🎉 Exercism Research is now launched. Help Exercism, help science and have some fun at research.exercism.io 🎉
Avatar of vuinguyen

vuinguyen's solution

to Transpose in the Objective-C Track

Published at Jul 13 2018 · 0 comments
Test suite

Given an input text output it transposed.

Roughly explained, the transpose of a matrix:


is given by:


Rows become columns and columns become rows. See https://en.wikipedia.org/wiki/Transpose.

If the input has rows of different lengths, this is to be solved as follows:

  • Pad to the left with spaces.
  • Don't pad to the right.

Therefore, transposing this matrix:


results in:


And transposing:


results in:


In general, all characters from the input should also be present in the transposed output. That means that if a column in the input text contains only spaces on its bottom-most row(s), the corresponding output row should contain the spaces in its right-most column(s).


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/transpose/Transpose.m

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


Reddit r/dailyprogrammer challenge #270 [Easy]. https://www.reddit.com/r/dailyprogrammer/comments/4msu2x/challenge_270_easy_transpose_the_input_text

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("TransposeExample.h")
# import "TransposeExample.h"
# else
# import "Transpose.h"

@interface TransposeTest : XCTestCase


@implementation TransposeTest

- (void)testEmptyInput {
    XCTAssertEqualObjects([Transpose transpose:@[]], @[]);

- (void)testTwoCharactersInARow {
    NSArray<NSString *> *input = @[@"A1"];
    NSArray<NSString *> *expected = @[@"A", @"1"];
    XCTAssertEqualObjects([Transpose transpose:input], expected);

- (void)testTwoCharactersInAColumn {
    NSArray<NSString *> *input = @[@"A", @"1"];
    NSArray<NSString *> *expected = @[@"A1"];
    XCTAssertEqualObjects([Transpose transpose:input], expected);

- (void)testSimple {
    NSArray<NSString *> *input = @[@"ABC", @"123"];
    NSArray<NSString *> *expected = @[@"A1", @"B2", @"C3"];
    XCTAssertEqualObjects([Transpose transpose:input], expected);

- (void)testSingleLine {
    NSArray<NSString *> *input = @[@"Single line."];
    NSArray<NSString *> *expected = @[@"S", @"i", @"n", @"g", @"l", @"e", @"", @"l", @"i", @"n", @"e", @"."];
    XCTAssertEqualObjects([Transpose transpose:input], expected);

- (void)testFirstLineLongerThanSecondLine {
    NSArray<NSString *> *input = @[@"The fourth line.", @"The fifth line."];
    NSArray<NSString *> *expected = @[@"TT", @"hh", @"ee", @"", @"ff", @"oi", @"uf", @"rt", @"th", @"h", @" l", @"li", @"in", @"ne", @"e.", @"."];
    XCTAssertEqualObjects([Transpose transpose:input], expected);

- (void)testSecondLineLongerThanFirstLine {
    NSArray<NSString *> *input = @[@"The first line.", @"The second line."];
    NSArray<NSString *> *expected = @[@"TT", @"hh", @"ee", @"", @"fs", @"ie", @"rc", @"so", @"tn", @" d", @"l", @"il", @"ni", @"en", @".e", @" ."];
    XCTAssertEqualObjects([Transpose transpose:input], expected);

- (void)testSquare {
    NSArray<NSString *> *input = @[@"HEART", @"EMBER", @"ABUSE", @"RESIN", @"TREND"];
    NSArray<NSString *> *expected = @[@"HEART", @"EMBER", @"ABUSE", @"RESIN", @"TREND"];
    XCTAssertEqualObjects([Transpose transpose:input], expected);

- (void)testRectangle {
    NSArray<NSString *> *input = @[@"FRACTURE", @"OUTLINED", @"BLOOMING", @"SEPTETTE"];
    NSArray<NSString *> *expected = @[@"FOBS", @"RULE", @"ATOP", @"CLOT", @"TIME", @"UNIT", @"RENT", @"EDGE"];
    XCTAssertEqualObjects([Transpose transpose:input], expected);

- (void)testTriangle {
    NSArray<NSString *> *input = @[@"T", @"EE", @"AAA", @"SSSS", @"EEEEE", @"RRRRRR"];
    NSArray<NSString *> *expected = @[@"TEASER", @" EASER", @"  ASER", @"   SER", @"    ER", @"     R"];
    XCTAssertEqualObjects([Transpose transpose:input], expected);

- (void)testManyLines { //!OCLINT
    NSArray<NSString *> *input = @[@"Chor. Two households, both alike in dignity,",
                                   @"In fair Verona, where we lay our scene,",
                                   @"From ancient grudge break to new mutiny,",
                                   @"Where civil blood makes civil hands unclean.",
                                   @"From forth the fatal loins of these two foes",
                                   @"A pair of star-cross'd lovers take their life;",
                                   @"Whose misadventur'd piteous overthrows",
                                   @"Doth with their death bury their parents' strife.",
                                   @"The fearful passage of their death-mark'd love,",
                                   @"And the continuance of their parents' rage,",
                                   @"Which, but their children's end, naught could remove,",
                                   @"Is now the two hours' traffic of our stage;",
                                   @"The which if you with patient ears attend,",
                                   @"What here shall miss, our toil shall strive to mend."
    NSArray<NSString *> *expected = @[
                                      @"hnrhr hohnhshh",
                                      @"o oeopotedi ea",
                                      @"rfmrmash  cn t",
                                      @".a e ie fthow",
                                      @" ia fr weh,whh",
                                      @"Trnco miae  ie",
                                      @"w ciroitr btcr",
                                      @" eeih a uote",
                                      @"hrnl sdtln  is",
                                      @"oot ttvh tttfh",
                                      @"un bhaeepihw a",
                                      @"e,ro -trsui ol",
                                      @"h uofcu sarhu",
                                      @"owddarrdan o m",
                                      @"lhg to'egccuwi",
                                      @"sr als t  ists",
                                      @",ebk 'phool'h,",
                                      @"  reldi ffd",
                                      @"bweso tb  rtpo",
                                      @"oea ileutterau",
                                      @"t kcnoorhhnatr",
                                      @"hl isvuyee'fi",
                                      @" atv es iisfet",
                                      @"ayoior trr ino",
                                      @"l  lfsoh  ecti",
                                      @"ion   vedpn  l",
                                      @"   nekt te  rh",
                                      @"ncuse ra-tau l",
                                      @" et  tormsural",
                                      @"dniuthwea'g t",
                                      @"iennwesnr hsts",
                                      @"g,ycoi tkrttet",
                                      @"n ,l r s'a anr",
                                      @"i  ef  'dgcgdi",
                                      @"t  aol   eoe,v",
                                      @"y  nei sl,u; e",
                                      @",  .sf to l",
                                      @"     e rv d  t",
                                      @"     ; ie    o",
                                      @"       f, r",
                                      @"       e  e  m",
                                      @"       .  m  e",
                                      @"          o  n",
                                      @"          v  d",
                                      @"          e  .",
                                      @"          ,"
    XCTAssertEqualObjects([Transpose transpose:input], expected);

//  Transpose.m
//  Transpose
//  Created by Vui Nguyen on 1/8/17.
//  Copyright © 2017 Sunfish Empire. All rights reserved.

#import "Transpose.h"

@implementation Transpose

// Function that, given an input matrix, returns the matrix, transposed.
// For example, given the matrix["ABC", "123"], it will give us the matrix["A1", "B2", C3"].
// We will use this input matrix example in the comments throughout this function.
+(NSArray*) transpose: (NSArray*)inputMatrix
    NSLog(@"inputMatrix: @%@", inputMatrix);
    // The outputMatrix to return, which will be the inputMatrix, transposed
    NSMutableArray * outputMatrix = [[NSMutableArray alloc] init];
    // This is an NSInteger to prevent loss of precision when we do a calculation
    // later with singleString.length, which returns an NSInteger
    NSInteger maxStringLengthInputMatrix = 0;
    // Create an Enumerator to make it easier to loop through the inputMatrix
    NSEnumerator * inputMatrixEnumerator = [inputMatrix objectEnumerator];
    // find the largest string (longest row) in inputMatrix
    int index = 0;
    NSString * singleString = @"";
    while (singleString = [inputMatrixEnumerator nextObject])
        if (singleString.length > maxStringLengthInputMatrix)
            maxStringLengthInputMatrix = singleString.length;
        index = index + 1;
    // initialize strings for outputMatrix
    for (index = 0; index < maxStringLengthInputMatrix; index++)
        outputMatrix[index] = @"";
    // When working with input rows of different lengths, we
    // need a way to pad to the left of the transposed row with spaces,
    // but NOT pad to the right. This array helps us with keeping
    // track of rows that have spaces to the right, so that the spaces
    // can be removed, but leave padding spaces to the left alone.
    // For example, a transposed row like this is okay: "   E" (several spaces
    // to the left of E). But this is NOT okay: "E    " (several spaces to the
    // right of E)
    NSMutableArray * lastNonSpaceIndexArray = [[NSMutableArray alloc] init];
    // initialize array of "last nonspace character indices"
    for (index = 0; index < maxStringLengthInputMatrix; index++)
        [lastNonSpaceIndexArray addObject: [NSNumber numberWithInt:-1]];

    // character from the inputMatrix that will be inserted into the transposed
    // outputMatrix at the correct index
    NSString * singleCharInputMatrix;
    // This nested for-loop takes a row from the inputMatrix and inserts it into the
    // corresponding column in the outputMatrix.
    // For example, the string "ABC" in row 0 of the inputMatrix, will be inserted as
    // "A" in row 0, column 0; "B" in row 1, column 0, and "C" in row 2, column 0 of
    // the outputMatrix.
    int indexInputMatrix  = 0;
    int indexOutputMatrix = 0;
    for (indexInputMatrix = 0; indexInputMatrix < inputMatrix.count; indexInputMatrix++)
        for (indexOutputMatrix = 0; indexOutputMatrix < outputMatrix.count; indexOutputMatrix++)
            // If the current row/string in the inputMatrix is shorter
            // than the other rows in the inputMatrix, then we don't want
            // to access the index beyond the valid string in that row, otherwise
            // we'll throw an error. So, we say the "character" at that index is
            // a space even though there's nothing there. Later on, we will add
            // this space character to the outputMatrix
            // at the correct, corresponding index in outputMatrix.
            if (indexOutputMatrix >= [inputMatrix[indexInputMatrix] length])
                singleCharInputMatrix = @" ";
                // All we're trying to do here is get the character from the current row of inputMatrix,
                // so that later we can append it to the right spot in the outputMatrix
                const unichar singleChar = [inputMatrix[indexInputMatrix] characterAtIndex:indexOutputMatrix];
                singleCharInputMatrix = [NSString stringWithCharacters: &singleChar length:1];
                // if the character from the inputMatrix is NOT a space, we're going to update
                // the lastNonSpaceIndexArray with its position, to help keep track of spaces
                // to the RIGHT of a valid transposed row, so we can remove those extra spaces
                // later
                if (![singleCharInputMatrix isEqual: @" "])
                    lastNonSpaceIndexArray[indexOutputMatrix] = [NSNumber numberWithInteger:indexInputMatrix];
            // Finally, this is where we append the character from inputMatrix into
            // the transposed outputMatrix at the correct index
            // For example, if "ABC" is row 0 of our inputMatrix, and we just grabbed
            // the letter A, then we will now insert A into row 0, column 0 of outputMatrix
            outputMatrix[indexOutputMatrix] = [outputMatrix[indexOutputMatrix] stringByAppendingString:singleCharInputMatrix];
        }   // end outputMatrix loop
    }   // end outermost loop of nested for loop (inputMatrix loop)
    // After filling out the outputMatrix, we have to do some cleanup. That's because
    // some rows may have padded spaces to the right, which we will remove here
    // in this for-loop.
    for (indexOutputMatrix = 0; indexOutputMatrix < outputMatrix.count; indexOutputMatrix++)
        // if a row in outputMatrix has padded spaces to the right, remove those extra spaces
        if (( [lastNonSpaceIndexArray[indexOutputMatrix] integerValue] + 1) < [outputMatrix[indexOutputMatrix] length])
            // Because String operations in Objective-C are so hard, we have this convoluted statement that
            // takes the valid string to the left of the padded spaces, and returns the string. The
            // padded spaces get left behind.  This will also blank output rows which contain only spaces.
            NSString * cleanedUpString = [outputMatrix[indexOutputMatrix] substringToIndex:([lastNonSpaceIndexArray[indexOutputMatrix] integerValue] + 1)];
            // set the row in the outputMatrix to the new and improved, cleaned up string
            outputMatrix[indexOutputMatrix] = cleanedUpString;
    NSLog(@"Final outputMatrix: @%@", outputMatrix);
    return outputMatrix;


Community comments

Find this solution interesting? Ask the author a question to learn more.

What can you learn from this solution?

A huge amount can be learned from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

  • What compromises have been made?
  • Are there new concepts here that you could read more about to improve your understanding?