Category: Science

Code of the day: C, Get the Digits from a Number


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Today’s Code of the Day is about how to find the digit of a number at a given position. I decided to go with the C / C++ programming languages to time, but you can easily translate the algorithms to other languages.

C Code.

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#include <stdio.h>
#include <math.h>
 
// Function: getDigitFromNum returns a digit at a given index of a integer.
// Goes from right to left with the starting index of 0.
int getDigitFromNum( int num, int digit ){
    num /= pow( 10, digit );
    return num % 10;
}
// Function: getDigitFromDec returns a digit at a given index of a double.
// Goes from left to right with the starting index of 0.
int getDigitFromDec( double dec, int digit ){
    dec *= pow( 10, digit );
    return (int)dec % 10;
}
// Function: getDigitFromNum returns the decimal values of a double.
double getDecimals( double dec ){
    return dec - (int)dec;
}

Example:
Input File

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// Programmed by Larry Battle, bateru.com/news
// Date: April 26, 2011
// Purpose: To demonstrate how to get a digit by a position index.
#include <stdio.h>
#include <math.h>
 
int getDigitFromNum( int num, int digit ){
    num /= pow( 10, digit );
    return num % 10;
}
int getDigitFromDec( double dec, int digit ){
    dec *= pow( 10, digit );
    return ((int)dec) % 10;
}
double getDecimals( double dec ){
    return dec - (int)dec;
}
 
void main(){
    double dec = 1234.56789;
    printf( "\n%d", getDigitFromNum( (int)dec, 0 ));
    printf( "\n%d", getDigitFromDec( dec, 2 ));
    printf( "\n%f", getDecimals( dec ));
}

Output:

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0.567890
Larry Battle

Larry Battle

I love to program, and discover new tech. Check out my <a href="http://stackoverflow.com/users/527776/larry-battle">stackoverflow</a> and <a href="https://github.com/LarryBattle">github</a> accounts.

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Learn Sig Figs in No Time


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Hey Everybody,
I made a video for one of my old post, What are Sig Figs? Tutorial
Enjoy. 🙂

Larry Battle

Larry Battle

I love to program, and discover new tech. Check out my <a href="http://stackoverflow.com/users/527776/larry-battle">stackoverflow</a> and <a href="https://github.com/LarryBattle">github</a> accounts.

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Code of the Day: Javascript Significant Figures


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Did you know that JavaScript automatically trims off trailing zeros on numbers with decimals?
Well now you do, and in order to save precision you must wrap the number inside a string.
Example:

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var a = "0.0100";
var b = 0.0100;                           // To save space the interpreter cuts off the last two zeros.
console.log( a === b.toString() );	// displays false because a = "0.0100" and b = 0.01
 
console.log( 1000 === 1000.000 );	// displays true
console.log( 0.1 === 0.1000 );		// displays true

Some might argue that you can use num.toPrecision() to save the trailing zeros. But what most are unaware of is that toPrecision() returns a string.

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var num = (120.0).toPrecision(8);
console.log( num );                      // displays 120.00000
console.log( typeof num );            // displays string
console.log( +num );                    // displays 120
// +num is a shortcut for parseInt( num, 10);

Now let’s see what happens with leading zero’s.

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//A leading 0 in front of a number converts it base 8.
// a.k.a. parseInt( num, 8);
console.log(0011);		// Displays 9
console.log( 010.1000 );		// Error! Octals can only be whole numbers.
console.log( 00000.1000 );	// Error for the same reason.
// The problem is that a leading zero will cause the number to be converted to octal.
// Thus, an error is generated because the decimal point is neither an operation nor semicolon.

Using numbers as strings can be useful when trying to find the Significant Digits, like in the following function.

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// Returns the significant digits of a number.
// @param {Number|String} num
// @return Returns -1 if invalid input, otherwise will return a positive number.
var getSigFigs = function (num) {
  if (!isFinite(Number(num))) {
    return -1;
  }
  var n = String(num).trim(),
  FIND_FRONT_ZEROS_SIGN_DOT_EXP = /^[\D0]+|\.|([e][^e]+)$/g,
  FIND_RIGHT_ZEROS = /0+$/g;
 
  if (!/\./.test(num)) {
    n = n.replace(FIND_RIGHT_ZEROS, "");
  }
  return n.replace(FIND_FRONT_ZEROS_SIGN_DOT_EXP, "").length;
};

Usage:

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console.log( getSigFigFromNum( "0.01230" ) == 5);

Test cases

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// Try using Qunit to run tests.
var assert = {};
assert.equal = function (a, b) {
  if (a != b) {
    console.error("FAIL: --> %s != %s", a, b);
  } else {
    console.log("Pass: %s == %s", a, b);
  }
};
var test = function (name, fn) {
  console.log("\n#Testing: %s", name);
  fn();
};
test("Invalid numbers", function () {
  assert.equal(getSigFigs("742400g"), -1);
  assert.equal(getSigFigs("g742400"), -1);
  assert.equal(getSigFigs("Infinity"), -1);
  assert.equal(getSigFigs("-Infinity"), -1);
  assert.equal(getSigFigs("NaN"), -1);
});
var testWithPrefixes = function (prefixes, nums, fn) {
  prefixes.forEach(function (prefix) {
    nums.forEach(function (num) {
      //assert.equal( "Testing " + expected + "" prefix+expected, fn(prefix+num));
    });
  });
};
test("Whole Numbers", function () {
  assert.equal(getSigFigs("742400"), 4);
  assert.equal(getSigFigs("742000"), 3);
  assert.equal(getSigFigs("740000"), 2);
  assert.equal(getSigFigs("700000"), 1);
});
test("Negative Whole Numbers", function () {
  assert.equal(getSigFigs("-742400"), 4);
  assert.equal(getSigFigs("-742000"), 3);
  assert.equal(getSigFigs("-740000"), 2);
  assert.equal(getSigFigs("-700000"), 1);
});
test("+Whole numbers", function () {
  assert.equal(getSigFigs("+742400"), 4);
  assert.equal(getSigFigs("+742000"), 3);
  assert.equal(getSigFigs("+740000"), 2);
  assert.equal(getSigFigs("+700000"), 1);
});
test("Decimals", function () {
  assert.equal(getSigFigs("0.07284"), 4);
  assert.equal(getSigFigs("0.0728"), 3);
  assert.equal(getSigFigs("0.072"), 2);
  assert.equal(getSigFigs("0.07"), 1);
  assert.equal(getSigFigs("123.07"), 5);
});
test("Negative Decimals", function () {
  assert.equal(getSigFigs("-0.07284"), 4);
  assert.equal(getSigFigs("-0.0728"), 3);
  assert.equal(getSigFigs("-0.072"), 2);
  assert.equal(getSigFigs("-0.07"), 1);
  assert.equal(getSigFigs("-10.07"), 4);
});
test("Decimals", function () {
  assert.equal(getSigFigs("+0.07284"), 4);
  assert.equal(getSigFigs("+0.0728"), 3);
  assert.equal(getSigFigs("+0.072"), 2);
  assert.equal(getSigFigs("+0.07"), 1);
});
test("Zero", function () {
  assert.equal(getSigFigs("0.0"), 0);
  assert.equal(getSigFigs("-0.0"), 0);
  assert.equal(getSigFigs("0"), 0);
  assert.equal(getSigFigs("+0"), 0);
  assert.equal(getSigFigs("-0"), 0);
});
 
test("E notated", function () {
  assert.equal(getSigFigs("1.3e32"), 2);
  assert.equal(getSigFigs("-1.23e-32"), 3);
  assert.equal(getSigFigs("-1.023e+120"), 4);
  assert.equal(getSigFigs("+5.023e+32"), 4);
  assert.equal(getSigFigs("+5.0230e+32"), 5);
});

Can you think of any other useful examples for number wrapped in strings?

Larry Battle

Larry Battle

I love to program, and discover new tech. Check out my <a href="http://stackoverflow.com/users/527776/larry-battle">stackoverflow</a> and <a href="https://github.com/LarryBattle">github</a> accounts.

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Book Review: Computers Ltd: What They Really Can’t Do


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Hey everybody,
I would like to share a book review that I wrote on amazon.com.
Book: “Computers Ltd.: What They Really Can’t Do (Popular Science)” (2003)
Author: David Harel

Computers Ltd.: What They Really Can't Do

“This book sets the record straight. Computers can’t fix all of the world problems because they’re limited by time and space.

The author starts off by defining algorithms and how computer programs work. He’s then explores common problems in computer science using a fair amount of algebra and graphs, like NP complete problems, the travelling salesman problem, the Turing test, tower of Hanoi, and etc.

Restraining from being a complete pessimist, discussions mainly addressing Cryptography are included. Cryptography shows how computational complexity can be used for the greater good, as it’s nearly impossible to break the encryption within a reasonable amount of time for any data encoded in RSA.

Lastly, the author ends the book with his take on hot areas in computing, such as Quantum Computers, Artificial Intelligence and evolutionary (generic) programming.
Overall, I enjoyed this pocket size book and recommend it for those interested in expanding their knowledge in Computer Science.”
-Larry Battle

Larry Battle

Larry Battle

I love to program, and discover new tech. Check out my <a href="http://stackoverflow.com/users/527776/larry-battle">stackoverflow</a> and <a href="https://github.com/LarryBattle">github</a> accounts.

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