CODEWITHSUNDEEP
cfor-loopsyntax
ettoralk
ettoralk

Reputation: 13

C for loop, supposed to be able to omit initialization inside loop itself, doesn't work as intended

so I'm slowly trying to learn C from scratch. I'm at a point in the book I'm using where an exercise is proposed: use nested loops to find Pythagorean triplets. Now I'll show the code.

#include <stdio.h>

int main(void){
int lato1=1;
int lato2=1;
int ipotenusa=1;
  for(;lato1 <= 500; lato1++){
    for(;lato2 <= 500; lato2++){
      for(;ipotenusa <= 500; ipotenusa++){

   
        if (((lato1 * lato1)+(lato2 * lato2))==(ipotenusa*ipotenusa)){
          printf("Tripletta %10d %10d %10d\n",lato1,lato2,ipotenusa);
        }
      }
    }
  }


  return 0;
}

now, apart from terrible formatting and style, apart from the shi*t optimization, the code as shown, doesnt work. It only execute the inner most loop, and then the program ends.

If, however, I initialize each variable/counter inside each loop then it works. Why? I read that for loop initialization is valid even whitout no arguments (;;) but in this case I just wanted to initialize those variables before the loop, let's say because I wanted to access those value after the loop is done, it just doesn't seem to work like it's supposed to.

English is not my primary language so apologies in advance for any mystake.

Can somebody explain what is the problem?

Edit 1: So, I don't know if it was my bad English or something else. I said, if I declare and initialize the variables before the loop, like in the code I've shown you, it only goes through the inner most loop (ipotenusa) and it does so with the following output values: 1 1 1 then 1 1 2 then 1 1 3 and so on, where the only increasing number is the last one (ipotenusa); AFTER we reach 1 1 500 the programs abruptily ends. I then said that if I initialize the variables as normal, meaning inside the for loop instruction, then it works as intended.

Even if declared earlier there is NO reason it should't work. The variable's value it's supposed to increase. Up until now the only useful answer was to initialize a variable outside the loop but assign a value to it in the loop statement, but this is not at the end the answer I need, because I should be able to skip the initialization inside the loop statement altogether.

EDIT 2: I was wrong and You guys were right, language barrier (most likely foolishness, rather) was certainly the cause of the misunderstanding lol. Sorry and Thanks for the help!

Upvotes: 1

Views: 674

Answers (4)

Ankit Mishra
Ankit Mishra

Reputation: 590

You are using variables that are initialized outside of the for loops. Usually what happens is when we use inner loops(nested loops) we intended to initialize it with base value everytime but in your case it is never going to do that. Since you have initialized variable out side of the for loops so it will persist its value through out its life time of course which is main function in this case.

Upvotes: 0

John C
John C

Reputation: 1981

You accidentally answered your own question:

...in this case I just wanted to initialize those variables before the loop, let's say because I wanted to access those value after the loop is done...

Think about the value of ipotenusa...or have your program print out the value of each variable at the start of each loop for debugging purposes. It'll look something like the following log.

lato1: 1
lato2: 1
ipotenusa: 1
ipotenusa: 2
ipotenusa: 3
ipotenusa: 4
ipotenusa: 5
lato2: 2
lato2: 3
lato2: 4
lato2: 5
lato1: 2
lato1: 3
lato1: 4
lato1: 5

The inner loops never "reset," because you're saving the value. You're not wrong on either count, but you've set up two requirements that conflict with each other, and the C runtime can only give you one.

Basically, lato2 and ipotenusa need to be initialized (but not necessarily declared) in their for statements, so that they're set up to run again.

int lato1=1;
int lato2;
int ipotenusa;
for(;lato1 <= 5; lato1++){
  for(lato2 = 1;lato2 <= 5; lato2++){
    for(ipotenusa = 1;ipotenusa <= 5; ipotenusa++){
      if (((lato1 * lato1)+(lato2 * lato2))==(ipotenusa*ipotenusa)){
        printf("Tripletta %10d %10d %10d\n",lato1,lato2,ipotenusa);
      }
    }
  }
}

It's worth pointing out that, even though the specification might (or might not; I don't remember and am too fuzzy to look it up, right now) say that the loop variables only exist during the loop, I've never seen an implementation actually do that, so you'll see a lot of code out there that defines the variable inside of for and uses it after. It's probably not great and a static checker like lint might complain, but it's common enough that it'll pass most code reviewers, to give you a sense of whether it can be done.

Upvotes: 1

gnasher729
gnasher729

Reputation: 52538

Your code does exactly what you told it to do.

If you initialise lato2 with a value of 1, what makes you think that the initialisation will be repeated at the start of the inner for loop? Of course it isn't. You told the compiler that you didn't want it to be initialised again. After that loop executed the first time, lato2 has a value of 501, and it will keep that value forever.

Even if it hadn't produced a bug, putting the initialisation and the for loop so far apart is very, very bad style (wouldn't pass a code review in a professional setting and therefore would have to be changed).

for (int lato2 = 1; lato2 <= 500; ++lato2) is clear, obvious, and works.

Upvotes: 1

ZBay
ZBay

Reputation: 332

One approach would be - declaring variables outside of the loop and initialize them inside. This way, you can access the variables after the loops.

#include <stdio.h>

int main(void){
int lato1, lato2, ipotenusa;
for(lato1=1; lato1 <= 500; lato1++){
  for(lato2=1; lato2 <= 500; lato2++){
    for(ipotenusa=1; ipotenusa <= 500; ipotenusa++){

   
      if (((lato1 * lato1)+(lato2 * lato2))==(ipotenusa*ipotenusa)){         
        printf("Tripletta %10d %10d %10d\n",lato1,lato2,ipotenusa);
      }
    }
  }
}


  return 0;
}

Upvotes: 0

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