Nested for loop only runs inner loop (c++)

Question

I am quite new to c++, and I believe the answer to my problem is very, very simple.

I've been using the Eclipse IDE, but have recently changed to a simple text editor and using the command line for compiling. (As i currently don't have my own computer, and I am not allowed to install anything on the one I am using).

However, while writing a program, I noticed that whenever I had nested loops, it would only run the inner loop.

I've tried compiling my code using different online compilers, which results in the same problem.

Because of this, I believe that the problem is related to something simple, that Eclipse was taking care of automatically.

#include <iostream>

int main() {
  for (int i; i<3; i++) {
    for (int j; j<3; j++) {
      std::cout << j << std::endl;
    }
  }
  return 0;
}

Above is the simplest example, I could think of, that produces the problem. The expected output is 0, 1, 2, 0, 1, 2, 0, 1, 2, however it only outputs 0, 1, 2 when I compile and run it.

First of all, use for(int i = 0;.. , you are not guaranteed to have zero value there in general. Same with j. — MasterAler
– MasterAler, Commented Apr 8, 2019 at 23:51

lost_in_the_source · Accepted Answer · 2019-04-08 22:47:50Z

5

You're not initializing the i and j variables to 0, so the variables start off by having undefined values. Fix to:

for (int i = 0; i < 3; i++) {
    for (int j = 0; j < 3; j++) {
        std::cout << j << std::endl;
    }
}

answered Apr 8, 2019 at 22:47

lost_in_the_source

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2 Comments

dandan Over a year ago

This is so obvious, I can't believe I missed it. Guess I was tunnel-visioning on the IDE-change too much :) Thanks!

Jarod42 Over a year ago

Worst than undefined values, it is UB to read uninitialized variables.

Jack · Accepted Answer · 2019-04-08 22:48:37Z

0

The problem is that you are using uninitialized variables, which leave them with undefined values

for (int i; i < 3; i++) {
         ^

Try with

for (int i = 0; i < 3; i++) {

answered Apr 8, 2019 at 22:48

Jack

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3 Comments

dandan Over a year ago

This is so obvious, I can't believe I missed it. Guess I was tunnel-visioning on the IDE-change too much :) Thanks!

drescherjm Over a year ago

@dandan I am surprised you didn't get a warning from the compilers you tested.

Jarod42 Over a year ago

Worst than undefined values, it is UB to read uninitialized variables.

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