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Why the loop stops when i = -1?

for (i = len; i--;)

Full code is here http://rosettacode.org/wiki/Longest_increasing_subsequence#C .

Thx!!

halachkin
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  • What does the second part of a for loop do, and what does i-- mean? Answer those, and you've got your answer. – Scott Mermelstein Mar 26 '14 at 18:04
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    The loop stops at i==0, however i-- decrements the value once _after_ it has been compared and the loop has decided to exit, making i end up at the value -1. – Joachim Isaksson Mar 26 '14 at 18:04

6 Answers6

8

The loop stops when the value of i-- is 0. Since i-- returns the value before the decrement, when i-- is 0, i is -1.

clcto
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2

I don't see an answer worthy of being right, though Joachim Isaksson's comment is a concise explanation.

The loop stops when it evaluates a value of 0 or false.

The i-- is what's throwing you off. That's "post-decrement", i.e. the operator returns the current value of i (in this case, for the loop to evaluate), and immediately thereafter, decrements the current value. Pseudocode for postdecrement follows:

int retval = i;
i = i - 1;
return retval;

(Based on Eric Lippert's comments, technically, the compiler could do postdecrement as i = i - 1; return i + 1;. The point is that the value emitted by the operator is no longer the value of i.)

(as opposed to predecrement, which in pseudocode is simply):

i = i - 1;
return i;

So when the loop is evaluating i, it sees 0 so exits, but immediately after evaluating the operator has decremented i to -1.

So, to answer your question:

The loop doesn't stop when i = -1. It stops when i = 0, but then i is decremented to -1.

Scott Mermelstein
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  • I note that the pre-decrement *also* evaluates `i` and then after evaluation, decrements it. If you think that's false then ask yourself the question: *how could the new value be computed without first evaluating the old value*? The difference between the pre- and post- operators is not *when `i` is evaluated relative to the mutation*. Rather it is *what the value of the operator is*. – Eric Lippert Mar 26 '14 at 19:31
  • @EricLippert I think the issue you took was with my use of the word "evaluate", which was sloppy. The for loop evaluates the value returned by the operator. I made an edit and included pseudocode. Does that fit better? – Scott Mermelstein Mar 26 '14 at 19:40
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    My point was rather nit-picky, I admit. But I see a *lot* of misconceptions about this operator. I note that your pseudo-code might imply an ordering of side effects which would be warrented in C# or Java but not in C. C promises you that the value of the operator will be either the new or the old value, but does not promise you *when the mutation will happen*, just that it will happen sometime before the next sequence point. – Eric Lippert Mar 26 '14 at 20:13
  • A more detailed exegesis of the various situations you can get into can be found here: http://stackoverflow.com/questions/17926794/incrementing-pointers-exact-sequence/17935062#17935062 – Eric Lippert Mar 26 '14 at 20:14
  • Thanks, that was an interesting (and well written) read. I keep forgetting about how loose the rules are for C compilers. – Scott Mermelstein Mar 26 '14 at 20:41
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Your loop stops when the expression i-- becomes 0 because any non-zero value in C is treated as true and a zero as false.
At each iteration i is checked for true or false and then decremented .

haccks
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0
 for (i = len; i--;)

here i-- is the exit condition. when i-- is false which is 0 the loop will exit.
since i-- is post decreament i value is used for condition checking. so when

i=0;

loop exits after that i=i-1 is executed as part of post decrement so 

i=-1;

when loop exits since first i's value is used after that it is decremented

LearningC
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0

FOR loops are composed of 3 parts:

for ( [Initialization] ; [Exit Condition] ; [Cycle Update] )

[Initialization] is used to assign a value to the loop index before the first iteration

[Exit Condition] is a boolean expression evaluated before each iteration. If it is true another iteration is executed, otherwise the FOR loop ends

[Cycle Update] is the instruction used to update the loop index at each iteration

a classical FOR statement looks like this:

FOR(I = len ; I > END_VALUE ; I--)

What is probably missing in your code is the exit condition, so the cycle update part is interpreted as an exit condition and consequently treated as a boolean expression. When the integer becomes 0 its interpreted boolean value corresponds to FALSE and the loop is ended. Any non-zero value is instead interpreted as a TRUE boolean and allows the execution of another iteration.

Michele Borriello
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-1

When a boolean value is needed, 0 is same as false. Anything other than 0 is true. So, it must stop when i is 0, not -1. You are seeing the value of i to be -1 because of the post-decrement operator.

R Sahu
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