CODEWITHSUNDEEP
cloopsfor-loop
nos
nos

Reputation: 20862

c/c++ for loop condition statement

folks,

I am adding a logic expression in the for loop and it is not behaving as I expected. Could you shine some light on me? Thanks very much.

The following code works fine

for (i=0;i<N;++i)
    if (a[i] == 1){
        ....
    }

and I tried to write it this way, it seems the for loop is fully skipped.

for (i=0;i<N && a[i]==1;++i){
    ....
}

What is wrong with the 2nd way?

Upvotes: 0

Views: 168

Answers (3)

theNotSoPro
theNotSoPro

Reputation: 318

You never enter the loop if a[0] != 1. That's a condition for your for loop to be executed just like how i < N is too.

Upvotes: 0

Mike
Mike

Reputation: 49363

This code:

for (i=0;i<N;++i)
    if (a[i] == 1){

Means, increment i, starting from 0, keep going until it's greater than or equal to N, for each element in a[] check to see if it's equal to 1

This code however:

while(i < N && a[i] == 1)

Says loop as long as i is less than N and a[i] is equal to 1. So as soon as either of those condtions is false it will break from the loop.

Upvotes: 0

Mats Petersson
Mats Petersson

Reputation: 129314

The loop continues while the condition is true. Remember that a for-loop for(A; B; C) can be replaced with [conceptually]:

A;
while(B)
{
   ...
   C;
}

So, you have:

i = 0;
while(i < N && a[i] == 1)
{
   ...
   i++;
}

So, if at the first instance a[i] is not 1, then you never enter the loop, and just go to whatever comes after. It's probably not what you wanted to do, which is why it's not doing what you wanted... ;)

Upvotes: 4

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