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KiligWY's solution

to Collatz Conjecture in the Objective-C Track

Published at Jul 13 2018 · 0 comments
Test suite

The Collatz Conjecture or 3x+1 problem can be summarized as follows:

Take any positive integer n. If n is even, divide n by 2 to get n / 2. If n is odd, multiply n by 3 and add 1 to get 3n + 1. Repeat the process indefinitely. The conjecture states that no matter which number you start with, you will always reach 1 eventually.

Given a number n, return the number of steps required to reach 1.


Starting with n = 12, the steps would be as follows:

  1. 12
  2. 6
  3. 3
  4. 10
  5. 5
  6. 16
  7. 8
  8. 4
  9. 2
  10. 1

Resulting in 9 steps. So for input n = 12, the return value would be 9.


There are two different methods of getting set up to run the tests with Objective-C:

  • Create an Xcode project with a test target which will run the tests.
  • Use the ruby gem objc as a test runner utility.

Both are described in more detail here: http://exercism.io/languages/objective-c

Submitting Exercises

When submitting an exercise, make sure your solution file is in the same directory as the test code.

The submit command will look something like:

exercism submit <path-to-exercism-workspace>/objective-c/collatz-conjecture/CollatzConjecture.m

You can find the Exercism workspace by running exercism debug and looking for the line beginning with Workspace.


An unsolved problem in mathematics named after mathematician Lothar Collatz https://en.wikipedia.org/wiki/3x_%2B_1_problem

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.


#import <XCTest/XCTest.h>

#if __has_include("CollatzConjectureExample.h")
# import "CollatzConjectureExample.h"
# else
# import "CollatzConjecture.h"

@interface CollatzConjectureTest : XCTestCase


@implementation CollatzConjectureTest

- (void)testZeroStepsForOne {
    XCTAssertEqual(0, [CollatzConjecture stepsForNumber:1]);

- (void)testDivideIfEven {
    XCTAssertEqual(4, [CollatzConjecture stepsForNumber:16]);

- (void)testEvenAndOddSteps {
    XCTAssertEqual(9, [CollatzConjecture stepsForNumber:12]);

- (void)testLargeNumberOfEvenAndOddSteps {
    XCTAssertEqual(152, [CollatzConjecture stepsForNumber:1000000]);

- (void)testZeroIsAnError {
    XCTAssertEqual([CollatzConjecture stepsForNumber:0], NSNotFound);

- (void)testNegativeValueIsAnError {
    XCTAssertEqual([CollatzConjecture stepsForNumber:-15], NSNotFound);



#import <Foundation/Foundation.h>

@interface CollatzConjecture : NSObject

+ (NSInteger)stepsForNumber:(NSInteger)number;



#import "CollatzConjecture.h"

@implementation CollatzConjecture

+ (NSInteger)stepsForNumber:(NSInteger)number {
    if (number <= 0) {
        return NSNotFound;
    NSInteger steps = 0;
    while (number != 1) {
        if (number % 2 == 0) {
            number /= 2;
        } else {
            number = number * 3 + 1;
    return steps;


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