for loop: make increment and accessing in one loop - c

Question

I do not know why this is giving me garbage:
#include <stdio.h>
int main(){
int a[10];
for(
int i =0;
i++<10;
a[i-1]=i
)
{
printf("%i:%i\n", i-1, a[i-1]);
}
}
Which gives me:
0:1676584240
1:32609
2:0
3:0
4:-1577938528
5:21992
6:-1577938864
7:21992
8:2114427248
9:32766
The indices looks correct, and even the assignment inside the loop, is correct (e.g. printf("%i\n",a[0]) gives 1 which is correct, after loop). But inside the body of for loop, the printf, despite having correct indices, gives wrong values (some garbage). Why is that?
EDIT, after some answers with ... && (a[i]=i), I have tried to do that with other statements:
for(
int i = 0;
i<10 && (a[i]=i);
(i++) && printf("%i\n", a[i])
);
But that does not print anything, just gives warning:
warning: value computed is not used [-Wunused-value]
i++ && printf("%i\n",a[i])
Why? when I can make "true" statement of (a[i]=i), why cannot I make "true" statment of (i++)?

Answer 1

doing
for(
int i =0;
i++<10;
a[i-1]=i
)
{
printf("%i:%i\n", i-1, a[i-1]);
}
you set the entries after you print them because a[i-1]=i is executed after the body of the for, so you print non initialized array entries
your code is also 'complicated' because you increase i too earlier in the test part of for, a 'standard' way to do what you (probably) want is :
for(int i = 0; i < sizeof(a)/sizeof(*a); ++i) {
a[i]=i+1;
printf("%i:%i\n", i, a[i]);
}
if you really want to not have a[i]=i+1; in the body you can do that :
for(int i = 0; (i < sizeof(a)/sizeof(*a)) && (a[i]=i+1); ++i) {
printf("%i:%i\n", i, a[i]);
}
to avoid a warning when compiling do ... && ((a[i]=i+1) != 0);
note (a[i]=i+1) is not false because values at least 1, in case you wanted to do a[i]=i the test to use can be (i < sizeof(a)/sizeof(*a)) && (a[i]=i, 1) to not be false when i is 0
but that does not help to read the code ^^

Answer 2

A for loop consists of 4 parts:
Initialisation
Condition
Body code
Action
Example:
for (initialisation; condition; action)
{
Body code
}
The execution order is as I enumerated it above.
In your example,
for (int i = 0; i++ < 10; a[i-1])
{
printf("%i:%i\n");
}
Initialisation - i = 1
Condition - i++ < 10; --> 0 < 10 = true, i = 1
Body code - printf while i=1, a[0] is still uninitialised!
Action - a[i-1] = i, a[0] = 1 (only now a[0] gets initialised!

Answer 3

In your code you first execute the code in the brackets, then you assign the array. It has to be done the right opposite way.
#include <stdio.h>
int main()
{
int a[10];
for(int i =0; i++, i <= 10 && (a[i-1] = i);)
{
printf("%i:%i\n", i-1, a[i-1]);
}
}
https://godbolt.org/z/r-DvJi
or better
#include <stdio.h>
int main()
{
int a[10];
for(int i = 0; i < 10 && (a[i] = i + 1); i++)
{
printf("%i:%i\n", i, a[i]);
}
}