Strange behaviour in declaration inside a while condition C++

Question

I am implementing a python like split() function in C++ to train myself. I got the idea from this SO thread: Parse (split) a string in C++ using string delimiter (standard C++)

In this code:

while ((pos = s.find(delimiter)) != std::string::npos) {
    token = s.substr(0, pos);
    std::cout << token << std::endl;
    s.erase(0, pos + delimiter.length());
}

The value os pos is assigned inside the condition of the while loop.

I tried the same thing:

#include <iostream>
#include <string>
#include <algorithm>
#include <vector>

std::vector<std::string> split(std::string inp, std::string delimeter){
    std::vector<std::string> res;
    while (size_t pos = inp.find(delimeter) <= inp.length()){
        std::cout << inp << "   " << pos << std::endl ;
        if (inp.substr(0, delimeter.length()) == delimeter) {
            inp.erase(0, delimeter.length());
            continue;
        }
        res.push_back(inp.substr(0, pos));
        inp.erase(0, pos);
    }
    return res;
}

int main() {
    for (auto i : split(",,ab,c,,d", ",")){
        std::cout << i << " ";
    }
    std::cout << std::endl;
}

My output is:

,,ab,c,,d   1
,ab,c,,d   1
ab,c,,d   1
b,c,,d   1
,c,,d   1
c,,d   1
,,d   1
,d   1
a b c

My question is why poes it say that the position of , in string ,,ab,c,,d 1 is 1?

And why is the position in ab,c,,d is 1 as well?

I modified the code like this:

#include <iostream>
...
    size_t pos = 0;
    while (pos <= inp.length()){
        pos = inp.find(delimeter);
        ...
}

int main() {
    for (auto i : split(",,ab,c,,d", ",")){
        std::cout << i << " ";
    }
    std::cout << std::endl;
}

Where ... remains unchanged and now it works like a charm, the output is:

,,ab,c,,d   0
,ab,c,,d   0
ab,c,,d   2
,c,,d   0
c,,d   1
,,d   0
,d   0
d   18446744073709551615
ab c d

Just as I expected.

So my question is: Why cant i declare a variable inside a while condition? Isnt the condition is evaluated at all cycle (thus the declaration happen again?) Even at the very first cycle i get the result 1 which is wrong. Why is that?

Your statement about the first code sample is wrong. And that makes the question confusing! `pos` has been declared before the loop. — juanchopanza, Oct 13 '16 at 14:10
In the first code `pos` is not declared in the for loop. it is declared somewhere before it as it should be. — NathanOliver, Oct 13 '16 at 14:11
"Why cant i declare a variable inside a while condition?" you can. just remember that the scope of the variable is reset on each iteration of the loop — UKMonkey, Oct 13 '16 at 14:12
@juanchopanza Yes sorry you are right, it is declared somewhere else, but it is assigned inside the condition of the for loop. Thats what i wanted to say. — Gábor Erdős, Oct 13 '16 at 14:19

AnT · Accepted Answer · 2016-10-13T14:34:58.400

5

while (size_t pos = inp.find(delimeter) <= inp.length()){

is interpreted as

while (size_t pos = (inp.find(delimeter) <= inp.length())){

while you need a completely different grouping

while ((size_t pos = inp.find(delimeter)) <= inp.length()){

The latter is illegal in C++ though.

It is not possible to declare a variable in while condition and at the same time make it participate in a more complicated conditional expression (like a comparison with another value). When you declare a variable in C++ condition all you can have is its initial value converted to bool.

The modified code, where pos is declared before the loop, properly implements your intent.

edited Oct 13 '16 at 14:34

answered Oct 13 '16 at 14:12

AnT

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So i was checking a condition and got a boolen 1. That explains it, and its so clear.. I was only sitting here for like and hour to find it out.. Thanks a lot, i ll mark this as correct as soon as i can (10 minutes) – Gábor Erdős Oct 13 '16 at 14:15

Strange behaviour in declaration inside a while condition C++

1 Answers1