Convert simple loop into do-while

Question

How would I convert the following while loop into a proper do-while without double counting the first time?

void ctype()
{
    char c;
    while ((c = getchar()) != '.')
        printf("'%c' is %s a letter.\n", c, isalpha(c)? "indeed" : "not");
    printf("'%c' is %s a letter.\n", c, isalpha(c)? "indeed" : "not");
}

What I have thus far is:

void ctype()
// Print the letters up through the period, but quit on the period
{
    char c = getchar();
    do {
        printf("'%c' is %s a letter.\n", c, isalpha(c)? "indeed" : "not");
    } while ((c = getchar()) != '.') 
}

But this double-getchar's on the first item. What would be the proper way to do this? It's almost like I want the equivalent of a post-increment on the getchar() in the while loop.

Sample input/ouput of the while loop, which is currently correct:

$ run
.
'.' is not a letter.
$ run
Hello.
'H' is indeed a letter.
'e' is indeed a letter.
'l' is indeed a letter.
'l' is indeed a letter.
'o' is indeed a letter.
'.' is not a letter.

What makes `do-while` more "proper" than `while`? You don't use `do-while` if you may need to stop before the first iteration. — Barmar, Jan 25 '21 at 05:19
@Barmar It's not, this is an academic exercise to see how to do the conversion. — David542, Jan 25 '21 at 05:19
Also note that your code will fail badly if the user presses the end-of-file keyboard sequence. — Some programmer dude, Jan 25 '21 at 05:20
@Barmar when would it stop before the first iteration? Ignoring ctrl-d/crl-c/no-input, the min case would be a user entering in `.` in which case it would do one loop. — David542, Jan 25 '21 at 05:23
I misunderstood, and thought you didn't want to include the `.` if that's the first thing they type. — Barmar, Jan 25 '21 at 05:24
@Someprogrammerdude I mean if a user entered in `.` the first getchar would eat the `.` and then the `while` loop would not exit but require an additional input from the user. — David542, Jan 25 '21 at 05:24
What do you mean, "double-getchar's on the first item"? Could you give an example or two of what you want the code to do and what your code does? — Beta, Jan 25 '21 at 05:27

klutt · Accepted Answer · 2021-01-25T07:50:52.407

8

It can be done like this:

char c;
do {
    c = getchar();
    printf("'%c' is %s a letter.\n", c, isalpha(c)? "indeed" : "not");
} while(c != '.');

In the general case, you can always change

while(<expr>) {
    // Body
}

to

do {
    if(<expr>) break;

    // Body
} while(1);

Note that this is just for a plain conversion to a do-while. There are other flaws in the code. To correct these at the same time:

int c;
do {
    c = getchar();
    if(c == EOF) break;
    printf("'%c' is %s a letter.\n", c, isalpha(c)? "indeed" : "not");
} while(c != '.');

edited Jan 25 '21 at 07:50

answered Jan 25 '21 at 05:28

klutt

30,332
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I see, thank you. And what way would you suggest doing it? – David542 Jan 25 '21 at 05:28
I see, and one last question. Why are you doing `int c` instead of `char c`, or is that a trivial distinction? Does that allow doing multi-byte chars, or something else? Finally, just change the `if` statements to occur at the end (I want to print the trailing period). – David542 Jan 25 '21 at 05:33
2

@David542 Otherwise you cannot check for EOF – klutt Jan 25 '21 at 05:34
@dxiv You're right. Corrected it now. Read too fast. – klutt Jan 25 '21 at 05:39
1

`c` needs to be an `int` first and foremost because that's the return type of `getchar()`. Secondarily because between `1` and `1 << (CHAR_BIT - 1)` of its return values are outside the range of `char`. – John Bollinger Jan 25 '21 at 05:46
@David542 https://stackoverflow.com/a/35356684/918959 – Antti Haapala -- Слава Україні Jan 25 '21 at 07:42
You should *break* before `printf` on EOF – Antti Haapala -- Слава Україні Jan 25 '21 at 07:44

dxiv · Answer 2 · 2021-01-25T06:03:08.283

5

Another way to check for both '.' and EOF with no duplicate calls to either getchar or printf.

    int c;
    while ((c = getchar()) != EOF)
    {
        printf("'%c' is %s a letter.\n", c, isalpha(c)? "indeed" : "not");
        if(c == '.') break;
    }

edited Jan 25 '21 at 06:03

answered Jan 25 '21 at 05:30

dxiv

16,984
2
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1

When `char` is _unsigned_, `while ((c = getchar()) != EOF)` can lead to an infinite loop unless `char c;` --> `int c;` also changed. – chux - Reinstate Monica Jan 25 '21 at 05:52
@chux-ReinstateMonica Right, thanks. Edited to add an explicit `int c;`. – dxiv Jan 25 '21 at 06:05
1

Also when `char` is signed and negative, `isalpha` will have undefined behaviour... – Antti Haapala -- Слава Україні Jan 25 '21 at 07:43

score 2 · Answer 3 · edited Jan 25 '21 at 05:40

2

Following code is equivalent to code written using while loop:

void ctype()
{ 
    char c;
    do {
       c = getchar();
       printf("'%c' is %s a letter.\n", c, isalpha(c)? "indeed" : "not");
    } while (ch != '.');
}

edited Jan 25 '21 at 05:40

klutt

30,332
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answered Jan 25 '21 at 05:21

g-217

2,069
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No, it's not equivalent. The OPs "while" loop does *NOT* printf() if c =='.'; your loop does. – paulsm4 Jan 25 '21 at 05:24
3

Check the first code snippet using while loop, it will print '.' after loop ends. – g-217 Jan 25 '21 at 05:26
https://stackoverflow.com/a/35356684/918959 - also the argument to `isalpha` must be unsigned char (or int, hence `int c`) – Antti Haapala -- Слава Україні Jan 25 '21 at 07:42

score 1 · Answer 4 · answered Jan 25 '21 at 09:50

1

There's nothing wrong with do - while but generally while is more readable, so stick with the latter if you have the option. Also there's generally nothing wrong with an initial call to the function outside the loop, if that makes the loop code clearer.

There are more concerning problems with this code - you don't check for EOF, you don't discard line feed and you have assignment inside a control or loop condition.

The most readable form is perhaps this:

int c = getchar();

while (c!='.' && c!=EOF)
{
  /* handle the case where c == '\n' */

  /* other stuff */

  c = getchar(); 
}

answered Jan 25 '21 at 09:50

Lundin

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thanks for the feedback. What's wrong with doing `you have assignment inside a control or loop condition` ? I thought that was a common practice in "c code" ? – David542 Jan 25 '21 at 19:08
@David542 There's many problems with it. The code may turn harder to read, you might get accidental mix up between `=` and `==` and you may end up with multiple side effects in the same expression which in turn leads to brittle code. Consider for example `if( (c=getchar()) == c ) // typed same character twice?` which might look like sensible code at a glance, but is actually an undefined behavior bug. – Lundin Jan 26 '21 at 08:46

Convert simple loop into do-while

4 Answers4