Optimizing/Reducing For-loops

Question

I am learning C and I want to have an idea on how can I reduce the for-loops, to make it more presentable.

#include<stdio.h>

int main() {

char input[81];
int i;
char input2[9][9];
int col,row;
printf("Enter sudoku board:");
scanf("%s",&input);


for (i=0;i<9;i++) {
    for (col=0;col<9;col++) 
        input2[0][col]=input[i];
    for (col=0;col<9;col++) 
        input2[1][col]=input[i];
    for (col=0;col<9;col++) 
        input2[2][col]=input[i];
    for (col=0;col<9;col++) 
        input2[3][col]=input[i];
    for (col=0;col<9;col++) 
        input2[4][col]=input[i];
    for (col=0;col<9;col++) 
        input2[5][col]=input[i];
    for (col=0;col<9;col++) 
        input2[6][col]=input[i];
    for (col=0;col<9;col++) 
        input2[7][col]=input[i];
    for (col=0;col<9;col++) 
        input2[8][col]=input[i];
}

this shows that the user would enter 81 numbers in an array then the numbers entered will be transferred to a 9x9 array. thanks in advance!:)

...your loops appear to set all cells of input2 to input[8]. — Chowlett
– Chowlett, Commented Aug 18, 2011 at 9:58

Sander De Dycker · Accepted Answer · 2011-08-18 09:59:18Z

2

Your code is not doing what you expect it to do.

It sounds like you wanted something like :

for (row = 0; row < 9; ++row) {
    for (col = 0; col < 9; ++col) {
        input2[row][col] = input[(row * 9) + col];
    }
}

EDIT : Btw, when letting the user enter a string, make sure that there is enough room in the buffer to hold the entire string, plus the terminating '\0' character ! Your input buffer needs to have room for at least 82 char's.

answered Aug 18, 2011 at 9:59

Sander De Dycker

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4 Comments

Trixie Cruz Over a year ago

Why did you use ++row and ++col instead of using row++ and col++? thanks

Sander De Dycker Over a year ago

@Trixie : force of habit from using C++. Prefix increment can be more efficient because it avoids a copy operation. For intrinsic types like int, it doesn't really matter either way, but it's a good idea to use prefix increment whenever possible nevertheless (just to get used to it for those cases where it does matter). Some more details can be found in C++ FAQ Lite 13.15

Sander De Dycker Over a year ago

@Trixie : because there are 9 items in each row, so to skip one row, you need to skip 9 items (two-dimensional arrays are stored in row major order).

Sander De Dycker Over a year ago

The wiki article I mentioned in the previous comment explains that quite clearly. Have a read through it.

codaddict · Accepted Answer · 2011-08-18 10:16:38Z

1

Since you want to allow the user to enter 81 digits ( as characters), the size of the char array to hold them must be 82 or more.

char input[82];

Also your scanf should not have the &:

scanf("%s",input);

Since you want to transform the 1D array entered by user to a 2D array you can do:

for (row=0;row<9;row++) {
   for (col=0;col<9;col++) 
      input2[row][col] = input[row*9+col];

edited Aug 18, 2011 at 10:16

answered Aug 18, 2011 at 9:59

codaddict

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Comments

goldenmean · Accepted Answer · 2011-08-18 10:00:01Z

0

As a first step below code should work -

int i,j;
for(i=0;i<9;i++)
{
    for(j=0;j<9;j++)
    {
        input2[i][j]=input[i];

    }
}

EDIT: I have given solution for making the for loops 'more readable', to do whatever functionality OP mentioned. Have not suggested any thing other than that like whether his original code is correct or incorrect?

answered Aug 18, 2011 at 10:00

goldenmean

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Comments

Eric Z · Accepted Answer · 2011-08-18 10:08:54Z

What you want is to fill in a 9 * 9 matrix based on 81 number of inputs. As is mentioned, you need allocate one more character for the terminating '\0'. That's what C-style array is famous for.

   int i = 0, row = 0, col = 0;
   // set matrix row by row
   for (row = 0; row < 9; row++)
   {
      // for each row, set from left to right
      for (col = 0; col < 9; col++) 
      {
        input2[row][col] = input[i++];   
      }
   }

Improvement: you don't have to pre-allocate a string to hold that 81 characters. You can choose to directly set to the resulting array you want.

       int i = 0, row = 0, col = 0;
       // set matrix row by row
       for (row = 0; row < 9; row++)
       {
          // for each row, set from left to right
          for (col = 0; col < 9; col++) 
          {
            scanf("%c", &input2[row][col]);
          }
       }

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