w                 # e is q times w
    q = []                    # q is now an empty list
    print '%s * %s = %s' % (q,w,e)
                              # q is an empty list, so this is wrong

Obviously you're not trying to assign q to an empty list each time. You need to make a list BEFORE the loop, then keep adding to it.

some_list = []                # some_list is an empty list
for _ in range(99):           # this will go 0, 1, ... , 98
    q = random.randint(0, 99) # q is a random number 0-99
    w = random.randint(0, 99) # w is a random number 0-99
    e = q * w                 # e is q times w
    some_list.append(e)       # add e to the list!
    print "%s * %s = %s" % (q,w,e)
                              # print "q * w = e"

Note that when you get a bit more advanced, you can wrap this up nicely as a list comprehension so you don't have to initialize it empty then add to it.

some_list = [random.randint(0,99) * random.randint(0,99) for _ in range(99)]
# equivalent to your code

But this is still a bit past your comfort level. One step at a time! :)

Things to note here that I changed:

range is a half-open range, e.g. range(1,100) is 99 numbers, 1-99. It does not include 100.
You don't actually USE any of the numbers in your range, you just want to run your loop that many times. The common idiom for this is for _ in range(some_number). You use an underscore (_) to represent to other coders that you don't use the variable here (note that _ does not represent anything special to Python, it's just convention)
for _ in range(N) is a quick way to do something N times.
list.append adds to a list =)

score 0 · Answer 3 · answered Sep 09 '14 at 01:50

A few notes on style:

don't define a list inside a for loop, q=[] in this case, if you want it to store values acrovss the several iterations of the for loop.
don't use the same name to define multiple objects inside a loop, q in this case being used both for storing a random number and as a list.

Ways to adding to list:

Use append() function of the list object to append a value to the list
Overload + operator to concatenate a new value to the original list

So, here's how i would change your code:

First define the list outside the loop:
```
import random
e = []
```

Inside the for loop, use append() function to add value to the master list, e:

for x in range(0,100):
    q=random.randint(0,99)
    w=random.randint(0,99)
    e_val=q*w    # new local variable that resets at each iteration of the loop
    e.append(e_val)
    print '%s * %s = %s'%(q,w,e_val)

Alternatively, you could have replaced the line e.append(e_val) with the following:
```
e += [e_val]
```

Storing items in a list for python

3 Answers3