CODEWITHSUNDEEP
c++while-loop
Gábor Erdős
Gábor Erdős

Reputation: 3689

Strange behaviour in declaration inside a while condition C++

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?

Upvotes: 0

Views: 94

Answers (1)

AnT stands with Russia
AnT stands with Russia

Reputation: 320491

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.

Upvotes: 5

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