I am trying to write a subroutine that determines whether or not the number passed in is prime, and it's not working correctly. The numbers I'm passing in should not be identified as prime. Is there a logic error, or something about Perl that I'm missing?
sub isPrime {
my ( $n ) = #_;
for ( my $i = 3 ; $i < $n ; $i++ ) {
if ( $n % $i == 0 ) {
return 0;
}
else {
return 1;
}
}
}
At the moment your function is checking just if n is not divisible by 3 because it calls return immediately after the fisrt test.
Try to make the function return 0 within the for loop, and return 1 outside it, or set a flag for the number being prime that is initially true and return its value after the loop.
You should also start your for loop at 2, not at 3, otherwise you aren't testing for even numbers.
Here is my code I wrote in about 40 minutes. Don't hate if it is inefficient, I am still learning perl.
print ("This is a prime number checker!\n");
print ("Enter a number below to check it:\n");
$y = 0;
$num = <>;
for ($i = $num; $i > 0; $i--) {
if ($num % $i == 0) {
$y += 1;
}
}
if ($y > 2) {
print ("$num is not a prime!");
} else {
print ("$num is a prime!");
}
Related
I have a code in Perl which takes in a number and adds up all the prime numbers up to that number. I keep on getting the value 0 which means it is not updating my $sum variable, but I don't know what else to do.
sub checkPrime {
my($numb) = #_;
$primeCheck = "prime\n";
if ($numb == 1) {
$primeCheck = "notPrime\n";
}
for ($i = 2; $i < $numb; $i++) {
$mod = $numb % $i;
if ($mod == 0) {
$primeCheck = "notPrime\n"
}
}
return $primeCheck;
}
sub sumOfPrimes {
my($input) = #_;
$sum = 0;
for ($i = 2; $i <= $input; $i++) {
if (checkPrime($i) eq "prime") {
$sum = $sum + $i;
}
}
return $sum;
}
print sumOfPrimes(10);
You are not comparing the correct string. You include a newline character (\n) when you set the value, but not when you compare it. Change:
if (checkPrime($i) eq "prime")
to:
if (checkPrime($i) eq "prime\n")
That is the simplest change, but you probably don't need to have \n in there at all.
To sum prime numbers you need to identify if the number is a prime number. Let's create a function which returns 1 if the number is prime and 0 otherwise.
sub isPrime {
my $n = shift;
return 0 unless $n > 1;
for( my $i = 2; $i < $n; $i++ ) {
return 0 if $n % $i == 0;
}
return 1;
}
Now go through the list of numbers and sum only those which is prime
$sum += $num if isPrime($num);
I tried writing a simple code to find whether a number can be expressed as the sum of primes or not, in Perl. The sample code is as shown:
sub funcIsPrime {
my $num = $_[0];
my $isPrime = 1;
for($i= 2; $i <= $num/2; $i++){
if($num%$i == 0){
$isPrime = 0;
last;
}
}
return $isPrime;
}
#my $num = <>;
my $num = 20;
for($i = 2; $i <= $num/2; $i++){
print "$i\t";
my $j = $num-$i;
print "$j\n";
if(funcIsPrime($i) and funcIsPrime($j)){ # Line x
print "$num = $i + $j\n";
}
}
The function call statements in Line x do not execute. The same line when put outside the loop works fine. What can be the possible solution? Please help. Thank you.
The main issue is missing my in variable declarations. Perl won't let you run the program if you include use warnings; and use strict;:
Global symbol "$i" requires explicit package name (did you forget to declare "my $i"?) at test.pl line 22.
Execution of test.pl aborted due to compilation errors.
Here's simplified working code (you can search for factors up to the square root of n, by the way, although this isn't a perfect or efficient prime test by any means):
use strict;
use warnings;
sub isPrime {
my $num = $_[0];
for (my $i = int sqrt $num; $i > 1; $i--) {
if ($num % $i == 0) {
return 0;
}
}
return 1;
}
my $num = 20;
for (my $i = 2; $i <= $num / 2; $i++) {
my $j = $num - $i;
if (isPrime($i) && isPrime($j)) {
print "$num = $i + $j\n";
}
}
Output
20 = 3 + 17
20 = 7 + 13
I heard from multiple tutorials that there is no Boolean variable. Instead I can use 1 for true or 0 for false. However, I have 2 methods to get the boolean values. The output is the same.. But I do not know which method is correct for collecting the return boolean values.
Let me give you an example
I made a script,call.pl to call the function from another script,script.pl and the script.pl will return the 1 or 0. I perform the if conditional to evaluate.
If it is true, it will says that it is even otherwise it is odd.
Method 1
script.pl
sub checkevenodd {
my ($num) = #_;
chomp($num);
my $remainder = $num % 2;
if ($remainder == 0)
{
return 1;
}
else
{
return 0
}
}
1;
call.pl
require "script.pl";
my $no = 123;
if (checkevenodd($no) == 1)
{
print "it is even";
}
else
{
print "it is odd";
}
method 2
script.pl
sub checkevenodd {
my ($num) = #_;
chomp($num);
my $remainder = $num % 2;
if ($remainder == 0)
{
return 1;
}
else
{
return 0
}
}
1;
call.pl
require "script.pl";
my $no = 123;
if (checkevenodd($no))
{
print "it is even";
}
else
{
print "it is odd";
}
I use the function to check whether is it 1 or 0... Then if it is 1, it is even or else odd.
So which method is best for receiving the boolean value from the function?? I do not want to create a variable. Instead I want to return 1 or 0.. How to get the 1 or 0.. Is it correct method??
When you write:
if (checkevenodd($no) == 1)
You are not checking for a Boolean value. You are checking for the value 1. In this case it will work (because checkevenodd() only ever returns 0 or 1) but, in general, you should only ever check the truth of a Boolean expression, not its value. It's much better to write:
if (checkevenodd($no))
A couple of other points.
checkevenodd is not a good name for this subroutine. When I have subroutines that return Boolean values, I always try to give them a name that starts with is_. Your subroutine returns true if the number is even, so I would call this subroutine is_even().
Your subroutine is far more complex than it needs to be. I would write it as:
sub is_even {
my ($num) = #_;
# For an even number, $num % 2 is zero - which is false.
# Therefore use ! to make it true.
return ! $num % 2;
}
How about something like this?
sub even_odd {
my ($num) = #_;
my $remainder = $num % 2;
return $remainder ? 0 : 1;
}
and using it as a complete script:
my $no = 123;
if (even_odd($no))
{
print "it is even";
}
else
{
print "it is odd";
}
sub even_odd {
my ($num) = #_;
my $remainder = $num % 2;
return $remainder ? 0 : 1;
}
returns: it is odd
I have a Perl program containing the following methods:
det To find the determinant of a matrix.
identityMatrix Return an n by n identity matrix (1s on the main diagnal, rest 0s).
matrixAdd To add two matrices together.
matrixScalarMultiply To multiply an integer by a matrix.
I can easily find the determinant of, for example, a matrix A - I where
(It is 0)
But what if I want to find the determinant of A - RI?
In this case, I want my program to solve for the characteristic polynomial solution (a.k.a. the determinant containing variables) along these lines instead of an integer value:
Any suggestions on how to handle this? Code below:
#!/usr/bin/perl
#perl Solver.pl
use strict;
use warnings;
### solve the characteristic polynomial det(A - RI)
my #A = ( # 3x3, det = -3
[1, 3, -3],
[1, 0, 0],
[0, 1, 0],
);
# test_matrix = A - I
my $test_matrix = matrixAdd( \#A,
matrixScalarMultiply( identityMatrix(3), -1 ) );
print "\nTest:\n";
for( my $i = 0; $i <= $#$test_matrix; $i++ ){
print "[";
for( my $j = 0; $j <= $#$test_matrix; $j++ ){
$j == $#$test_matrix ? print $test_matrix->[$i][$j], "]\n" :
print $test_matrix->[$i][$j], ", ";
}
}
my $dd = det ($test_matrix);
print "det = $dd \n";
# recursively find determinant of a real square matrix
# only call on n by n matrices where n >= 2
#
# arg0 = matrix reference
sub det{
my ($A) = #_;
#base: 2x2 matrix
if( $#$A + 1 == 2 ){ #recall $#$A == last index of A
return $A->[0][0]*$A->[1][1] - $A->[1][0]*$A->[0][1];
}
#cofactor expansion for matrices > 2x2
my $answer = 0;
for( my $col = 0; $col <= $#$A; $col++ ){
my $m = (); #sub matrix
my $multiplier = $A->[0][$col];
if( $col % 2 == 1 ){ #+, -, +, -, ...
$multiplier *= -1;
}
for( my $i = 1; $i <= $#$A; $i++ ){
#j is indexer for A, k for m
for( my ($j, $k) = (0, 0); $j <= $#$A; $j++ ){
$m->[$i-1][$k++] = $A->[$i][$j] unless $j == $col;
}
}
$answer += $multiplier*det( $m );
}#end cofactor expansion
return $answer;
}#end det()
# return reference to an n by n identity matrix
# can do this in Perl!
#
# arg0 = dimension 'n'
sub identityMatrix{
my $n = shift;
my #ret;
for (my $i = 0; $i < $n; $i++ ){
for (my $j = 0; $j < $n; $j++ ){
$ret[$i][$j] = $i == $j ? 1 : 0;
}
}
return \#ret;
}
# return reference to an n by n matrix which is the sum
# of two different n by n matrices, "a" and "b"
#
# arg0, 1 = references to the pair of matrices to add
sub matrixAdd{
my #ret;
my ($a, $b) = ($_[0], $_[1]);
for (my $i = 0; $i <= $#$a; $i++ ){
for (my $j = 0; $j <= $#$a; $j++ ){
$ret[$i][$j] = $a->[$i][$j] + $b->[$i][$j];
}
}
return \#ret;
}
# return reference to a matrix multiplied by a given scalar
#
# arg0 = reference to matrix
# arg1 = scalar to multiply by
sub matrixScalarMultiply{
my #ret;
my ($a, $multiplier) = ($_[0], $_[1]);
for (my $i = 0; $i <= $#$a; $i++ ){
for (my $j = 0; $j <= $#$a; $j++ ){
$ret[$i][$j] = $a->[$i][$j] * $multiplier;
}
}
return \#ret;
}
This is called symbolic math and is in the wheelhouse of tools like Mathematica. For Perl, there are packages like Math::Synbolic but I couldn't tell you how easy they are to use.
On the other hand, if you are just interested in what values of R have a determinant of zero and not that interested in what the characteristic polynomial looks like, then you are looking for the eigenvalues of A. There are some Perl libraries for that, too.
I'm new at Perl and im having trouble at designing a certain function in Perl.
The Function should find and return all Increasing and Decreasing Strips.
What does that mean? Two Positions are neighbors if they're neighboring numbers. i.e. (2,3) or (8,7). A Increasing Strip is an increasing Strip of neighbors. i.e. (3,4,5,6). Decreasing Strip is defined similar. At the beginning of every Array a 0 gets added and at the end the length of the array+1. Single Numbers without neighbors are decreasing. 0 and n+1 are increasing.
So if i have the array (0,3,4,5,9,8,6,2,1,7,10) i should get the following results:
Increasing Strips are: (3,4,5) (10) (0)
Decreasing Strips are: (9,8), (6), (2,1) (7)
I tried to reduce the problem to only getting all Decreasing Strips, but this is as far as i get: http://pastebin.com/yStbgNme
Code here:
sub getIncs{
my #$bar = shift;
my %incs;
my $inccount = 0;
my $i=0;
while($i<#bar-1){
for($j=$i; 1; $j++;){
if($bar[$j] == $bar[$j+1]+1){
$incs{$inccount} = ($i,$j);
} else {
$inccount++;
last;
}
}
}
//edit1: I found a Python-Program that contains said function getStrips(), but my python is sporadic at best. http://www.csbio.unc.edu/mcmillan/Media/breakpointReversalSort.txt
//edit2: Every number is exactly one Time in the array So there can be no overlap.
use strict;
my #s = (0,3,4,5,9,8,6,2,1,7,10);
my $i = 0;
my $j = 0; #size of #s
my $inc = "Increasing: ";
my $dec = "Decreasing: ";
# Prepend the beginning with 0, if necessary
if($s[0] != 0 || #s == 0 ) { unshift #s, 0; }
$j = #s;
foreach(#s) {
# Increasing
if( ($s[$i] == 0) || ($i == $j-1) || ($s[$i+1] - $s[$i]) == 1 || ($s[$i] - $s[$i-1] == 1)) {
if($s[$i] - $s[$i-1] != 1) { $inc .= "("; }
$inc .= $s[$i];
if($s[$i+1] - $s[$i] != 1) { $inc .= ")"; }
if($s[$i+1] - $s[$i] == 1) { $inc .= ","; }
}
#Decreasing
if( ($s[$i]-$s[$i-1] != 1) && ($s[$i+1] - $s[$i] != 1) && ($s[$i] != 0) && ($i != $j-1) ) {
if($s[$i-1] - $s[$i] != 1) { $dec .= "("; }
$dec .= $s[$i];
if($s[$i] - $s[$i+1] != 1) { $dec .= ")"; }
if($s[$i] - $s[$i+1] == 1) { $dec .= ","; }
}
$i++;
}
$inc =~ s/\)\(/\),\(/g;
$dec =~ s/\)\(/\),\(/g;
print "$inc\n";
print "$dec\n";
Result:
Increasing: (0),(3,4,5),(10)
Decreasing: (9,8),(6),(2,1),(7)