In a class in a console application I wrote a method that reads many integer inputs and sums them up. The inputs calls for another method written in another class (Input.ReadIntegerConsole()) that's supposed to read the input, and then make the operations of the first class continue if the input is an integer, or otherwise to write "invalid choice", this way:
private void ReadInputAndSumNumbers()
{
Console.Write("\nNumber you choose? ");
int.TryParse(Console.ReadLine(), out numberChosen);
numberChosen = Input.ReadIntegerConsole();
sum += numberChosen;
etc. The problem is that this way, when I run the application, the console asks: Number you choose? then whatever I input, the program does nothing. At this point I will put another input and only now the ReadIntegerConsole method starts: if it is an integer the operations continue, otherwise the program displays "invalid choice". How to do it so the program does everything in one step instead of two steps, so it reads my input and immediately verifies if it is an integer or not and decides to continue or to display "invalid choice"? How to do it without having to write two inputs? I know for sure that the Input.ReadIntegerConsole code is written correctly and that's why I didn't display it here. I tried writing in many different parts of the class that numberChosen = Input.ReadIntegerConsole() but it has never worked. I tried also writing numberChosen=int.Parse(ConsoleReadLine())=Input.ReadIntegerConsole() but of course it doesn't work.
Console.Write("\nNumber you choose? ");
int.TryParse(Console.ReadLine(), out numberChosen);
numberChosen = Input.ReadIntegerConsole();
sum += numberChosen;
You didn't provide the implementation of Input.ReadIntegerConsole but its a fair guess that it also calls Console.ReadLine and parses it into an int (same as the line above it). So basically, your code is doing:
Print a message
Wait for user input, parse into an int and store in numberChosen
Call Input.ReadIntegerConsole which likely waits for user input, parses it into an int and returns it, and store it in numberChosen
Add the number to sum
So when you input the first number, it does exactly what you think, but Input.ReadIntegerConsole asks for another input and overwrites the original one (hence if you enter junk, it says "invalid value"). This second read from the console explains why the program appears to do nothing, since its waiting for more input.
Removing the manual Console.ReadLine gets rid of the redundant read/parse, which is why it fixed your code. Its not that it works without Console.ReadLine, its that the method you are using hides a call to it.
If you're asking what I think you're asking, I think you want this:
private void ReadInputAndSumNumbers()
{
Console.Write("\nNumber you choose? ");
int numberChosen;
if(int.TryParse(Console.ReadLine(), out numberChosen)) {
sum += numberChosen;
}
else {
Console.WriteLine("Invalid choice.");
}
}
int.TryParse is a method with an out parameter, which means the result of parsing will be assigned to numberChosen. It returns a bool, however: true if successful (string passed is parseable into an int), or false if unsuccessful. So what it's saying is:
If int.TryParse can create an int from the input string, the variable numberChosen has the result stored when the method finishes; add numberChosen to sum.
else, it was unsuccessful, and write "Invalid choice".
Out parameters can get a little confusing, but they're powerful. But if I did a poor job explaining, here are some links: Int32.TryParse, out (C# reference)
I'm looking at some C# source code trying to trace a bug and relying on my basic understanding of programming and msdn to decipher, as I have no experience developing with it. At a key point that the buggy behavior must pass through, I found the following:
public static bool isObjectSpecialCheck(object someObject)
{
string someParam = getParam(someObject);
if (String.IsNullOrEmpty(someParam))
{
someParam = getParamSomewhereElse(someObject);
}
if (!(string.IsNullOrEmpty(someParam)))
{
try
{
if (!string.IsNullOrEmpty(paramIsSpecial(someParam)))
return (true);
else
return (false);
}
catch (System.Exception ex)
{
GlobalConstants.Log(log, "Error", "isObjectSpecialCheck", ex);
return (false);
}
}
return (false);
}
I have swapped out the original variables with dummies to try and keep the question abstract. What I'm noticing is that .isNullOrEmpty is used three distinct ways:
if (String.IsNullOrEmpty(someParam))
if (!(string.IsNullOrEmpty(someParam)))
if (!string.IsNullOrEmpty(paramIsSpecial(someParam)))
The first uses String as the type, first letter capitalized, and does not use a negation.
The second has the negation outside parenthesis and is only passing in a defined variable to the method.
The third has the negation right beside the expression, with the IsNullOrEmpty being passed a function.
So I guess my questions are: Do these distinctions make a difference in general? Do they appear to be required/intentional in the above code? If they do make slight difference but the above choices appear to be style-choices from different contributors, what are the potential logical errors that could result?
The bug I am tracking down might occur if the above always returned true, even when it shouldn't. I'm wondering if 99% of the time the above would return true or false as expected but would return a false true if a specific value were given (maybe 0 or a string literal 'NULL', etc).
Obviously it matters if the negation operator is there or missing. None of the other differences are relevant.
string is simply an alias for System.String. It doesn't matter which you use.
The use of parenthesis around the method call in the second example isn't needed; it is no different than omitting them, as in the third example.
string is an alias for System.String, so there is no difference between those two, similar to how int and Int32 are the same thing.
The negation syntax also does not matter, as it is applying negation to the entire quantity when the extra parentheses are used. Remove it if it is confusing or distracting, leave it in otherwise.
Unfortunately, it sounds like you are going to need to set break points and walk through the code until you find out where the logic is wrong or the data you are processing is wrong. You might also consider putting Console debugging statements in the 3 branches of concern and then viewing the results to see which path(s) are being taken by your data.
One potential issue could be that String.IsNullOrEmpty(str) would return false if str is only whitespace (ex. " "). If your logic is wanting to treat a space as "empty", you would want to trim the string first. It's imposible to tell if this is the actual issue with the limited details you can provide, but something to lookout for none the less.
I am validating input on a form and attempting to prompt the user of improper input(s) based on the combination of controls used.
For example, I have 2 combo boxes and 3 text boxes. The 2 combo boxes must always have a value other than the first (default) value, but one of three, or two of three, or all text boxes can be filled to make the form valid.
In one such scenario I have a 6 line if statement to try to make the test easily readable:
if ((!String.Equals(ComboBoxA.SelectedValue.ToString(), DEFAULT_COMBO_A_CHOICE.ToString())
&& !String.IsNullOrEmpty(TextBoxA.Text)
&& !String.Equals(ComboBoxB.SelectedValue.ToString(), DEFAULT_COMBO_B_CHOICE.ToString()))
||
(!String.IsNullOrEmpty(TextBoxB.Text)
|| !String.IsNullOrEmpty(TextBoxC.Text)))
{
//Do Some Validation
}
I have 2 questions:
Should this type of if statement be avoided at all cost?
Would it be better to enclose this test in another method? (This would be a good choice as this validation will happen in more than one scenario)
Thanks for your input(s)!
In such a case I find it helps to move some of the logic out of the if statement and into some more meaningfully named booleans. Eg.
bool comboBoxASelected = !String.Equals(ComboBoxA.SelectedValue.ToString(), DEFAULT_COMBO_A_CHOICE.ToString());
bool comboBSelected = !String.Equals(ComboBoxB.SelectedValue.ToString(), DEFAULT_COMBO_B_CHOICE.ToString());
bool textBoxAHasContent = !String.IsNullOrEmpty(TextBoxA.Text);
bool textBoxBHasContent = !String.IsNullOrEmpty(TextBoxB.Text);
bool textBoxCHasContent = !String.IsNullOrEmpty(TextBoxC.Text);
bool primaryInformationEntered = comboBoxASelected && textBoxAHasContent && comboBSelected;
bool alternativeInformationEntered = textBoxBHasContent || textBoxCHasContent;
if (primaryInformationEntered || alternativeInformationEntered)
{
//Do Some Validation
}
Obviously, name the combo and text boxes to reflect their actual content. When someone has to work their way through the logic several months down the line they'll thank you.
a) Doesn't have to necesarily be avoided at all costs. The code works. But it is certainly messy, confusing and I would say could be difficult to maintain.
b) Yes. Give it a relevant name so that the code reader knows what is going on there.
I personally wouldn't have a big issue with code like this. (Your last set of parentheses seem unnecessary.)
Generally, I'd like to keep my if statements simpler. But all your conditions are simple ones. If you really need to test that many tests, then I'd keep it like it is.
It is not very readable yes. But you can shorten it:
!String.Equals(ComboBoxA.SelectedValue.ToString(), DEFAULT_COMBO_A_CHOICE.ToString()
could also be written as:
ComboBoxA.SelectedValue.ToString()!=DEFAULT_COMBO_A_CHOICE
I presume DEFAULT_COMBO_A_CHOICE is already of string to ToString si superflous.
also the parenthese around
(!String.IsNullOrEmpty(TextBoxB.Text)
|| !String.IsNullOrEmpty(TextBoxC.Text))
are not necessary.
IMO such conditions should be avoided (though not at all costs). They are very difficult to read an maintain.
There are several ways of doing that
Try and group the conditions according to the behavior they represent. For example
if (OrderDetailsSelected() && ShippingAddressProvided() )
{
This way you can also avoid the duplication of the conditions within your form.
Secondly, you can use the Boolean Algebra to simplify the expression and
Use Extract Method refactoring to move conditions, which are difficult to read in functions to avoid duplication and make them more readable.
For ex. The condition
String.Equals(ComboBoxB.SelectedValue.ToString(), DEFAULT_COMBO_B_CHOICE.ToString())
can be extracted into a function
private bool IsDefaultA() { return ... }
Possible Duplicates:
While vs. Do While
When should I use do-while instead of while loops?
I've been programming for a while now (2 years work + 4.5 years degree + 1 year pre-college), and I've never used a do-while loop short of being forced to in the Introduction to Programming course. I have a growing feeling that I'm doing programming wrong if I never run into something so fundamental.
Could it be that I just haven't run into the correct circumstances?
What are some examples where it would be necessary to use a do-while instead of a while?
(My schooling was almost all in C/C++ and my work is in C#, so if there is another language where it absolutely makes sense because do-whiles work differently, then these questions don't really apply.)
To clarify...I know the difference between a while and a do-while. While checks the exit condition and then performs tasks. do-while performs tasks and then checks exit condition.
If you always want the loop to execute at least once. It's not common, but I do use it from time to time. One case where you might want to use it is trying to access a resource that could require a retry, e.g.
do
{
try to access resource...
put up message box with retry option
} while (user says retry);
do-while is better if the compiler isn't competent at optimization. do-while has only a single conditional jump, as opposed to for and while which have a conditional jump and an unconditional jump. For CPUs which are pipelined and don't do branch prediction, this can make a big difference in the performance of a tight loop.
Also, since most compilers are smart enough to perform this optimization, all loops found in decompiled code will usually be do-while (if the decompiler even bothers to reconstruct loops from backward local gotos at all).
I have used this in a TryDeleteDirectory function. It was something like this
do
{
try
{
DisableReadOnly(directory);
directory.Delete(true);
}
catch (Exception)
{
retryDeleteDirectoryCount++;
}
} while (Directory.Exists(fullPath) && retryDeleteDirectoryCount < 4);
Do while is useful for when you want to execute something at least once. As for a good example for using do while vs. while, lets say you want to make the following: A calculator.
You could approach this by using a loop and checking after each calculation if the person wants to exit the program. Now you can probably assume that once the program is opened the person wants to do this at least once so you could do the following:
do
{
//do calculator logic here
//prompt user for continue here
} while(cont==true);//cont is short for continue
This is sort of an indirect answer, but this question got me thinking about the logic behind it, and I thought this might be worth sharing.
As everyone else has said, you use a do ... while loop when you want to execute the body at least once. But under what circumstances would you want to do that?
Well, the most obvious class of situations I can think of would be when the initial ("unprimed") value of the check condition is the same as when you want to exit. This means that you need to execute the loop body once to prime the condition to a non-exiting value, and then perform the actual repetition based on that condition. What with programmers being so lazy, someone decided to wrap this up in a control structure.
So for example, reading characters from a serial port with a timeout might take the form (in Python):
response_buffer = []
char_read = port.read(1)
while char_read:
response_buffer.append(char_read)
char_read = port.read(1)
# When there's nothing to read after 1s, there is no more data
response = ''.join(response_buffer)
Note the duplication of code: char_read = port.read(1). If Python had a do ... while loop, I might have used:
do:
char_read = port.read(1)
response_buffer.append(char_read)
while char_read
The added benefit for languages that create a new scope for loops: char_read does not pollute the function namespace. But note also that there is a better way to do this, and that is by using Python's None value:
response_buffer = []
char_read = None
while char_read != '':
char_read = port.read(1)
response_buffer.append(char_read)
response = ''.join(response_buffer)
So here's the crux of my point: in languages with nullable types, the situation initial_value == exit_value arises far less frequently, and that may be why you do not encounter it. I'm not saying it never happens, because there are still times when a function will return None to signify a valid condition. But in my hurried and briefly-considered opinion, this would happen a lot more if the languages you used did not allow for a value that signifies: this variable has not been initialised yet.
This is not perfect reasoning: in reality, now that null-values are common, they simply form one more element of the set of valid values a variable can take. But practically, programmers have a way to distinguish between a variable being in sensible state, which may include the loop exit state, and it being in an uninitialised state.
I used them a fair bit when I was in school, but not so much since.
In theory they are useful when you want the loop body to execute once before the exit condition check. The problem is that for the few instances where I don't want the check first, typically I want the exit check in the middle of the loop body rather than at the very end. In that case, I prefer to use the well-known for (;;) with an if (condition) exit; somewhere in the body.
In fact, if I'm a bit shaky on the loop exit condition, sometimes I find it useful to start writing the loop as a for (;;) {} with an exit statement where needed, and then when I'm done I can see if it can be "cleaned up" by moving initilizations, exit conditions, and/or increment code inside the for's parentheses.
A situation where you always need to run a piece of code once, and depending on its result, possibly more times. The same can be produced with a regular while loop as well.
rc = get_something();
while (rc == wrong_stuff)
{
rc = get_something();
}
do
{
rc = get_something();
}
while (rc == wrong_stuff);
It's as simple as that:
precondition vs postcondition
while (cond) {...} - precondition, it executes the code only after checking.
do {...} while (cond) - postcondition, code is executed at least once.
Now that you know the secret .. use them wisely :)
do while is if you want to run the code block at least once. while on the other hand won't always run depending on the criteria specified.
I see that this question has been adequately answered, but would like to add this very specific use case scenario. You might start using do...while more frequently.
do
{
...
} while (0)
is often used for multi-line #defines. For example:
#define compute_values \
area = pi * r * r; \
volume = area * h
This works alright for:
r = 4;
h = 3;
compute_values;
-but- there is a gotcha for:
if (shape == circle) compute_values;
as this expands to:
if (shape == circle) area = pi *r * r;
volume = area * h;
If you wrap it in a do ... while(0) loop it properly expands to a single block:
if (shape == circle)
do
{
area = pi * r * r;
volume = area * h;
} while (0);
The answers so far summarize the general use for do-while. But the OP asked for an example, so here is one: Get user input. But the user's input may be invalid - so you ask for input, validate it, proceed if it's valid, otherwise repeat.
With do-while, you get the input while the input is not valid. With a regular while-loop, you get the input once, but if it's invalid, you get it again and again until it is valid. It's not hard to see that the former is shorter, more elegant, and simpler to maintain if the body of the loop grows more complex.
I've used it for a reader that reads the same structure multiple times.
using(IDataReader reader = connection.ExecuteReader())
{
do
{
while(reader.Read())
{
//Read record
}
} while(reader.NextResult());
}
I can't imagine how you've gone this long without using a do...while loop.
There's one on another monitor right now and there are multiple such loops in that program. They're all of the form:
do
{
GetProspectiveResult();
}
while (!ProspectIsGood());
I like to understand these two as:
while -> 'repeat until',
do ... while -> 'repeat if'.
I've used a do while when I'm reading a sentinel value at the beginning of a file, but other than that, I don't think it's abnormal that this structure isn't too commonly used--do-whiles are really situational.
-- file --
5
Joe
Bob
Jake
Sarah
Sue
-- code --
int MAX;
int count = 0;
do {
MAX = a.readLine();
k[count] = a.readLine();
count++;
} while(count <= MAX)
Here's my theory why most people (including me) prefer while(){} loops to do{}while(): A while(){} loop can easily be adapted to perform like a do..while() loop while the opposite is not true. A while loop is in a certain way "more general". Also programmers like easy to grasp patterns. A while loop says right at start what its invariant is and this is a nice thing.
Here's what I mean about the "more general" thing. Take this do..while loop:
do {
A;
if (condition) INV=false;
B;
} while(INV);
Transforming this in to a while loop is straightforward:
INV=true;
while(INV) {
A;
if (condition) INV=false;
B;
}
Now, we take a model while loop:
while(INV) {
A;
if (condition) INV=false;
B;
}
And transform this into a do..while loop, yields this monstrosity:
if (INV) {
do
{
A;
if (condition) INV=false;
B;
} while(INV)
}
Now we have two checks on opposite ends and if the invariant changes you have to update it on two places. In a certain way do..while is like the specialized screwdrivers in the tool box which you never use, because the standard screwdriver does everything you need.
I am programming about 12 years and only 3 months ago I have met a situation where it was really convenient to use do-while as one iteration was always necessary before checking a condition. So guess your big-time is ahead :).
It is a quite common structure in a server/consumer:
DOWHILE (no shutdown requested)
determine timeout
wait for work(timeout)
IF (there is work)
REPEAT
process
UNTIL(wait for work(0 timeout) indicates no work)
do what is supposed to be done at end of busy period.
ENDIF
ENDDO
the REPEAT UNTIL(cond) being a do {...} while(!cond)
Sometimes the wait for work(0) can be cheaper CPU wise (even eliminating the timeout calculation might be an improvement with very high arrival rates). Moreover, there are many queuing theory results that make the number served in a busy period an important statistic. (See for example Kleinrock - Vol 1.)
Similarly:
DOWHILE (no shutdown requested)
determine timeout
wait for work(timeout)
IF (there is work)
set throttle
REPEAT
process
UNTIL(--throttle<0 **OR** wait for work(0 timeout) indicates no work)
ENDIF
check for and do other (perhaps polled) work.
ENDDO
where check for and do other work may be exorbitantly expensive to put in the main loop or perhaps a kernel that does not support an efficient waitany(waitcontrol*,n) type operation or perhaps a situation where a prioritized queue might starve the other work and throttle is used as starvation control.
This type of balancing can seem like a hack, but it can be necessary. Blind use of thread pools would entirely defeat the performance benefits of the use of a caretaker thread with a private queue for a high updating rate complicated data structure as the use of a thread pool rather than a caretaker thread would require thread-safe implementation.
I really don't want to get into a debate about the pseudo code (for example, whether shutdown requested should be tested in the UNTIL) or caretaker threads versus thread pools - this is just meant to give a flavor of a particular use case of the control flow structure.
This is my personal opinion, but this question begs for an answer rooted in experience:
I have been programming in C for 38 years, and I never use do / while loops in regular code.
The only compelling use for this construct is in macros where it can wrap multiple statements into a single statement via a do { multiple statements } while (0)
I have seen countless examples of do / while loops with bogus error detection or redundant function calls.
My explanation for this observation is programmers tend to model problems incorrectly when they think in terms of do / while loops. They either miss an important ending condition or they miss the possible failure of the initial condition which they move to the end.
For these reasons, I have come to believe that where there is a do / while loop, there is a bug, and I regularly challenge newbie programmers to show me a do / while loop where I cannot spot a bug nearby.
This type of loop can be easily avoided: use a for (;;) { ... } and add the necessary termination tests where they are appropriate. It is quite common that there need be more than one such test.
Here is a classic example:
/* skip the line */
do {
c = getc(fp);
} while (c != '\n');
This will fail if the file does not end with a newline. A trivial example of such a file is the empty file.
A better version is this:
int c; // another classic bug is to define c as char.
while ((c = getc(fp)) != EOF && c != '\n')
continue;
Alternately, this version also hides the c variable:
for (;;) {
int c = getc(fp);
if (c == EOF || c == '\n')
break;
}
Try searching for while (c != '\n'); in any search engine, and you will find bugs such as this one (retrieved June 24, 2017):
In ftp://ftp.dante.de/tex-archive/biblio/tib/src/streams.c , function getword(stream,p,ignore), has a do / while and sure enough at least 2 bugs:
c is defined as a char and
there is a potential infinite loop while (c!='\n') c=getc(stream);
Conclusion: avoid do / while loops and look for bugs when you see one.
while loops check the condition before the loop, do...while loops check the condition after the loop. This is useful is you want to base the condition on side effects from the loop running or, like other posters said, if you want the loop to run at least once.
I understand where you're coming from, but the do-while is something that most use rarely, and I've never used myself. You're not doing it wrong.
You're not doing it wrong. That's like saying someone is doing it wrong because they've never used the byte primitive. It's just not that commonly used.
The most common scenario I run into where I use a do/while loop is in a little console program that runs based on some input and will repeat as many times as the user likes. Obviously it makes no sense for a console program to run no times; but beyond the first time it's up to the user -- hence do/while instead of just while.
This allows the user to try out a bunch of different inputs if desired.
do
{
int input = GetInt("Enter any integer");
// Do something with input.
}
while (GetBool("Go again?"));
I suspect that software developers use do/while less and less these days, now that practically every program under the sun has a GUI of some sort. It makes more sense with console apps, as there is a need to continually refresh the output to provide instructions or prompt the user with new information. With a GUI, in contrast, the text providing that information to the user can just sit on a form and never need to be repeated programmatically.
I use do-while loops all the time when reading in files. I work with a lot of text files that include comments in the header:
# some comments
# some more comments
column1 column2
1.234 5.678
9.012 3.456
... ...
i'll use a do-while loop to read up to the "column1 column2" line so that I can look for the column of interest. Here's the pseudocode:
do {
line = read_line();
} while ( line[0] == '#');
/* parse line */
Then I'll do a while loop to read through the rest of the file.
Being a geezer programmer, many of my school programming projects used text menu driven interactions. Virtually all used something like the following logic for the main procedure:
do
display options
get choice
perform action appropriate to choice
while choice is something other than exit
Since school days, I have found that I use the while loop more frequently.
One of the applications I have seen it is in Oracle when we look at result sets.
Once you a have a result set, you first fetch from it (do) and from that point on.. check if the fetch returns an element or not (while element found..) .. The same might be applicable for any other "fetch-like" implementations.
I 've used it in a function that returned the next character position in an utf-8 string:
char *next_utf8_character(const char *txt)
{
if (!txt || *txt == '\0')
return txt;
do {
txt++;
} while (((signed char) *txt) < 0 && (((unsigned char) *txt) & 0xc0) == 0xc0)
return (char *)txt;
}
Note that, this function is written from mind and not tested. The point is that you have to do the first step anyway and you have to do it before you can evaluate the condition.
Any sort of console input works well with do-while because you prompt the first time, and re-prompt whenever the input validation fails.
Even though there are plenty of answers here is my take. It all comes down to optimalization. I'll show two examples where one is faster then the other.
Case 1: while
string fileName = string.Empty, fullPath = string.Empty;
while (string.IsNullOrEmpty(fileName) || File.Exists(fullPath))
{
fileName = Guid.NewGuid().ToString() + fileExtension;
fullPath = Path.Combine(uploadDirectory, fileName);
}
Case 2: do while
string fileName = string.Empty, fullPath = string.Empty;
do
{
fileName = Guid.NewGuid().ToString() + fileExtension;
fullPath = Path.Combine(uploadDirectory, fileName);
}
while (File.Exists(fullPath));
So there two will do the exact same things. But there is one fundamental difference and that is that the while requires an extra statement to enter the while. Which is ugly because let's say every possible scenario of the Guid class has already been taken except for one variant. This means I'll have to loop around 5,316,911,983,139,663,491,615,228,241,121,400,000 times.
Every time I get to the end of my while statement I will need to do the string.IsNullOrEmpty(fileName) check. So this would take up a little bit, a tiny fraction of CPU work. But do this very small task times the possible combinations the Guid class has and we are talking about hours, days, months or extra time?
Of course this is an extreme example because you probably wouldn't see this in production. But if we would think about the YouTube algorithm, it is very well possible that they would encounter the generation of an ID where some ID's have already been taken. So it comes down to big projects and optimalization.
Even in educational references you barely would find a do...while example. Only recently, after reading Ethan Brown beautiful book, Learning JavaScript I encountered one do...while well defined example. That's been said, I believe it is OK if you don't find application for this structure in you routine job.
It's true that do/while loops are pretty rare. I think this is because a great many loops are of the form
while(something needs doing)
do it;
In general, this is an excellent pattern, and it has the usually-desirable property that if nothing needs doing, the loop runs zero times.
But once in a while, there's some fine reason why you definitely want to make at least one trip through the loop, no matter what. My favorite example is: converting an integer to its decimal representation as a string, that is, implementing printf("%d"), or the semistandard itoa() function.
To illustrate, here is a reasonably straightforward implementation of itoa(). It's not quite the "traditional" formulation; I'll explain it in more detail below if anyone's curious. But the key point is that it embodies the canonical algorithm, repeatedly dividing by 10 to pick off digits from the right, and it's written using an ordinary while loop... and this means it has a bug.
#include <stddef.h>
char *itoa(unsigned int n, char buf[], int bufsize)
{
if(bufsize < 2) return NULL;
char *p = &buf[bufsize];
*--p = '\0';
while(n > 0) {
if(p == buf) return NULL;
*--p = n % 10 + '0';
n /= 10;
}
return p;
}
If you didn't spot it, the bug is that this code returns nothing — an empty string — if you ask it to convert the integer 0. So this is an example of a case where, when there's "nothing" to do, we don't want the code to do nothing — we always want it to produce at least one digit. So we always want it to make at least one trip through the loop. So a do/while loop is just the ticket:
do {
if(p == buf) return NULL;
*--p = n % 10 + '0';
n /= 10;
} while(n > 0);
So now we have a loop that usually stops when n reaches 0, but if n is initially 0 — if you pass in a 0 — it returns the string "0", as desired.
As promised, here's a bit more information about the itoa function in this example. You pass it arguments which are: an int to convert (actually, an unsigned int, so that we don't have to worry about negative numbers); a buffer to render into; and the size of that buffer. It returns a char * pointing into your buffer, pointing at the beginning of the rendered string. (Or it returns NULL if it discovers that the buffer you gave it wasn't big enough.) The "nontraditional" aspect of this implementation is that it fills in the array from right to left, meaning that it doesn't have to reverse the string at the end — and also meaning that the pointer it returns to you is usually not to the beginning of the buffer. So you have to use the pointer it returns to you as the string to use; you can't call it and then assume that the buffer you handed it is the string you can use.
Finally, for completeness, here is a little test program to test this version of itoa with.
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char *argv[])
{
int n;
if(argc > 1)
n = atoi(argv[1]);
else {
printf("enter a number: "); fflush(stdout);
if(scanf("%d", &n) != 1) return EXIT_FAILURE;
}
if(n < 0) {
fprintf(stderr, "sorry, can't do negative numbers yet\n");
return EXIT_FAILURE;
}
char buf[20];
printf("converted: %s\n", itoa(n, buf, sizeof(buf)));
return EXIT_SUCCESS;
}
I ran across this while researching the proper loop to use for a situation I have. I believe this will fully satisfy a common situation where a do.. while loop is a better implementation than a while loop (C# language, since you stated that is your primary for work).
I am generating a list of strings based on the results of an SQL query. The returned object by my query is an SQLDataReader. This object has a function called Read() which advances the object to the next row of data, and returns true if there was another row. It will return false if there is not another row.
Using this information, I want to return each row to a list, then stop when there is no more data to return. A Do... While loop works best in this situation as it ensures that adding an item to the list will happen BEFORE checking if there is another row. The reason this must be done BEFORE checking the while(condition) is that when it checks, it also advances. Using a while loop in this situation would cause it to bypass the first row due to the nature of that particular function.
In short:
This won't work in my situation.
//This will skip the first row because Read() returns true after advancing.
while (_read.NextResult())
{
list.Add(_read.GetValue(0).ToString());
}
return list;
This will.
//This will make sure the currently read row is added before advancing.
do
{
list.Add(_read.GetValue(0).ToString());
}
while (_read.NextResult());
return list;