I am working on an introductory smalltalk program. The goal is to print all elements of an array of integers forward, backward, and then print only the elements of the array that end in a certain digit.
I have already done the first two, but I'm stuck on how I should handle the last goal.
Here is my code:
|myArray|
myArray _ Array new: 15.
1 to: 15 do: [:i | myArray at:i put:i*2].
myArray printNl.
myArray reverse printNl.
Is this for a class or are you following a tutorial? What other concepts have your learned (either in this class or in other languages)? If you couldn't send the #'printNl' message, what would you do?
I think that the purpose of this exercise is to introduce you to using loops (to iterate forwards and backwards), and to conditionals inside a loop (have you been introduced to conditionals and branching yet?).
If you had a loop that printed each element of the array, you could add a conditional (such as an #'ifTrue:' message) to isolate the printing.
It depends a bit on the dialect you are using.
I think the important thing is to realize that an Integer is not a String and therefore, they have different behaviour.
|myArray tail|
myArray := Array new: 15.
tail := '6'.
1 to: 15 do: [:i | myArray at:i put:i*2].
myArray do:[:entry | (entry asString matchesTail:tail) ifTrue:[entry "whatever your need to do with it"] ].
Related
I have a method that shifts all the items, in an array, to the left by one position. In my post condition I need to ensure that my items have shifted to the left by one. I have already compared the first element of the old array to the last element of the new array. How do i across loop through the old array from 2 until count, loop through the new array from 1 until count-1 and compare them? This is my implementation so far.
items_shifted:
old array.deep_twin[1] ~ array[array.count]
and
across 2 |..| (old array.deep_twin.count) as i_twin all
across 1 |..| (array.count-1) as i_orig all
i_twin.item ~ i_orig.item
end
end
end
I expected the result to be true but instead I get a contract violation pointing to this post condition. I have tested the method out manually by printing out the array before and after the method and I get the expected result.
In the postcondition that fails, the loop cursors i_twin and i_orig iterate over sequences 2 .. array.count and 1 .. array.count - 1 respectively, i.e. their items are indexes 2, 3, ... and 1, 2, .... So, the loop performs comparisons 2 ~ 1, 3 ~ 2, etc. (at run-time, it stops on the first inequality). However, you would like to compare elements, not indexes.
One possible solution is shown below:
items_shifted:
across array as c all
c.item =
if c.target_index < array.upper then
(old array.twin) [c.target_index + 1]
else
old array [array.lower]
end
end
The loop checks that all elements are shifted. If the cursor points to the last element, it compares it against the old first element. Otherwise, it tests whether the current element is equal to the old element at the next index.
Cosmetics:
The postcondition does not assume that the array starts at 1, and uses array.lower and array.upper instead.
The postcondition does not perform a deep twin of the original array. This allows for comparing elements using = rather than ~.
Edit: To avoid potential confusion caused by precedence rules, and to highlight that comparison is performed for all items between old and new array, a better variant suggested by Eric Bezault looks like:
items_shifted:
across array as c all
c.item =(old array.twin)
[if c.target_index < array.upper then
c.target_index + 1
else
array.lower
end]
end
I am using below commands to find the size of array
$size_x = #x;
or
$size_x = $#x+1 ;
Both work well when I use in simple statement. But when I use them in the loop, the size becomes one number bigger. Why it is happening in that way. Below is example:
for ($i=1;$i<=10;$i++){
if (1**2+2**2>=1){
#x[$i] =2+3;
$size_x = #x;
}
print "my size is $size_x\n";
}
here is results:
my size is 2
my size is 3
my size is 4
my size is 5
my size is 6
my size is 7
my size is 8
my size is 9
my size is 10
my size is 11
The answer should be from 1 to 10 instead of 2 to 11, i think. What is a better way to get size correctly? Thanks.
After reading your code, I honestly can't figure out what you're trying to do here, but if you're trying to create an array with 11 elements and assign all of them to 5 except for the first one, then you've done an excellent job. Perhaps it would help to see a visualization of the array you've created:
[undef, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5]
If that's really what you were hoping for, there are faster/simpler/better ways to do the same thing, but I have to believe you were trying to accomplish something else. At a very basic level, arrays in Perl are 0-indexed:
Normal arrays are ordered lists of scalars indexed by number, starting
with 0.
Having said that, you very rarely see the C-style for loop in Perl code, and that's because it's very rarely necessary. More often, you would see something like this:
for (0 .. 9) {
# do something...
}
That code uses a foreach-style loop and the range operator (..) to generate a sequence (technically, a list of values from left to right, counting up by one). Enough about loops. Let's talk about your strange logic.
12 + 22 will always be 5, which will always be greater than 1. The same goes for 2 + 3. There's nothing dynamic about this code, which is why I have to think that you meant to use the loop iterator in your calculations somehow. Otherwise, you could easily fill a 10-element array with a bunch of 5's in one line:
my #x = (5) x 10;
Now, one last point that applies to this code and all Perl code you write from this point forward. Every single file must include the following two lines at the top:
use strict;
use warnings;
I believe that Perl arrays are zero-based, meaning that the first element is stored and accessed at index 0. Here is your for loop:
for ($i=1; $i<=10; $i++) {
if (1**2+2**2>=1) {
#x[$i] =2+3;
$size_x = #x;
}
print "my size is $size_x\n";
}
When you make the first assignment #x[1] = 2 + 3, the array is considered to already have an (empty) element in position zero, so the size is returned as being 2.
The easiest way to avoid this problem from happening would be to refactor your for loop to start at index 0:
for ($i=0; $i<10; $i++) {
...
}
When you start the index from 1 the size automatically increases by 1. Try starting the index from 0 or
#x[$i-1] =2+3;
Just as a side note:
Array elements in Perl are denoted with a starting $, not an #.
So #x is the whole array, while $x[$i] is the $i'th element of the array #x.
use warnings; should warn about this.
(I know this is more a comment than an answer, but as a SO newbie I'm not yet allowed to comment. Sorry.)
I'm a total Perl newb, but still cannot believe I cannot figure this out with all the info I've read through online, but, I've burned too much time and am suffering from block at this point. Hoping to learn something based on my real life example...
Ok, I think I have an array of arrays, created like this:
my #array1 = ();
my #array2 = ();
my $ctr1 = 0;
my $col;
[sql query]
while(($col)=$sth->fetchrow_array() ) {
$array1[$ctr1]=$col;
$ctr1++;
}
print STDERR "#array1";
##results in 10 rows, a mac address in each
##00:00:00:00:00:00 00:11:11:11:11:11 22:22:22:22:22:22 33:33:33:33:33:33 ...
Now I do another query. While looping through results, I am looking for those 10 mac addresses. When I find one, I add a row to array2 with the mac and the sequential number accumulated to the point, like this:
[sql query]
while(($col)=$sth->fetchrow_array() ) {
$ctr2++;
if( my ($matched) = grep $_ eq $col, #array1 ) {
push( #array2, ($col,$ctr2) );
}
}
print STDERR "#array2";
##results in 10 rows, a mac address and an integer in each
##00:00:00:00:00:00 2 00:11:11:11:11:11 24 22:22:22:22:22:22 69 33:33:33:33:33:33 82 ...
Now the easy part. I want to loop through array2, grabbing the mac address to use as part of a sql query. Therein lies the problem. I am so ignorant as to exactly what I am doing that even though I had it almost working, I can't get back to that point. Ignorance is definitely not bliss.
When I loop through array2, I am getting a host of errors, based on the different forms of the statement. The one I think is right is listed below along with the error message...
my $ctr3 = 0;
foreach $ctr3 (#array2) {
my $chkmac = $array2[$ctr3][0]; <--- gacks here with the error below - line 607
[SQL query]
[Thu May 30 14:05:09 2013] [error] Can't use string ("00:66:55:77:99:88") as an ARRAY ref while "strict refs" in use at /path/to/test.cgi line 607.\n
I believe the issue is that my array of arrays is not an array of arrays. If it were, it would work as coded, or so I think from the reading... That said, I cannot fathom what I am dealing with otherwise. This will be a head slapper I'm all but sure, but I am stumped.... Little help, please?
TIA
O
For an array of arrays you want to use an array reference, e.g.
push #array2, [$col, $ctr2];
When accessing an element within an array refernce, you'll want to use the -> operator. Also, when looping through an array, it's not necessary to index back into that same array. So the last part would look more like:
foreach $ctr3 (#array2) {
my $chkmac = $ctr3->[0];
....
When you do the foreach there, $ctrl3 won't have the index in it, it'll have the value. So you should just need to do $ctrl3->[0]. Note the -> which dereferences the array reference (#array2 is actually an array of array references).
EDIT: As AKHolland pointed out, #array2 actually isn't an array of array references, although that's what it should be. You also need to change:
push( #array2, ($col, $ctr2) );
To
push( #array2, [$col, $ctr2] );
This makes an array reference, rather than a list. A list in this context just collapses down into regular arguments to push, meaning you're pushing two separate strings into #array2.
You are correct that your array of arrays is not an array of arrays, since in Perl there is no such thing. So what do you have instead? There's two ways to see.
First, when you print #array2, you come up with a string composed of alternating MACs and counts, separated by spaces. Since the spaces sort-of-signify the division between array elements, we might surmise that what we've got is a single array of heterogeneous elements, such that element 0 is a MAC, element 1 is a count, element 2 is another MAC, and so on.
The other perspective is to look at how #array2 is constructed:
push( #array2, ($col,$ctr2) );
From the documentation for push, we find that push ARRAY LIST works by appending the elements of LIST to the end of ARRAY. This has the effect of flattening the list into the array such that its original identity as a list is lost. You can add all the parentheses you want, when Perl expects a list it flattens all of them away.
So how do you achieve the effect you want? The List-of-Lists documentation has a detailed treatment, but the short answer is that you make a list of array references. Array references are scalars and are therefore legal elements in an array. But they retain their identify as array references!
The anonymous array reference constructor is the square bracket []. In order to push an array reference containing the elements $col and $ctr2 onto the end of #array2, you simply do this:
push( #array2, [$col, $ctr2] );
The code you wrote for accessing a particular element of the array-reference-in-an-array now works. But since I've already written a bunch of paragraphs on the subject, let me finish by explaining what was wrong originally and how changing the push statements suddenly makes it work.
The expression $array2[$ctr3][0] is sometimes written as $array2[$ctr3]->[0] to clarify what it's actually doing. What it does is to take the value of $array2[$ctr3] and treat it as an array reference, taking its 0 element. If we take $ctr3 to be 0 (as it would be at the top of the loop) the value of $array2[$ctr3] is the first element, 00:00:00:00:00:00. When you then subsequently ask Perl to treat 00:00:00:00:00:00 as an array reference, Perl dies because it doesn't know how to treat a string as an array reference.
When instead the value of $array2[$ctr3] is an array reference because that is what you pushed onto #array2 when constructing it, Perl is able to do as you ask, dereferencing the array reference and looking at element 0 of the resulting array, whose value happens to be 00:00:00:00:00:00.
I'm trying to check if my arrays are returning nonsense by accessing out of bounds elements, in fortran. And I want to check these values are less than one, and if they are, change them to one.
This is the piece of my code causing issues:
lastNeighLabel=(/clusterLabel(jj-1,kk,ll), clusterLabel(jj,kk-1,ll), clusterLabel(jj,kk,ll-1)/)
LastNeighLabel contains the cluster label (between 1 and n, where n isthe total number of unique seperate clusters found) for the last neighbour in the x,y,z direction respectively.
When jj or kk or ll are 1, they try and access the 0th element in the array, and as FORTRAN counts from 1 in arrays, it tries to destroy the universe. I'm currently in a tangled mess of about 8 if/elseif statements trying to code for every eventuality. But I was hoping there was a way of operating on each element. So basically I'd like to say where((/jj-1,kk-1,ll-1/).lt.1) do clusterLabel(jj-1,kk,ll)=0 etc depending on which element is causing the problem.
But I can't think of a way to do that because where will only manipulate the variables passed to it, not a different array at the same index. Or am I wrong?
Will gladly edit if this doesn't make sense.
It is not obligatory that Fortran accesses arrays starting from one. Any starting value is allowed. If it more convenient to you to have a zero indexed array, declare the array as:
real, dimension (0:N-1, 0:M-1) :: array
Or
real, dimension (0:N, 0:M) :: array
and have the 0 indices be extra to catch special cases.
This might be another solution to your problem, since zero index values would be legal.
Another possible way to approach this, is to create an extended cluster label array (with index bounds starting at 0), which is equal to the cluster label array with a layer of zeroes tacked on the outside. You can then let your loop run safely over all values of jj, kk, and ll. It depends on the size of the array if this is a feasible solution.
integer :: extended_cluster_label(0:size(cluster_label,1), &
0:size(cluster_label,2), &
0:size(cluster_label,3) &
)
extended_cluster_label(0,:,:) = 0
extended_cluster_label(:,0,:) = 0
extended_cluster_label(:,:,0) = 0
extended_cluster_label(1:, 1:, 1:) = cluster_label
Maybe you could use a function?
real function f(A,i,j,k)
real :: A(:,:,:)
integer :: i,j,k
if (i==0.or.j==0.or.k==0) then
f=0
else
f=A(i,j,k)
endif
end function f
and then use f(clusterLabel,jj-1,kk,ll) etc.
I am trying to take my array of numbers based on a variable that determines its size and sort it.
The array is created using the random numbers seed on Fortran 95. However when I try to sort it I run into big trouble. It compiles fine, but the array is printed with a lot of asterisks in it.
In addition I wanted to print my array sideways, (for instance something like this: 1 2 3 4 etc.) but I even failed at doing that. I realize that it must be done using the Advance="no" within a DO loop, but apparently that is erroneous as well.
Below is the code that I am using. If anyone is willing to let me know where I may be wrong I would be very grateful. Thanks for your time.
SUBROUTINE Sorter(num, numinteger)
INTEGER, INTENT(OUT):: num(100)
INTEGER, INTENT(IN):: numinteger
DO i=1, (numinteger-1)
min=num(i)
pos=i
DO j=i,numinteger
IF (num(j)<min)THEN
min=num(j)
pos=j
END IF
END DO
temp=num(i)
num(i)=min
num(pos)=temp
END DO
PRINT*, " "
PRINT*, "Sorted Numbers"
DO i=1, numinteger
WRITE(*,23,ADVANCE="NO") num
23 FORMAT (I2)
END DO
END SUBROUTINE
Thanks!
You don't have any spaces between your numbers, but you are also looping over the array, but not incrementing an index... you are asking the computer to print the entire array on each interation.
I think this should be: WRITE(*,23,ADVANCE="NO") num(i)