How to reverse every other sublist of a list?












5












$begingroup$


I have a list with sublists list = {{1, 2}, {3, 4}, {5, 6}, {7, 8}}. I would like to reverse every other sublist, starting from the second one. The result would look like result = {{1, 2}, {4, 3}, {5, 6}, {8, 7}}. What would be a simple way to do this?






list-manipulation







share|improve this question









$endgroup$

















    5












    $begingroup$


    I have a list with sublists list = {{1, 2}, {3, 4}, {5, 6}, {7, 8}}. I would like to reverse every other sublist, starting from the second one. The result would look like result = {{1, 2}, {4, 3}, {5, 6}, {8, 7}}. What would be a simple way to do this?






    list-manipulation







    share|improve this question









    $endgroup$















      5












      5








      5


      2



      $begingroup$


      I have a list with sublists list = {{1, 2}, {3, 4}, {5, 6}, {7, 8}}. I would like to reverse every other sublist, starting from the second one. The result would look like result = {{1, 2}, {4, 3}, {5, 6}, {8, 7}}. What would be a simple way to do this?






      list-manipulation







      share|improve this question









      $endgroup$




      I have a list with sublists list = {{1, 2}, {3, 4}, {5, 6}, {7, 8}}. I would like to reverse every other sublist, starting from the second one. The result would look like result = {{1, 2}, {4, 3}, {5, 6}, {8, 7}}. What would be a simple way to do this?






      list-manipulation




      list-manipulation






      share|improve this question













      share|improve this question











      share|improve this question




      share|improve this question










      asked 2 days ago









      nanjunnanjun

      44429




      44429






















          4 Answers
          4






          active

          oldest

          votes


















          13












          $begingroup$

          Use MapAt, which accepts the same syntax as Part:



          MapAt[Reverse, list, {2 ;; ;; 2}]
          
(* {{1, 2}, {4, 3}, {5, 6}, {8, 7}} *)





          share|improve this answer









          $endgroup$





















            9












            $begingroup$

            Using Part and Span might not seem overly elegant but it is fast:



            list = RandomReal[{-1, 1}, {100000, 10}];
            

            
a = MapAt[Reverse, list, {2 ;; ;; 2}]; // RepeatedTiming // First
            

            
b = ReplacePart[list, i_?EvenQ :> Reverse@list[[i]]]; // RepeatedTiming // First
            

            
c = Module[{result = list},
            
     result[[2 ;; ;; 2]] = list[[2 ;; ;; 2, -1 ;; 1 ;; -1]];
            
     result
            
     ]; // RepeatedTiming // First
            

            
a == b == c



            0.11



            0.317



            0.0036



            True







            share|improve this answer











            $endgroup$









            • 2




              $begingroup$
              A bit cleaner, s/b comparable in speed: Riffle[list[[;; ;; 2]], list[[2 ;; ;; 2, -1 ;; 1 ;; -1]]]
              $endgroup$
              – ciao
              yesterday



















            4












            $begingroup$

            Another method is to use ReplacePart:



            ReplacePart[{{1, 2}, {3, 4}, {5, 6}, {7, 8}},
            
    i_ ? EvenQ :> Reverse@list[[i]]
            
]
            
(* {{1,2},{4,3},{5,6},{8,7}} *)





            share|improve this answer









            $endgroup$





















              3












              $begingroup$

              This will work



              list = {{1, 2}, {3, 4}, {5, 6}, {7, 8}};
              

              
Table[If[EvenQ[i], Reverse[list[[i]]], list[[i]]], {i, 1, Length[list]}]
              

              
(*{{1, 2}, {4, 3}, {5, 6}, {8, 7}}*)
              
```





              share|improve this answer








              New contributor




              cphys is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
              Check out our Code of Conduct.






              $endgroup$














                Your Answer





                StackExchange.ifUsing("editor", function () {
                return StackExchange.using("mathjaxEditing", function () {
                StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix) {
                StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["$", "$"], ["\\(","\\)"]]);
                });
                });
                }, "mathjax-editing");

                StackExchange.ready(function() {
                var channelOptions = {
                tags: "".split(" "),
                id: "387"
                };
                initTagRenderer("".split(" "), "".split(" "), channelOptions);

                StackExchange.using("externalEditor", function() {
                // Have to fire editor after snippets, if snippets enabled
                if (StackExchange.settings.snippets.snippetsEnabled) {
                StackExchange.using("snippets", function() {
                createEditor();
                });
                }
                else {
                createEditor();
                }
                });

                function createEditor() {
                StackExchange.prepareEditor({
                heartbeatType: 'answer',
                autoActivateHeartbeat: false,
                convertImagesToLinks: false,
                noModals: true,
                showLowRepImageUploadWarning: true,
                reputationToPostImages: null,
                bindNavPrevention: true,
                postfix: "",
                imageUploader: {
                brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
                contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
                allowUrls: true
                },
                onDemand: true,
                discardSelector: ".discard-answer"
                ,immediatelyShowMarkdownHelp:true
                });


                }
                });














                draft saved

                draft discarded


















                StackExchange.ready(
                function () {
                StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmathematica.stackexchange.com%2fquestions%2f194901%2fhow-to-reverse-every-other-sublist-of-a-list%23new-answer', 'question_page');
                }
                );

                Post as a guest















                Required, but never shown

























                4 Answers
                4






                active

                oldest

                votes








                4 Answers
                4






                active

                oldest

                votes









                active

                oldest

                votes






                active

                oldest

                votes









                13












                $begingroup$

                Use MapAt, which accepts the same syntax as Part:



                MapAt[Reverse, list, {2 ;; ;; 2}]
                
(* {{1, 2}, {4, 3}, {5, 6}, {8, 7}} *)





                share|improve this answer









                $endgroup$


















                  13












                  $begingroup$

                  Use MapAt, which accepts the same syntax as Part:



                  MapAt[Reverse, list, {2 ;; ;; 2}]
                  
(* {{1, 2}, {4, 3}, {5, 6}, {8, 7}} *)





                  share|improve this answer









                  $endgroup$
















                    13












                    13








                    13





                    $begingroup$

                    Use MapAt, which accepts the same syntax as Part:



                    MapAt[Reverse, list, {2 ;; ;; 2}]
                    
(* {{1, 2}, {4, 3}, {5, 6}, {8, 7}} *)





                    share|improve this answer









                    $endgroup$



                    Use MapAt, which accepts the same syntax as Part:



                    MapAt[Reverse, list, {2 ;; ;; 2}]
                    
(* {{1, 2}, {4, 3}, {5, 6}, {8, 7}} *)






                    share|improve this answer












                    share|improve this answer



                    share|improve this answer










                    answered 2 days ago









                    marchmarch

                    17.6k22770




                    17.6k22770























                        9












                        $begingroup$

                        Using Part and Span might not seem overly elegant but it is fast:



                        list = RandomReal[{-1, 1}, {100000, 10}];
                        

                        
a = MapAt[Reverse, list, {2 ;; ;; 2}]; // RepeatedTiming // First
                        

                        
b = ReplacePart[list, i_?EvenQ :> Reverse@list[[i]]]; // RepeatedTiming // First
                        

                        
c = Module[{result = list},
                        
     result[[2 ;; ;; 2]] = list[[2 ;; ;; 2, -1 ;; 1 ;; -1]];
                        
     result
                        
     ]; // RepeatedTiming // First
                        

                        
a == b == c



                        0.11



                        0.317



                        0.0036



                        True







                        share|improve this answer











                        $endgroup$









                        • 2




                          $begingroup$
                          A bit cleaner, s/b comparable in speed: Riffle[list[[;; ;; 2]], list[[2 ;; ;; 2, -1 ;; 1 ;; -1]]]
                          $endgroup$
                          – ciao
                          yesterday
















                        9












                        $begingroup$

                        Using Part and Span might not seem overly elegant but it is fast:



                        list = RandomReal[{-1, 1}, {100000, 10}];
                        

                        
a = MapAt[Reverse, list, {2 ;; ;; 2}]; // RepeatedTiming // First
                        

                        
b = ReplacePart[list, i_?EvenQ :> Reverse@list[[i]]]; // RepeatedTiming // First
                        

                        
c = Module[{result = list},
                        
     result[[2 ;; ;; 2]] = list[[2 ;; ;; 2, -1 ;; 1 ;; -1]];
                        
     result
                        
     ]; // RepeatedTiming // First
                        

                        
a == b == c



                        0.11



                        0.317



                        0.0036



                        True







                        share|improve this answer











                        $endgroup$









                        • 2




                          $begingroup$
                          A bit cleaner, s/b comparable in speed: Riffle[list[[;; ;; 2]], list[[2 ;; ;; 2, -1 ;; 1 ;; -1]]]
                          $endgroup$
                          – ciao
                          yesterday














                        9












                        9








                        9





                        $begingroup$

                        Using Part and Span might not seem overly elegant but it is fast:



                        list = RandomReal[{-1, 1}, {100000, 10}];
                        

                        
a = MapAt[Reverse, list, {2 ;; ;; 2}]; // RepeatedTiming // First
                        

                        
b = ReplacePart[list, i_?EvenQ :> Reverse@list[[i]]]; // RepeatedTiming // First
                        

                        
c = Module[{result = list},
                        
     result[[2 ;; ;; 2]] = list[[2 ;; ;; 2, -1 ;; 1 ;; -1]];
                        
     result
                        
     ]; // RepeatedTiming // First
                        

                        
a == b == c



                        0.11



                        0.317



                        0.0036



                        True







                        share|improve this answer











                        $endgroup$



                        Using Part and Span might not seem overly elegant but it is fast:



                        list = RandomReal[{-1, 1}, {100000, 10}];
                        

                        
a = MapAt[Reverse, list, {2 ;; ;; 2}]; // RepeatedTiming // First
                        

                        
b = ReplacePart[list, i_?EvenQ :> Reverse@list[[i]]]; // RepeatedTiming // First
                        

                        
c = Module[{result = list},
                        
     result[[2 ;; ;; 2]] = list[[2 ;; ;; 2, -1 ;; 1 ;; -1]];
                        
     result
                        
     ]; // RepeatedTiming // First
                        

                        
a == b == c



                        0.11



                        0.317



                        0.0036



                        True








                        share|improve this answer














                        share|improve this answer



                        share|improve this answer








                        edited yesterday

























                        answered 2 days ago









                        Henrik SchumacherHenrik Schumacher

                        59.9k582167




                        59.9k582167








                        • 2




                          $begingroup$
                          A bit cleaner, s/b comparable in speed: Riffle[list[[;; ;; 2]], list[[2 ;; ;; 2, -1 ;; 1 ;; -1]]]
                          $endgroup$
                          – ciao
                          yesterday














                        • 2




                          $begingroup$
                          A bit cleaner, s/b comparable in speed: Riffle[list[[;; ;; 2]], list[[2 ;; ;; 2, -1 ;; 1 ;; -1]]]
                          $endgroup$
                          – ciao
                          yesterday








                        2




                        2




                        $begingroup$
                        A bit cleaner, s/b comparable in speed: Riffle[list[[;; ;; 2]], list[[2 ;; ;; 2, -1 ;; 1 ;; -1]]]
                        $endgroup$
                        – ciao
                        yesterday




                        $begingroup$
                        A bit cleaner, s/b comparable in speed: Riffle[list[[;; ;; 2]], list[[2 ;; ;; 2, -1 ;; 1 ;; -1]]]
                        $endgroup$
                        – ciao
                        yesterday











                        4












                        $begingroup$

                        Another method is to use ReplacePart:



                        ReplacePart[{{1, 2}, {3, 4}, {5, 6}, {7, 8}},
                        
    i_ ? EvenQ :> Reverse@list[[i]]
                        
]
                        
(* {{1,2},{4,3},{5,6},{8,7}} *)





                        share|improve this answer









                        $endgroup$


















                          4












                          $begingroup$

                          Another method is to use ReplacePart:



                          ReplacePart[{{1, 2}, {3, 4}, {5, 6}, {7, 8}},
                          
    i_ ? EvenQ :> Reverse@list[[i]]
                          
]
                          
(* {{1,2},{4,3},{5,6},{8,7}} *)





                          share|improve this answer









                          $endgroup$
















                            4












                            4








                            4





                            $begingroup$

                            Another method is to use ReplacePart:



                            ReplacePart[{{1, 2}, {3, 4}, {5, 6}, {7, 8}},
                            
    i_ ? EvenQ :> Reverse@list[[i]]
                            
]
                            
(* {{1,2},{4,3},{5,6},{8,7}} *)





                            share|improve this answer









                            $endgroup$



                            Another method is to use ReplacePart:



                            ReplacePart[{{1, 2}, {3, 4}, {5, 6}, {7, 8}},
                            
    i_ ? EvenQ :> Reverse@list[[i]]
                            
]
                            
(* {{1,2},{4,3},{5,6},{8,7}} *)






                            share|improve this answer












                            share|improve this answer



                            share|improve this answer










                            answered 2 days ago









                            Jason B.Jason B.

                            48.9k389197




                            48.9k389197























                                3












                                $begingroup$

                                This will work



                                list = {{1, 2}, {3, 4}, {5, 6}, {7, 8}};
                                

                                
Table[If[EvenQ[i], Reverse[list[[i]]], list[[i]]], {i, 1, Length[list]}]
                                

                                
(*{{1, 2}, {4, 3}, {5, 6}, {8, 7}}*)
                                
```





                                share|improve this answer








                                New contributor




                                cphys is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                                Check out our Code of Conduct.






                                $endgroup$


















                                  3












                                  $begingroup$

                                  This will work



                                  list = {{1, 2}, {3, 4}, {5, 6}, {7, 8}};
                                  

                                  
Table[If[EvenQ[i], Reverse[list[[i]]], list[[i]]], {i, 1, Length[list]}]
                                  

                                  
(*{{1, 2}, {4, 3}, {5, 6}, {8, 7}}*)
                                  
```





                                  share|improve this answer








                                  New contributor




                                  cphys is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                                  Check out our Code of Conduct.






                                  $endgroup$
















                                    3












                                    3








                                    3





                                    $begingroup$

                                    This will work



                                    list = {{1, 2}, {3, 4}, {5, 6}, {7, 8}};
                                    

                                    
Table[If[EvenQ[i], Reverse[list[[i]]], list[[i]]], {i, 1, Length[list]}]
                                    

                                    
(*{{1, 2}, {4, 3}, {5, 6}, {8, 7}}*)
                                    
```





                                    share|improve this answer








                                    New contributor




                                    cphys is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                                    Check out our Code of Conduct.






                                    $endgroup$



                                    This will work



                                    list = {{1, 2}, {3, 4}, {5, 6}, {7, 8}};
                                    

                                    
Table[If[EvenQ[i], Reverse[list[[i]]], list[[i]]], {i, 1, Length[list]}]
                                    

                                    
(*{{1, 2}, {4, 3}, {5, 6}, {8, 7}}*)
                                    
```






                                    share|improve this answer








                                    New contributor




                                    cphys is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                                    Check out our Code of Conduct.









                                    share|improve this answer



                                    share|improve this answer






                                    New contributor




                                    cphys is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                                    Check out our Code of Conduct.









                                    answered 2 days ago









                                    cphyscphys

                                    815




                                    815




                                    New contributor




                                    cphys is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                                    Check out our Code of Conduct.





                                    New contributor





                                    cphys is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                                    Check out our Code of Conduct.






                                    cphys is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                                    Check out our Code of Conduct.






























                                        draft saved

                                        draft discarded




















































                                        Thanks for contributing an answer to Mathematica Stack Exchange!


                                        • Please be sure to answer the question. Provide details and share your research!

                                        But avoid …



                                        • Asking for help, clarification, or responding to other answers.

                                        • Making statements based on opinion; back them up with references or personal experience.


                                        Use MathJax to format equations. MathJax reference.


                                        To learn more, see our tips on writing great answers.




                                        draft saved


                                        draft discarded














                                        StackExchange.ready(
                                        function () {
                                        StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmathematica.stackexchange.com%2fquestions%2f194901%2fhow-to-reverse-every-other-sublist-of-a-list%23new-answer', 'question_page');
                                        }
                                        );

                                        Post as a guest















                                        Required, but never shown





















































                                        Required, but never shown














                                        Required, but never shown












                                        Required, but never shown







                                        Required, but never shown

































                                        Required, but never shown














                                        Required, but never shown












                                        Required, but never shown







                                        Required, but never shown







                                        -

                                        Popular posts from this blog

                                        GameSpot

                                        Image
                                        body.skin-minerva .mw-parser-output table.infobox caption{text-align:center} GameSpot 戰地風雲：惡名昭彰2在Gamespot的評論 网站类型 新聞 持有者 CBS 创始人 Pete Deemer Vince Broady Jon Epstein 网站 http://www.gamespot.com/ 注册 Optional (free and paid) 推出时间 1996年5月1日 [1] GameSpot （中国大陆：游戏基地），於1996年5月由Pete Deemer和Vince Broady創立，是一個提供新聞、評論、預告片、下載及其他的相關資訊的電子遊戲網站。GameSpot被一間後來被CNET收購的企業ZDNet所收購。根據Alexa，GameSpot.com是200個網路擁擠最嚴重的網站之一。 除了由GameSpot員工創作的內容，網站還允許用戶寫評論、網誌、之後在網路論壇分享。一些在CNET旗下的GameFAQs分享。 2004年， GameSpot被Spike TV的觀眾選上「電子遊戲賞節目」贏得「最傑出遊戲網站。 [2] 其他的遊戲網站還有IGN、1UP.com、GameSpy是它最大的競爭對手。2008年，根據Compete.com的統計，「gamespot.com」吸引了最少6000萬人的點擊率。 [3] GameSpot的主頁鏈結了到最近新聞、評論、預告、和一些有關遊戲機的入口：Wii、任天堂DS、電腦遊戲、Xbox 360、PSP、PlayStation 2、PlayStation 3。它還有一列「最受歡迎遊戲名單」，還有給用戶快速獲得遊戲資訊的搜尋器。GameSpot 還包括一些小範圍的遊戲機：任天堂64、GameCube、Game Boy Color、Game Boy Advance、Xbox、PlayStation、SEGA Saturn、Dreamcast、Neo Geo Pocket Color、N-Gage、手機遊戲。 目录 1 歷史 1.1 國際歷史 1.2 著名的員工 2 評論和分...
                                        Read more

                                        日野市

                                        Image
                                        body.skin-minerva .mw-parser-output table.infobox caption{text-align:center} 日野市 日野市 日文轉寫  • 日文 日野市  • 平假名 ひのし  • 罗马字 Hino-shi 金剛寺（關東三十六不動尊靈場第9號）的不動堂 市旗 徽章 日野市在東京都的位置 日野市 日野市在日本的位置 坐标： 35°40′17″N 139°23′42″E  /  35.6714°N 139.395°E  / 35.6714; 139.395 国家   日本 地方 關東地方 都道府縣 東京都 接鄰行政區 府中市、國立市、立川市、昭島市、八王子市、多摩市 政府  • 市長 大坪冬彦 面积  •  总计 27.53 平方公里（10.63 平方英里） 人口 （2014年12月1日）  • 總計 183,323  • 密度 6,660/平方公里（17,200/平方英里） 象徵  • 市樹 樫  • 市花 菊花  • 市鳥 普通翠鳥 时区 日本標準時間 （UTC+9） 地方公共團體編號 13212-8 邮政编码 〒 191-8686 市役所地址 日野市神明一丁目12番1號 電話號碼 +81-42-585-1111 法人編號 1000020132128 網站 http://www.city.hino.lg.jp/ 人口：日野市官方網頁 日野市 （日语： 日野市 ／ ひのし   Hino shi   * / ? ）為一位于東京都（不含島嶼部分）中央地帶的城市。從東京站乘坐中央線特別快速列車45分鐘即可到該市。面積27.53km²。1963年（昭和38年）11月3日，該市開始實行市制，為全日本第559個市。汽車企業日野自動車的總部位於這裡。 往東京都特別區部的通勤率為20.9%，往八王子市的通勤率為12.0%（平成22年國勢調査）。 目录 1 概要 ...
                                        Read more

                                        Tu-95轟炸機

                                        Image
                                        body.skin-minerva .mw-parser-output table.infobox caption{text-align:center} 图-95 Ту–95 北约代号：熊（Bear） 一架在安格斯空军基地的图-95MS 概觀 類型 战略轰炸机 代號 北约代号： Bear （熊） 乘員 7名 駕駛員2名，機尾炮手1名，其他人員4名 首飛 1952年11月12日 服役 1956年 設計 圖波列夫設計局 產量 超过500架 現況 现役 主要用戶   蘇聯   俄羅斯 衍生機型 图-114、图-119、Tu-95 技术数据 長度 49.50米（162呎5吋） 翼展 51.10米（167呎8吋） 高度 12.12米（39呎9吋） 翼面積 310平方米（3,330平方呎） 空重 90,000公斤（198,000磅） 最大起飛重量 188,000公斤（414,500磅） 發動機 4具库兹涅佐夫NK-12MV型渦輪螺旋槳發動機 功率 4×11,000千瓦（14,800馬力） 性能數據 最大速度 925公里／時（500節，575哩／時） 爬升率 10米／秒（2,000呎／分） 最大升限 12,000米（39,000呎） 最大航程 15,000公里（8,100海浬，9,400哩） 翼負荷 606公斤／平方米（124磅／平方呎） 推重比 235 W／公斤（0.143馬力／磅） 武器装备 機炮 1具/2具AM-23型23公釐雷達控制機砲（機尾） 飛彈 空对地导弹： Kh-20、Kh-22 Kh-26、Kh-55 炸彈 60枚FAB-250炸彈，或 30枚FAB-500炸彈。 其他 载弹量（包括飞弹）：最多15,000公斤（33,000磅） Tu-95熊式MR型機 图-95 （俄语： Ту–95 ， 英语： Tu-95 ），北約代號： Bear （ 熊 ），是蘇聯圖波列夫設計局所研製，是全世界唯一服役的大型四渦輪螺旋槳發動機...
                                        Read more