Else statement inside for loop won't work

Question

EDIT: Problem is solved thank you all. I know the problem is not very well explained, sorry for that.

#include <stdio.h>


int main()

{
    int line=0;
    int longest=0;
    int v1[3][3]={{1,0,0},{1,1,1},{7,8,9}};
    for(int i=0 ; i<3;i++){
        for(int j=0; j<2 ;j++ ){
            if(v1[i][j]==0){
                
                int temp=1;
                for(int k=j+1; k< 3; k++){
                    printf("%d\n",k);
                    if( v1[i][k]==0 ){
                        temp++;
                    }
                    if( v1[i][k]=!0 ){
                        if( temp>longest ){
                            longest=temp;
                            line=i+1;
                        }
                    }
                }
            }
        }
    }
    printf("line:%d\nlenght:%d",line,longest);
}

I have a code like this. When I try to use else statement with if statement, else statement won't work when the requirements inside if statement are not met.

                    if( v1[i][k]==0 ){
                        temp++;
                    }
                    else{
                        if( temp>longest ){
                            longest=temp;
                            line=i+1;
                        }

But when I use another if statement it would run when v1[i][k]!=0 condition is met.

                    if( v1[i][k]==0 ){
                        temp++;
                    }
                    if( v1[i][k]!=0 ){
                        if( temp>longest ){
                            longest=temp;
                            line=i+1;
                        }

Replace `=!` => `!=`. I don't know how you claim it is working. — Eugene Sh., Feb 26 '21 at 18:59
@EugeneSh. `v1[i][k] = !0` is a valid statement, that's why it compiles, that's also why the code behaves differently when replacing with else. — Michaël Roy, Feb 26 '21 at 19:07
@EugeneSh. Reopen the question because it has nothing common with the typo. — Vlad from Moscow, Feb 26 '21 at 19:09
@VladfromMoscow You are probably right, as the question is about the other code — Eugene Sh., Feb 26 '21 at 19:10

Vlad from Moscow · Accepted Answer · 2021-02-26T19:43:19.477

In this statement

if( v1[i][k]=!0 ){

there is a typo. There should be

if( v1[i][k] != 0 ){

But in any case the first and the second programs have a logical error. This if statement

if( temp>longest )

must be placed outside the inner for loop.

for(int k=j+1; k< 3; k++){

And moreover the condition in the this loop

for(int j=0; j<2 ;j++ ){

shall be j < 3 instead of j < 2. Otherwise the situation when a row contains only the last element equal to zero will be ignored.

Here is a demonstrative program that shows how the for loops can be written.

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

int main(void) 
{
    enum { N = 10 };
    int a[N][N];

    srand( ( unsigned int )time( NULL ) );
    
    for ( int i = 0; i < N; i++ )
    {
        for ( int j = 0; j < N; j++ )
        {
            a[i][j] = rand() % 2;
        }
    }
    
    for ( int i = 0; i < N; i++ )
    {
        for ( int j = 0; j < N; j++ )
        {
            printf( "%d ", a[i][j] );
        }
        putchar( '\n' );
    }
    
    putchar( '\n' );
    
    int line = 0;
    int longest = 0;

    for ( int i = 0; i < N; i++ )
    {
        for ( int j = 0, k = 1; j < N - longest; j += k, k = 1 )
        {
            if ( a[i][j] == 0 )
            {
                while ( j + k < N && a[i][j + k] == 0 ) ++k;
                
                if ( longest < k )
                {
                    longest = k;
                    line = i + 1;
                }
            }
        }           
    }
    
    if ( line != 0 )
    {
        printf( "line: %d, length: %d\n", line, longest );
    }
    else
    {
        puts( "There is no row with elements equal to 0" );
    }
    
    return 0;
}

The program output might look like

0 0 0 1 1 0 0 1 0 0 
1 1 0 1 0 1 1 0 1 1 
1 1 0 1 1 0 0 0 1 1 
1 1 0 1 0 1 0 0 0 0 
1 1 0 1 0 0 0 0 1 1 
1 0 0 1 1 1 0 1 1 1 
1 0 0 1 0 0 0 0 0 0 
0 1 1 0 0 0 1 1 0 0 
0 1 0 1 1 0 0 1 1 0 
0 0 0 0 1 0 0 1 0 0 

line: 7, length: 6

tstanisl · Answer 2 · 2021-02-26T19:05:13.150

3

Replace =! with != An occasional error difficult to spot. In C '!=' means "not equal". While '=!' is a combination of assignment to logical negation. 'x=!0' is the same as 'x=1'

edited Feb 26 '21 at 19:05

answered Feb 26 '21 at 19:01

tstanisl

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4

Is it common? Never encountered. Also some compilers might warn when seeing assignment instead of comparison. And I think the question is in fact about the second snippet. – Eugene Sh. Feb 26 '21 at 19:03
@EugeneSh. Ok maybe not common but definitely not uncommon. It's high voted answer for query about difficult to spot errors. https://stackoverflow.com/a/14269532/4989451 – tstanisl Feb 26 '21 at 19:12
Compilers can definitely help finding it: https://wandbox.org/permlink/HXgseBzH5SBrOGh7 Anyway, as I said, the question is not about this code, but about the other one that does not have this typo – Eugene Sh. Feb 26 '21 at 19:18
the question was about second snipnet but when I did what Vlad from Moscow said else statements started working. Probably I was getting an logic error. – Erinç Utku Öztürk Feb 26 '21 at 19:27

Else statement inside for loop won't work

2 Answers2