问题
I am learning programming and I have started from C language. I was reading Let us C book. And I was going through this program in that book.
main( )
{
int a[5] = { 5, 1, 15, 20, 25 } ;
int i, j, k = 1, m ;
i = ++a[1] ;
j = a[1]++ ;
m = a[i++] ;
printf ( "\n%d %d %d", i, j, m ) ;
}
My understanding was, it will print i as 2, j as 1 and m as 15
But somehow it is printing as i as 3, j as 2 and m as 15? Why is it so?
Below is my understanding-
b = x++;
In this example suppose the value of variable ‘x’ is 5 then value of variable ‘b’ will be 5 because old value of ‘x’ is used.
b = ++y;
In this example suppose the value of variable ‘y’ is 5 then value of variable ‘b’ will be 6 because the value of ‘y’ gets modified before using it in a expression.
Is there anything wrong in my understanding?
回答1:
You hit the nail on the head. Your understanding is correct. The difference between pre and post increment expressions is just like it sounds. Pre-incrementation means the variable is incremented before the expression is set or evaluated. Post-incrementation means the expression is set or evaluated, and then the variable is altered. It's easy to think of it as a two step process.
b = x++;
is really:
b = x;
x++;
and
b = ++x;
is really:
x++;
b = x;
EDIT: The tricky part of the examples you provided (which probably threw you off) is that there's a huge difference between an array index, and its value.
i = ++a[1];
That means increment the value stored at a[1], and then set it to the variable i.
m = a[i++];
This one means set m to the value of a[i], then increment i. The difference between the two is a pretty big distinction and can get confusing at first.
Second EDIT: breakdown of the code
{
int a[5] = { 5, 1, 15, 20, 25 } ;
int i, j, k = 1, m ;
i = ++a[1] ;
j = a[1]++ ;
m = a[i++] ;
printf ( "\n%d %d %d", i, j, m ) ;
}
First:
i = ++a[1];
At this point we know a[1] = 1 (remember arrays are zero indexed). But we increment it first. Therefore i = 2.
j = a[1]++;
Remember we incremented a[1] before, so it is currently 2. We set j = 2, and THEN incremented it to 3. So j = 2 and now a[1] = 3.
m = a[i++];
We know i = 2. So we need to set m = a[2], and then increment i. At the end of this expression, m = 15, and i = 3.
In summary,
i = 3, j = 2, m = 15.
回答2:
Your understanding is not exactly correct. Pre-increment and post-increment operators are unary operators.
So, initially if b = 5, then ++b or b++ increments the value of b to 6. However, the difference between pre and post comes when you are using an assignment operator "=".
So,
if b=5
a=b++ // after this statement a=5 and b=6 as it is post increment
c=++b // after this statement c=7 and b=7
For clear understanding, you can divide the above statements as:
a=b;
b=b+1; //post increment
b=b+1; //pre increment
c=b;`
So, the example you gave:
main( )
{
int a[5] = { 5, 1, 15, 20, 25 } ;
int i, j, k = 1, m ;
i = ++a[1] ; // a[1] = 2 and i = 2
j = a[1]++ ; // j = 2 and a[1] = 3
m = a[i++] ; // m = a[2++] = 15, i now becomes 3
printf ( "\n%d %d %d", i, j, m ) ; // so i =3, j= 2 and m =15
}
For clarity, I am splitting the above code into multiple statements:
main( )
{
int a[5] = { 5, 1, 15, 20, 25 } ;
int i, j, k = 1, m ;
a[1] = a[1] + 1;
i = a[1];
j = a[1];
a[1] = a[1] + 1;
m = a[i]; // m = a[2] = 15
i = i + 1;
printf ( "\n%d %d %d", i, j, m ) ; // so i =3, j= 2 and m =15
}
I hope the above explanation clears your doubt and the output of the program you are running.
回答3:
Explanation:
Step 1: int a[5] = {5, 1, 15, 20, 25}; The variable arr is declared as an integer array with a size of 5 and it is initialized to a[0] = 5, a[1] = 1, a[2] = 15, a[3] = 20, a[4] = 25 .
Step 2: int i, j, m; The variable i,j,m are declared as an integer type.
Step 3: i = ++a[1]; becomes i = ++1; Hence i = 2 and a[1] = 2
Step 4: j = a[1]++; becomes j = 2++; Hence j = 2 and a[1] = 3.
Step 5: m = a[i++]; becomes m = a[2]; Hence m = 15 and i is incremented by 1(i++ means 2++ so i=3)
Step 6: printf("%d, %d, %d", i, j, m); It prints the value of the variables i, j, m
Hence the output of the program is 3, 2, 15
来源：https://stackoverflow.com/questions/16869020/pre-increment-vs-post-increment-in-array