C语言if与if-else实现疑问：打印矩形边框时为何必须使用if-else结构

Let's break down exactly what's happening here, because the difference between your original if-else if-else chain and the modified two separate if statements is all about condition exclusivity and value overwriting.

Original Code Logic (Working) #

Your original inner loop uses a chained if-else if-else structure, which means each condition is checked in order, and only one block of code runs per iteration:

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

• For any cell that's on the top edge (row=0) or left edge (columns=0), it gets set to * and the rest of the conditions are skipped entirely.
• If that's not true, it checks if the cell is on the bottom edge (row=Num-1) or right edge (columns=Num-1)—if yes, set to *.
• If neither condition is met, it sets the cell to a space.

No overwriting happens here—each cell only gets assigned once per loop iteration.

Modified Code Logic (Broken) #

In your modified code, the second if is independent, and its else only applies to that single if statement:

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

Let's walk through what happens to the first row (row=0) when Num=4:

1. For columns 0,1,2:
   • First if triggers (since row==0 is true), so the cell is set to *.
   • Second if checks: row==3 (false) and columns==3 (false for columns 0-2). So the else runs, overwriting the * with a space.
2. Only for column 3:
   • First if triggers (row==0 is true), setting the cell to *.
   • Second if also triggers (columns==3 is true), re-setting the cell to * (no harm here).

That's why your first row ends up as * * instead of ****—the middle cells get overwritten to spaces right after being set to *!

Fixing the Issue #

You have two simple, clean options to fix this:

1. Stick with the original chained if-else if-else structure: It’s straightforward and guarantees no accidental overwriting.
2. Combine all border conditions into a single if: This is even more concise and eliminates the problem entirely:

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

This way, any cell on any of the four borders gets set to *, and all inner cells get spaces—no conflicting assignments or overwriting to worry about.

内容的提问来源于stack exchange，提问作者Ethan Jelo Martinez