Add-ins:
Excel
PowerPoint

Find the greatest product of five consecutive digits in the 1000-digit number.

73167176531330624919225119674426574742355349194934

96983520312774506326239578318016984801869478851843

85861560789112949495459501737958331952853208805511

12540698747158523863050715693290963295227443043557

66896648950445244523161731856403098711121722383113

62229893423380308135336276614282806444486645238749

30358907296290491560440772390713810515859307960866

70172427121883998797908792274921901699720888093776

65727333001053367881220235421809751254540594752243

52584907711670556013604839586446706324415722155397

53697817977846174064955149290862569321978468622482

83972241375657056057490261407972968652414535100474

82166370484403199890008895243450658541227588666881

16427171479924442928230863465674813919123162824586

17866458359124566529476545682848912883142607690042

24219022671055626321111109370544217506941658960408

07198403850962455444362981230987879927244284909188

84580156166097919133875499200524063689912560717606

05886116467109405077541002256983155200055935729725

71636269561882670428252483600823257530420752963450

There are some optimizations one can consider but I would look at them only if the performance of the "full enumeration" code is not acceptable. The two changes I can think of are

(1) if we detect a zero in our set of 5 digits, we can skip ahead to a sequence starting with the first digit after the zero.

(2) once we have a product for the first 5 numbers, to calculate the product for the next 5 digits, we should divide the last result by the number we just dropped from consideration and multiply by the new number. This way we use 1 division and 1 multiplication rather than 4 multiplications. I don't know if this would improve performance or not since we are trading one division for 3 multiplications.

It may be possible to find a solution "by inspection" but I don't know if this is a generalizable solution. In this specific scenario, since we can compare our answer with the correct solution available from the Project Euler website it might be possible to location a pattern of 2 or more 9's with some 8s and 7s thrown in. But, if we did not have a way to compare our guesses with the known maximum, I am not sure if this approach would be reliable.

Below is the VBA code to step through all possible sequences. I use the Debug.Assert to verify that I did not lose some digits in copying the large number from the webpage into the VBE. I also generalize the code somewhat by putting the length of the sequence of interest in a constant. Since the performance was very quick, I skipped the improvements mentioned above. Finally, I used Debug.Print rather than MsgBox so that I could copy the result and paste it into the Project Euler webpage for verification.

Sub Euler8() Dim Nbr As String Nbr = "7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843" _ & "858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113" _ & "622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776" _ & "657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482" _ & "839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586" _ & "178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188" _ & "845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450" Debug.Assert Len(Nbr) = 1000 Dim I As Integer, Rslt As Long Const NbrChars As Integer = 5 For I = 1 To Len(Nbr) - NbrChars + 1 Step 1 Dim J As Integer, TempRslt As Long TempRslt = 1 For J = 1 To NbrChars TempRslt = TempRslt * CInt(Mid(Nbr, I + J - 1, 1)) Next J If TempRslt > Rslt Then Rslt = TempRslt Next I Debug.Print Rslt End Sub