CODEWITHSUNDEEP
cfor-loop
Ionut Hulub
Ionut Hulub

Reputation: 6326

C program interesting behaviour

I have stumbled upon a piece of code that generates some interesting results while debugging someone else's program.

I have created a small program to illustrate this behavior:

#include <stdio.h>

int main()
{
        char* word = "foobar"; int i, iterator = 0;
        for (i = 0; i < 6; i++ && iterator++)
          printf("%c", word[iterator]);
        return 0;
}

I know that this is not the right way to print a string. This is for demonstration purpose only.

Here I expected the output to be "foobar", obviously, but instead it is "ffooba". Basically it reads the first character twice, as if the first time iterator++ is executed nothing happens.

Can anyone explain why this happens?

Upvotes: 6

Views: 255

Answers (4)

jiych.guru
jiych.guru

Reputation: 664

I am reading K&R about logic operators,let me quote the origin words of book,which can explain your question. "Expressions connected by && or || are evaluated left to right, and evaluation stops as soon as the truth or falsehood of the result is known." Have a good understanding of these,the outputs wont puzzle.

Upvotes: 2

Basile Starynkevitch
Basile Starynkevitch

Reputation: 1

You really should learn to use a debugger like e.g. gdb and to compile with warnings and debugging info like gcc -Wall -g (assuming a Linux system). A recent gcc with -Wall gives you a warning about value computed is not used before the && operation.

The increment part of your for loop is strange. It is i++ && iterator++ (but it should be i++, iterator++ instead).

When i is 0 (on the first iteration), i++ gives 0 as result, so it is false, so the iterator++ is not executed.

Upvotes: 3

hmjd
hmjd

Reputation: 121961

The result of i++ is the current value of i which is zero on first iteration. This means iterator++ is not executed on first iteration due to short circuting (the right-hand side of && is only executed if the left-hand side is "true").

To fix you could use the comma operator (as already suggested or) use ++i which will return the value of i after the incremement (though comma operator is more obvious that both must always be evaluated).

Upvotes: 5

icktoofay
icktoofay

Reputation: 129001

The thing is iterator++ actually isn't executed the first time. The ++ operator returns the current value of a variable and then increments it, so the first time through, i++ will be equal to 0. && short-circuits, so iterator++ is not executed the first time.

To fix this, you could use the comma operator which unconditionally evaluates both, rather than the short-circuiting &&.

Upvotes: 11

Related Questions