对于C中的循环混淆

Question

This is a question from a practice test that I do not fully understand.

For the code fragment

int i = 0, j = 0, k = 0;
for (i=-1; i<=10; ++i){ 
    j = i; ++k; 
}

I am asked to find the values of the variables after executing the code.

The answers are:

 i = 11 j = 10 k = 12

I don't understand how, can someone please help?

Answer 1

Understanding the value of i after the loop is very simple, much simpler than the sorts of other answers here. The loop condition is i<=10 ... in order for the loop to terminate, that condition must be false . Clearly, the value of i that makes that false is 11.

The value of j at the end of the loop is the previous value of i , which is 10, and the value of k is the number of times the loop executed, which is 1 (for -1) + 1 (for 0) + 10 (for 1 thru 10) = 12.

Answer 2

i must be <= 10, so it is 11 to exit the loop and inside the last iteration of the loop, i = 10 = j. k is 1 after the first iteration, while i is -1. Running through the loop, you'll see:

k = 1, i = -1
k = 2, i = 0
k = 3, i = 1
k = 4, i = 2
k = 5, i = 3
k = 6, i = 4
k = 7, i = 5
k = 8, i = 6
k = 9, i = 7
k = 10, i = 8
k = 11, i = 9
k = 12, i = 10

Therefore k = 12

Answer 3

Here are the steps:

1. When the loop begins, all three variables are zero.
2. The loop initializer sets i to minus 1.
3. Loop test: i <= 10 is true, so loop is entered.
4. Inside the loop, j is set to i , so j is also minus 1.
5. k is incremented, so k becomes 1.
6. the iteration ends; i is incremented because of the ++i , so i becomes 0.
7. Loop test: since i is zero, i <= 10 is true, so the loop is entered again.

In this way, the loop continues, changing j , k , and i in that order. So when i becomes 10, j will be 9 and k 11. At that point:

1. The loop is entered for the last time.
2. j becomes 10 as well; k becomes 12
3. Then i gets incremented to 11. The loop condition i <= 10 is false, and the loop terminates.

So i is 11. j is 10, k is 12 when the loop terminates.

The key point is, after the first pass, every time the loop is entered, j is one less than i , and k is one greater than i . When the loop terminates, this is still the case.

Answer 4

for (i=-1; i<=10; ++i){ 
    j = i; ++k; 
}

Here is the loop :

i = i +1;     <-------+
   |                  |
check condition!------|--+
   |                  |  |
j = i;                |  |
   |                  |  | 
  k++;----------------+  |
   |                     |
   +<--------------------+
   |
other code

at last loop

  i = 10
   condition == true
   j = 10;
   k = 12;

Then

i= i+1 means i = 11 but the condition show false! loop end.

Answer 5

Take three variables separate. You can see the variable k would be incremented , the number of times the loop is executed. The no. of time sit would be executed from -1 to 10 it would have done 12 iterations

k = 1,  i = -1,  j=-1
k = 2,  i = 0,   j=0
k = 3,  i = 1,   j=1
k = 4,  i = 2,   j=2
k = 5,  i = 3,   j=3
k = 6,  i = 4,   j=4
k = 7,  i = 5,   j=5
k = 8,  i = 6,   j=6
k = 9,  i = 7,   j=7
k = 10, i = 8,   j=8
k = 11, i = 9,   j=9
k = 12, i = 10,  j=10

After This i has reached its limit, but it will first increment and then check, hence i=11, k=12 and j to a one less than the value of i ie j= 10