in reply to porting C code to Perl

Hello

I commented some parts of the code to make it clearer what the C code is trying to do.

#define N 100

int len = floor(10 * N/3) + 1;

// define an array, in C the size must be specified during compile
// time (just use @a and let the loop populate the array with <len>
// elements)
int A[len];
for(int i = 0; i < len; ++i) {
  A[i] = 2;
}

int nines    = 0;
int predigit = 0;

for(int j = 1; j < N + 1; ++j) {
  // iterate over the reversed array, transform each value
  // and calculate q -> 'map {...} reverse @a' ?
  int q = 0;
  for(int i = len; i > 0; --i) {       
    int x  = 10 * A[i-1] + q*i;        
    A[i-1] = x % (2*i - 1);            
    q = x / (2*i - 1);
  }

  A[0] = q%10;
  q  = q/10;
  if (9 == q) {
    ++nines;
  }
  else if (10 == q) {
    printf("%d", predigit + 1);

    // print '0'x$nines;
    for (int k = 0; k < nines; ++k) {     
      printf("%d", 0);
    }

    predigit, nines = 0;
  }
  else {
    printf("%d", predigit);
    predigit = q;
    if (0 != nines) {

      // print '9'x$nines;
      for (int k = 0; k < nines; ++k) {   
        printf("%d", 9);
      }

      nines = 0;
    }
  }
}

printf("%d", predigit);

[download]

Replies are listed 'Best First'.
Re^2: porting C code to Perl
by marioroy (Prior) on Oct 24, 2017 at 03:11 UTC

    Hi Monk::Thomas,

    The predigit value may set to 0 magically with some C compilers, I'm not sure. It's not the case on the Mac.

    predigit, nines = 0;

    [download]

    I changed the line to this.

    predigit = 0, nines = 0;

    [download]

    Basing on Discipulus's example, I too am interested and will try a plain Perl version. Then, will try parallelization via a number sequence for j. Thank you for the fun, fellow monks.

    Regards, Mario

[reply]
[d/l]
[select]
      The predigit value may set to 0 magically with some C compilers

      Since this value is defined as a global variable (not inside a function) and is an integral data type (not part of an object/struct) it is safe to assume 'predigit' is initialized as zero by the compiler. I guess the author of the C code wasn't sure and tried to initialize both variables (just in case) ... and got it wrong. However it doesn't matter, because the '= 0' doesn't actually add anything.

      test #1 - define at global scope

      #include <stdio.h>

int value1, value2 = 9;

void main(void) {
    printf("%d %d\n", value1, value2);
}

      [download]
      prints '0 9', not 9 9

      test #2 - define in function

      #include <stdio.h>

void func(void) {
    int value1, value2 = 9;
    printf("%d %d\n", value1, value2);
}

void main(void) {
    func();
}

      [download]
      may print '996873600 9' or '-1944996480 9' or '-51174016 9'
[reply]
[d/l]
[select]

        However it doesn't matter, because the '= 0' doesn't actually add anything.

        From testing, it seems to matter on the Mac platform including CentOS 7.3. I compared the output with this site. No warnings are emitted on CentOS 7.3.

        Without '= 0'

        predigit, nines = 0;

-- Mac OS X, Apple LLVM version 7.3.0 (clang-703.0.31)

demo.c:41:13: warning: expression result unused [-Wunused-value]
            predigit, nines = 0;
            ^~~~~~~~
1 warning generated.

0314159265358979323846264338327954288419716939937510582097494459230781
+6406286208998628734825342117067
                                 |                                    
+                |
                                 4                                    
+                7

-- Linux, gcc version 4.8.5 20150623

0314159265358979323846264338327954288419716939937510582097494459230781
+6406286208998628734825342117067
                                 |                                    
+                |
                                 4                                    
+                7

        [download]

        With '= 0'

        predigit = 0, nines = 0;

0314159265358979323846264338327950288419716939937510582097494459230781
+6406286208998628034825342117067
                                 |                                    
+                |
                                 0                                    
+                0

        [download]
[reply]
[d/l]
[select]
        it is safe to assume 'predigit' is initialized as zero by the compiler

        One should be careful with this assumption, I've seen several C compilers for microcontrollers that don't do this by default, in order to save on instruction memory (which can matter on tiny uC's).

[reply]

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