循环 C++ 中的条件语句

Question

for (i = 1; i <= a; i++){
    for (b = 1; b <= i; b++){
        cout << "X";
    }
    cout << endl;
}

if a=4, the result would be

X

XX

XXX

XXXX

while the output of this

for (i = 1; i <= a; i++){
    for (b = 1; b <= a - i; b++){
        cout << "A";
    }
    cout << endl;
}

will be

AAA

AA

A

i recognize the different in conditional statement but i can not explain it my self why so.

Answer 1

For your first example, the first for loop with run because 1 is less than a (4). Which then it will move on to the next for loop. So b = 1, and 1 equals to 1 which will run and print out X. Now that loop will end and the outer for loop will increment to 2, and repeat.

Answer 2

Take a look at your stop conditions for the inner loops:

b <= i;
b <= a - i;

Now, let's start with i being 1:

b <= 1;
b <= 3;

And, finish with i being 4:

b <= 4;
b <= 0;

Considering your b 's initializations:

b = 1;
b = 1;

Combine them all => your loop ranges for b s would be [1; 4] [1; 4] and [3; 1] [3; 1] .