Basic Question about If and If else implementation

November 19, 2021

I am trying to understand why if else is needed for this kind of implementation

Here is my working code

#include <stdio.h>

int main()
{
    int Num, row, columns; //Let Num = User Input 
    
    printf("ﾟ｡✧ʕっ•ᴥ•ʔっ PLease Enter a Number from 4 and 50 ⊂ʕ •ᴥ•⊂ ʔﾟ｡✧: \n");
    scanf("%d", &Num);
    
    char arr[Num][Num];
    
    if (Num >= 4 && Num <= 50){
        for (int row=0; row<Num; row++)
        {
            for (int columns=0; columns<Num; columns++){
                if (row == 0 || columns == 0){
                    arr[row][columns]='*';
                }
                else if (row == Num-1 || columns == Num-1){
                    arr[row][columns]='*'; 
                }
                else
                    arr[row][columns] = ' ';
            }
        }
    }
    else 
        printf("Error! Please enter a valid number :)");

    for(int i = 0; i < Num; i++){
        for(int j = 0; j < Num; j++ ){
            printf("%c", arr[i][j]);
        }
        printf("\n");
    }
 
    return 0;
}

The output of the code is (depending on inputed Num)

4
****
*  *
*  *
****

My question is why does the code not work if I don’t use if else here

for (int columns=0; columns<Num; columns++){
                if (row == 0 || columns == 0){
                    arr[row][columns]='*';
                }
                if (row == Num-1 || columns ==Num-1){
                    arr[row][columns]='*'; 
                }
                else
                    arr[row][columns] = ' ';
            }

The output will be like this

4
   *
   *
   *
****

I know that if-else statements are executed if the first if is false and won’t execute if is true.

Thank you for any responses in advance.

>Solution :

If we reformat the original:

if (row == 0 || columns == 0){
    arr[row][columns]='*';
}
else if (row == Num-1 || columns ==Num-1){
    arr[row][columns]='*'; 
}
else
    arr[row][columns] = ' ';

Into

if (row == 0 || columns == 0){
    arr[row][columns]='*';
}
else{
    if (row == Num-1 || columns ==Num-1){
        arr[row][columns]='*'; 
    }
    else{
        arr[row][columns] = ' ';
    }
}

Then you can clearly see that the difference. Especially considering the stand-alone else part.

With this, if row == 0 || columns == 0 is false then nothing is executed. But without the else if the stand-alone else part will happen for row == 0 and column == 0, overwriting what you set earlier.