Find the largest rectangle of 1’s with swapping of columns allowed












0















I came across this problem and solution at: http://www.geeksforgeeks.org/find-the-largest-rectangle-of-1s-with-swapping-of-columns-allowed/



But I couldn't get my head around the solution provided. Can someone please explain as to how the solution works ?



I already tried tracing on paper and stepping into code, but couldn't understand:
1. What role the sorting plays.
2. How the final area is being calculated without swapping any column as the question demands !



Any help is highly appreciated !










share|improve this question




















  • 1





    The question is not related to programming - it's about an algorithm which apply regardless of whether you write a program to implement it or you do it by hand using a piece of paper. I'll suggest you try to ask it at a math site instead. If you have problems implementing the algorithm or understanding the suggested implementation then this is the site to ask.

    – 4386427
    Sep 10 '15 at 8:47
















0















I came across this problem and solution at: http://www.geeksforgeeks.org/find-the-largest-rectangle-of-1s-with-swapping-of-columns-allowed/



But I couldn't get my head around the solution provided. Can someone please explain as to how the solution works ?



I already tried tracing on paper and stepping into code, but couldn't understand:
1. What role the sorting plays.
2. How the final area is being calculated without swapping any column as the question demands !



Any help is highly appreciated !










share|improve this question




















  • 1





    The question is not related to programming - it's about an algorithm which apply regardless of whether you write a program to implement it or you do it by hand using a piece of paper. I'll suggest you try to ask it at a math site instead. If you have problems implementing the algorithm or understanding the suggested implementation then this is the site to ask.

    – 4386427
    Sep 10 '15 at 8:47














0












0








0


0






I came across this problem and solution at: http://www.geeksforgeeks.org/find-the-largest-rectangle-of-1s-with-swapping-of-columns-allowed/



But I couldn't get my head around the solution provided. Can someone please explain as to how the solution works ?



I already tried tracing on paper and stepping into code, but couldn't understand:
1. What role the sorting plays.
2. How the final area is being calculated without swapping any column as the question demands !



Any help is highly appreciated !










share|improve this question
















I came across this problem and solution at: http://www.geeksforgeeks.org/find-the-largest-rectangle-of-1s-with-swapping-of-columns-allowed/



But I couldn't get my head around the solution provided. Can someone please explain as to how the solution works ?



I already tried tracing on paper and stepping into code, but couldn't understand:
1. What role the sorting plays.
2. How the final area is being calculated without swapping any column as the question demands !



Any help is highly appreciated !







algorithm matrix






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited Sep 10 '15 at 9:15







vinit

















asked Sep 10 '15 at 7:56









vinitvinit

132110




132110








  • 1





    The question is not related to programming - it's about an algorithm which apply regardless of whether you write a program to implement it or you do it by hand using a piece of paper. I'll suggest you try to ask it at a math site instead. If you have problems implementing the algorithm or understanding the suggested implementation then this is the site to ask.

    – 4386427
    Sep 10 '15 at 8:47














  • 1





    The question is not related to programming - it's about an algorithm which apply regardless of whether you write a program to implement it or you do it by hand using a piece of paper. I'll suggest you try to ask it at a math site instead. If you have problems implementing the algorithm or understanding the suggested implementation then this is the site to ask.

    – 4386427
    Sep 10 '15 at 8:47








1




1





The question is not related to programming - it's about an algorithm which apply regardless of whether you write a program to implement it or you do it by hand using a piece of paper. I'll suggest you try to ask it at a math site instead. If you have problems implementing the algorithm or understanding the suggested implementation then this is the site to ask.

– 4386427
Sep 10 '15 at 8:47





The question is not related to programming - it's about an algorithm which apply regardless of whether you write a program to implement it or you do it by hand using a piece of paper. I'll suggest you try to ask it at a math site instead. If you have problems implementing the algorithm or understanding the suggested implementation then this is the site to ask.

– 4386427
Sep 10 '15 at 8:47












2 Answers
2






active

oldest

votes


















3
















  1. What role the sorting plays.

  2. How the final area is being calculated without swapping any column as the question demands !




The sorting IS the swapping of columns. If we look at the 3rd row under step 2:



3 3 1 0 0


The sorted row corresponds to swapping the columns so that the column with the highest possible rectangle is placed first, after that comes the column that allows the second highest rectangle and so on. So, in the example there are 2 columns that can form a rectangle of height 3. That makes an area of 3*2=6. If we try to make the rectangle wider the height drops to 1, because there are no columns left that allow a higher rectangle on the 3rd row.



Edit: Avoiding unnecessary sorting



Using a standard O(n*log(n)) sorting algorithm gives us good performance.



It is possible to reduce the sorting needed by avoiding unnecessary sorting. The suggestions that follow won't reduce the O rating, but it will reduce the number of swaps substantially.



To reduce the number of swaps we need to abort the sorting of a row as soon as possible. To be able to do that I recommend using quicksort and always sort the left partition (higher numbers) first. Whenever the pivot times the partition size is smaller than the largest rectangle we have found so far, we know that the right partition (lower numbers) cannot hold the largest rectangle, so we can skip sorting the right partition.



Example:



Assume that the largest rectangle found so far has size 6 and the next row to sort looks like this:



1 3 0 3 0


We take the first element, 1 as pivot. The pivot times the partition size is 1 * 5 = 5, which is less than or equal to the largest size found. This means that we can skip the right partition, since it cannot yield a rectangle larger than 5.



3 3 (keep sorting this partition) - 1 0 0 (skip this partition)



Mergesort only allows us to skip parts of the final merge, so that's why I'd go with quicksort.






share|improve this answer


























  • Thanks, that was helpful !

    – vinit
    Sep 10 '15 at 14:21











  • how to restrict swaps to minimum. @Klas Lindbäck

    – Xax
    Jul 26 '16 at 7:40






  • 1





    @Xax Se question edit.

    – Klas Lindbäck
    Jul 26 '16 at 8:01













  • if you can pls elaborate a bit more @KlasLindbäck thanks

    – Xax
    Jul 26 '16 at 8:44






  • 1





    @Xax I've added an example.

    – Klas Lindbäck
    Jul 26 '16 at 9:31



















0














def rectangle(matrix):
R = len(matrix)
C = len(matrix[0])
mtrx =
for i in range(R):
row =
for j in range(C):
#creating a matrix of 0
row.append(0)
mtrx.append(row)


for i in range(C):
#copy first row matrix
mtrx[0][i] = matrix[0][i]
#counting the consecutive ones
for j in range(R):
if matrix[j][i] == 0:
mtrx[j][i] = 0
else:
mtrx[j][i] = mtrx[j-1][i]+1
#sort the rows
for i in range(R):
mtrx[i] = sorted(mtrx[i],reverse=True)
#Traverse the sorted matrix to find maximum area
max_area = 0
for i in range(R):
for j in range(C):
current = (j+1) * mtrx[i][j]
if current > max_area:
max_area = current
return max_area

matrix = [[0, 1, 0, 1, 0],
[0, 1, 0, 1, 1],
[1, 1, 0, 1, 0]]





share|improve this answer
























  • While this code may answer the question, it is better to explain how to solve the problem and provide the code as an example or reference. Code-only answers can be confusing and lack context.

    – Dima Kozhevin
    Jan 2 at 7:45











  • This is for people who understand how to read codes. I have left comment, and I implemented the algorithm from the link given in Python

    – Eye Sun
    Jan 3 at 0:04











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2 Answers
2






active

oldest

votes








2 Answers
2






active

oldest

votes









active

oldest

votes






active

oldest

votes









3
















  1. What role the sorting plays.

  2. How the final area is being calculated without swapping any column as the question demands !




The sorting IS the swapping of columns. If we look at the 3rd row under step 2:



3 3 1 0 0


The sorted row corresponds to swapping the columns so that the column with the highest possible rectangle is placed first, after that comes the column that allows the second highest rectangle and so on. So, in the example there are 2 columns that can form a rectangle of height 3. That makes an area of 3*2=6. If we try to make the rectangle wider the height drops to 1, because there are no columns left that allow a higher rectangle on the 3rd row.



Edit: Avoiding unnecessary sorting



Using a standard O(n*log(n)) sorting algorithm gives us good performance.



It is possible to reduce the sorting needed by avoiding unnecessary sorting. The suggestions that follow won't reduce the O rating, but it will reduce the number of swaps substantially.



To reduce the number of swaps we need to abort the sorting of a row as soon as possible. To be able to do that I recommend using quicksort and always sort the left partition (higher numbers) first. Whenever the pivot times the partition size is smaller than the largest rectangle we have found so far, we know that the right partition (lower numbers) cannot hold the largest rectangle, so we can skip sorting the right partition.



Example:



Assume that the largest rectangle found so far has size 6 and the next row to sort looks like this:



1 3 0 3 0


We take the first element, 1 as pivot. The pivot times the partition size is 1 * 5 = 5, which is less than or equal to the largest size found. This means that we can skip the right partition, since it cannot yield a rectangle larger than 5.



3 3 (keep sorting this partition) - 1 0 0 (skip this partition)



Mergesort only allows us to skip parts of the final merge, so that's why I'd go with quicksort.






share|improve this answer


























  • Thanks, that was helpful !

    – vinit
    Sep 10 '15 at 14:21











  • how to restrict swaps to minimum. @Klas Lindbäck

    – Xax
    Jul 26 '16 at 7:40






  • 1





    @Xax Se question edit.

    – Klas Lindbäck
    Jul 26 '16 at 8:01













  • if you can pls elaborate a bit more @KlasLindbäck thanks

    – Xax
    Jul 26 '16 at 8:44






  • 1





    @Xax I've added an example.

    – Klas Lindbäck
    Jul 26 '16 at 9:31
















3
















  1. What role the sorting plays.

  2. How the final area is being calculated without swapping any column as the question demands !




The sorting IS the swapping of columns. If we look at the 3rd row under step 2:



3 3 1 0 0


The sorted row corresponds to swapping the columns so that the column with the highest possible rectangle is placed first, after that comes the column that allows the second highest rectangle and so on. So, in the example there are 2 columns that can form a rectangle of height 3. That makes an area of 3*2=6. If we try to make the rectangle wider the height drops to 1, because there are no columns left that allow a higher rectangle on the 3rd row.



Edit: Avoiding unnecessary sorting



Using a standard O(n*log(n)) sorting algorithm gives us good performance.



It is possible to reduce the sorting needed by avoiding unnecessary sorting. The suggestions that follow won't reduce the O rating, but it will reduce the number of swaps substantially.



To reduce the number of swaps we need to abort the sorting of a row as soon as possible. To be able to do that I recommend using quicksort and always sort the left partition (higher numbers) first. Whenever the pivot times the partition size is smaller than the largest rectangle we have found so far, we know that the right partition (lower numbers) cannot hold the largest rectangle, so we can skip sorting the right partition.



Example:



Assume that the largest rectangle found so far has size 6 and the next row to sort looks like this:



1 3 0 3 0


We take the first element, 1 as pivot. The pivot times the partition size is 1 * 5 = 5, which is less than or equal to the largest size found. This means that we can skip the right partition, since it cannot yield a rectangle larger than 5.



3 3 (keep sorting this partition) - 1 0 0 (skip this partition)



Mergesort only allows us to skip parts of the final merge, so that's why I'd go with quicksort.






share|improve this answer


























  • Thanks, that was helpful !

    – vinit
    Sep 10 '15 at 14:21











  • how to restrict swaps to minimum. @Klas Lindbäck

    – Xax
    Jul 26 '16 at 7:40






  • 1





    @Xax Se question edit.

    – Klas Lindbäck
    Jul 26 '16 at 8:01













  • if you can pls elaborate a bit more @KlasLindbäck thanks

    – Xax
    Jul 26 '16 at 8:44






  • 1





    @Xax I've added an example.

    – Klas Lindbäck
    Jul 26 '16 at 9:31














3












3








3









  1. What role the sorting plays.

  2. How the final area is being calculated without swapping any column as the question demands !




The sorting IS the swapping of columns. If we look at the 3rd row under step 2:



3 3 1 0 0


The sorted row corresponds to swapping the columns so that the column with the highest possible rectangle is placed first, after that comes the column that allows the second highest rectangle and so on. So, in the example there are 2 columns that can form a rectangle of height 3. That makes an area of 3*2=6. If we try to make the rectangle wider the height drops to 1, because there are no columns left that allow a higher rectangle on the 3rd row.



Edit: Avoiding unnecessary sorting



Using a standard O(n*log(n)) sorting algorithm gives us good performance.



It is possible to reduce the sorting needed by avoiding unnecessary sorting. The suggestions that follow won't reduce the O rating, but it will reduce the number of swaps substantially.



To reduce the number of swaps we need to abort the sorting of a row as soon as possible. To be able to do that I recommend using quicksort and always sort the left partition (higher numbers) first. Whenever the pivot times the partition size is smaller than the largest rectangle we have found so far, we know that the right partition (lower numbers) cannot hold the largest rectangle, so we can skip sorting the right partition.



Example:



Assume that the largest rectangle found so far has size 6 and the next row to sort looks like this:



1 3 0 3 0


We take the first element, 1 as pivot. The pivot times the partition size is 1 * 5 = 5, which is less than or equal to the largest size found. This means that we can skip the right partition, since it cannot yield a rectangle larger than 5.



3 3 (keep sorting this partition) - 1 0 0 (skip this partition)



Mergesort only allows us to skip parts of the final merge, so that's why I'd go with quicksort.






share|improve this answer

















  1. What role the sorting plays.

  2. How the final area is being calculated without swapping any column as the question demands !




The sorting IS the swapping of columns. If we look at the 3rd row under step 2:



3 3 1 0 0


The sorted row corresponds to swapping the columns so that the column with the highest possible rectangle is placed first, after that comes the column that allows the second highest rectangle and so on. So, in the example there are 2 columns that can form a rectangle of height 3. That makes an area of 3*2=6. If we try to make the rectangle wider the height drops to 1, because there are no columns left that allow a higher rectangle on the 3rd row.



Edit: Avoiding unnecessary sorting



Using a standard O(n*log(n)) sorting algorithm gives us good performance.



It is possible to reduce the sorting needed by avoiding unnecessary sorting. The suggestions that follow won't reduce the O rating, but it will reduce the number of swaps substantially.



To reduce the number of swaps we need to abort the sorting of a row as soon as possible. To be able to do that I recommend using quicksort and always sort the left partition (higher numbers) first. Whenever the pivot times the partition size is smaller than the largest rectangle we have found so far, we know that the right partition (lower numbers) cannot hold the largest rectangle, so we can skip sorting the right partition.



Example:



Assume that the largest rectangle found so far has size 6 and the next row to sort looks like this:



1 3 0 3 0


We take the first element, 1 as pivot. The pivot times the partition size is 1 * 5 = 5, which is less than or equal to the largest size found. This means that we can skip the right partition, since it cannot yield a rectangle larger than 5.



3 3 (keep sorting this partition) - 1 0 0 (skip this partition)



Mergesort only allows us to skip parts of the final merge, so that's why I'd go with quicksort.







share|improve this answer














share|improve this answer



share|improve this answer








edited Jul 26 '16 at 9:30

























answered Sep 10 '15 at 10:54









Klas LindbäckKlas Lindbäck

30.5k44371




30.5k44371













  • Thanks, that was helpful !

    – vinit
    Sep 10 '15 at 14:21











  • how to restrict swaps to minimum. @Klas Lindbäck

    – Xax
    Jul 26 '16 at 7:40






  • 1





    @Xax Se question edit.

    – Klas Lindbäck
    Jul 26 '16 at 8:01













  • if you can pls elaborate a bit more @KlasLindbäck thanks

    – Xax
    Jul 26 '16 at 8:44






  • 1





    @Xax I've added an example.

    – Klas Lindbäck
    Jul 26 '16 at 9:31



















  • Thanks, that was helpful !

    – vinit
    Sep 10 '15 at 14:21











  • how to restrict swaps to minimum. @Klas Lindbäck

    – Xax
    Jul 26 '16 at 7:40






  • 1





    @Xax Se question edit.

    – Klas Lindbäck
    Jul 26 '16 at 8:01













  • if you can pls elaborate a bit more @KlasLindbäck thanks

    – Xax
    Jul 26 '16 at 8:44






  • 1





    @Xax I've added an example.

    – Klas Lindbäck
    Jul 26 '16 at 9:31

















Thanks, that was helpful !

– vinit
Sep 10 '15 at 14:21





Thanks, that was helpful !

– vinit
Sep 10 '15 at 14:21













how to restrict swaps to minimum. @Klas Lindbäck

– Xax
Jul 26 '16 at 7:40





how to restrict swaps to minimum. @Klas Lindbäck

– Xax
Jul 26 '16 at 7:40




1




1





@Xax Se question edit.

– Klas Lindbäck
Jul 26 '16 at 8:01







@Xax Se question edit.

– Klas Lindbäck
Jul 26 '16 at 8:01















if you can pls elaborate a bit more @KlasLindbäck thanks

– Xax
Jul 26 '16 at 8:44





if you can pls elaborate a bit more @KlasLindbäck thanks

– Xax
Jul 26 '16 at 8:44




1




1





@Xax I've added an example.

– Klas Lindbäck
Jul 26 '16 at 9:31





@Xax I've added an example.

– Klas Lindbäck
Jul 26 '16 at 9:31













0














def rectangle(matrix):
R = len(matrix)
C = len(matrix[0])
mtrx =
for i in range(R):
row =
for j in range(C):
#creating a matrix of 0
row.append(0)
mtrx.append(row)


for i in range(C):
#copy first row matrix
mtrx[0][i] = matrix[0][i]
#counting the consecutive ones
for j in range(R):
if matrix[j][i] == 0:
mtrx[j][i] = 0
else:
mtrx[j][i] = mtrx[j-1][i]+1
#sort the rows
for i in range(R):
mtrx[i] = sorted(mtrx[i],reverse=True)
#Traverse the sorted matrix to find maximum area
max_area = 0
for i in range(R):
for j in range(C):
current = (j+1) * mtrx[i][j]
if current > max_area:
max_area = current
return max_area

matrix = [[0, 1, 0, 1, 0],
[0, 1, 0, 1, 1],
[1, 1, 0, 1, 0]]





share|improve this answer
























  • While this code may answer the question, it is better to explain how to solve the problem and provide the code as an example or reference. Code-only answers can be confusing and lack context.

    – Dima Kozhevin
    Jan 2 at 7:45











  • This is for people who understand how to read codes. I have left comment, and I implemented the algorithm from the link given in Python

    – Eye Sun
    Jan 3 at 0:04
















0














def rectangle(matrix):
R = len(matrix)
C = len(matrix[0])
mtrx =
for i in range(R):
row =
for j in range(C):
#creating a matrix of 0
row.append(0)
mtrx.append(row)


for i in range(C):
#copy first row matrix
mtrx[0][i] = matrix[0][i]
#counting the consecutive ones
for j in range(R):
if matrix[j][i] == 0:
mtrx[j][i] = 0
else:
mtrx[j][i] = mtrx[j-1][i]+1
#sort the rows
for i in range(R):
mtrx[i] = sorted(mtrx[i],reverse=True)
#Traverse the sorted matrix to find maximum area
max_area = 0
for i in range(R):
for j in range(C):
current = (j+1) * mtrx[i][j]
if current > max_area:
max_area = current
return max_area

matrix = [[0, 1, 0, 1, 0],
[0, 1, 0, 1, 1],
[1, 1, 0, 1, 0]]





share|improve this answer
























  • While this code may answer the question, it is better to explain how to solve the problem and provide the code as an example or reference. Code-only answers can be confusing and lack context.

    – Dima Kozhevin
    Jan 2 at 7:45











  • This is for people who understand how to read codes. I have left comment, and I implemented the algorithm from the link given in Python

    – Eye Sun
    Jan 3 at 0:04














0












0








0







def rectangle(matrix):
R = len(matrix)
C = len(matrix[0])
mtrx =
for i in range(R):
row =
for j in range(C):
#creating a matrix of 0
row.append(0)
mtrx.append(row)


for i in range(C):
#copy first row matrix
mtrx[0][i] = matrix[0][i]
#counting the consecutive ones
for j in range(R):
if matrix[j][i] == 0:
mtrx[j][i] = 0
else:
mtrx[j][i] = mtrx[j-1][i]+1
#sort the rows
for i in range(R):
mtrx[i] = sorted(mtrx[i],reverse=True)
#Traverse the sorted matrix to find maximum area
max_area = 0
for i in range(R):
for j in range(C):
current = (j+1) * mtrx[i][j]
if current > max_area:
max_area = current
return max_area

matrix = [[0, 1, 0, 1, 0],
[0, 1, 0, 1, 1],
[1, 1, 0, 1, 0]]





share|improve this answer













def rectangle(matrix):
R = len(matrix)
C = len(matrix[0])
mtrx =
for i in range(R):
row =
for j in range(C):
#creating a matrix of 0
row.append(0)
mtrx.append(row)


for i in range(C):
#copy first row matrix
mtrx[0][i] = matrix[0][i]
#counting the consecutive ones
for j in range(R):
if matrix[j][i] == 0:
mtrx[j][i] = 0
else:
mtrx[j][i] = mtrx[j-1][i]+1
#sort the rows
for i in range(R):
mtrx[i] = sorted(mtrx[i],reverse=True)
#Traverse the sorted matrix to find maximum area
max_area = 0
for i in range(R):
for j in range(C):
current = (j+1) * mtrx[i][j]
if current > max_area:
max_area = current
return max_area

matrix = [[0, 1, 0, 1, 0],
[0, 1, 0, 1, 1],
[1, 1, 0, 1, 0]]






share|improve this answer












share|improve this answer



share|improve this answer










answered Jan 2 at 6:51









Eye SunEye Sun

12




12













  • While this code may answer the question, it is better to explain how to solve the problem and provide the code as an example or reference. Code-only answers can be confusing and lack context.

    – Dima Kozhevin
    Jan 2 at 7:45











  • This is for people who understand how to read codes. I have left comment, and I implemented the algorithm from the link given in Python

    – Eye Sun
    Jan 3 at 0:04



















  • While this code may answer the question, it is better to explain how to solve the problem and provide the code as an example or reference. Code-only answers can be confusing and lack context.

    – Dima Kozhevin
    Jan 2 at 7:45











  • This is for people who understand how to read codes. I have left comment, and I implemented the algorithm from the link given in Python

    – Eye Sun
    Jan 3 at 0:04

















While this code may answer the question, it is better to explain how to solve the problem and provide the code as an example or reference. Code-only answers can be confusing and lack context.

– Dima Kozhevin
Jan 2 at 7:45





While this code may answer the question, it is better to explain how to solve the problem and provide the code as an example or reference. Code-only answers can be confusing and lack context.

– Dima Kozhevin
Jan 2 at 7:45













This is for people who understand how to read codes. I have left comment, and I implemented the algorithm from the link given in Python

– Eye Sun
Jan 3 at 0:04





This is for people who understand how to read codes. I have left comment, and I implemented the algorithm from the link given in Python

– Eye Sun
Jan 3 at 0:04


















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