I want to clarify a few things about loops in procedural programming. So, I'm doing a unit on procedural programming in my IT course at sixth form and we need to write about three different types of loops, these are "Fixed for loop", "Pre-check while loop" and "Post-check while loop".
I think I have an idea of what these are however, I cannot find sufficient evidence to back up what I think I know.
Fixed for loop:
I think the fixed for loop is what I call the "generic" for loop, I think that this is where you define the counter variable with the for loop e.g.:
for (int i=0; i <= 6; i++ ){
print("Iteration " + i);
}
Pre-defined while loop: I think this is a while loop where the counter is defined before the loop itself e.g.:
int counter = 0;
while ( i <= 10){
print ( "Iteration " + i);
i ++;
}
However, if the above is true then I don't understand what a post-defined while loop would be.
If anyone would be kind enough to clarify what each of these loops are, I would much appreciate it.
My question in case it's not clear:
What do the following loops do and how are they defined: "Fixed for loop", "Pre-defined while" and "Post-defined while"
You can use a do...while loop which is basically the same as a while but the condition comes at the end rather than at the start.
int i = 0;
do {
print ( "Iteration " + i);
i ++;
} while ( i <= 10)
It's very useful if you want to perform a block of code at least once before checking the condition.
I could be wrong, but I believe there is a do-while construct. Using Java as example, you have
do {
statement(s)
} while (expression);
Where the condition is evaluated "post", or in other words, "after" the first iteration.
Related
I am wondering if it is good practice to increment a factorial within the increment statement of a for loop rather than the body of the for loop?
Below shows the factorial being incremented multiplicatively within the increment statement:
for(Count = 1; Count <= 10; Factorial *= Count, Count++);
An alternative is incrementing the factorial within the body of the for loop:
for(Count = 1; Count <= 10; Count++)
{
Factorial *= Count;
}
As the third parameter in a for loop is used for increments, is it a good or bad practice/code style to increment in this way? I believe it doesn't have any trade-off for readability + neatens up the code by saving some space, and although it doesn't change the outcome of the program, are there any negatives to using for loops in this way?
Do not move loop body into loop header.
The separation of loop components into loop body and loop header greatly improves readability.
Your readers will be able to tell loop's "payload" (i.e. Factorial *= Count) from loop's control (everything else) with a quick glance at your second code snippet. Your first code snippet would have your readers scratch their head for a few seconds before they realize that (1) there is a semicolon at the end of the for loop, and (2) the payload of the loop is in the header.
The fact that the compiler would probably generate the same code in both cases should not be a factor at all.
Due to my experience its more common to do it in the body of the for loop, because most actions are more than 1 line. And for such "easy" code it doesn't make a big difference in readability. The only important thing for you is to stay consistent.
I'm currently making my own programming language for fun using Ruby and wondering why having a "while" and "for" loop is something so universal. Why is it be bad to just have a single "loop" keyword?
ex)
loop (True) # acts as a while loop with a condition
loop var in 1..20 # acts as a for loop
loop var in list # acts as a for each loop
Would this somehow be disadvantageous or is the "while" and "for" loop simply just for semantic and readability?
In fact, there are even more loop-variants. For example do-while.
To answer your question:
Yes, all of them can be used to do the same thing. However, depending on the loop-variant you use, you implicitly say what this loop does.
For example, you only use a for-loop if you know how often you want to loop (e.g. for each item in a list (i know, there is also a foreach in many languages)). It is considered bad habit to modify the loop variable inside a for-loop.
So if you are not sure how often you will loop, you should most likely use something else than a for-loop... for example a while-loop.
Here are some simple code examples:
for(i = 0; i < someList.Length; i++) {
// do some stuff x times
}
while(someValue < someOtherValue) {
someValue = someValue * someValue;
}
For me it is clear, that i should not use a for-loop for the 2nd loop.
To sum up: Yes, you only need one loop construct. However, I personally (!) like to have the advantage to actually say something just by the choice which loop i take.
Would this somehow be disadvantageous or is the "while" and "for" loop simply just for semantic and readability?
I think that your idea would not be disadvantageous and, yes, while and for are chosen for semantics/readability (or even historical reasons). Then, there are slightly different meanings in every language. Often, the for loop implies that you have a control variable. From the classic basic
for i=1 to 10 ... (next i)
to pascal, and even in more advanced constructs (iterators) from python, this idea is respected. Moreover, in some languages (pascal, for example) a for implies that the low-high limits are pre-calculated. This pascal code for example:
B := 5;
for i := 1 to B do begin
B := 25
end;
can give warnings or errors, or can lead to 5 iterations even if in the body of the loop the upper limit is modified. The C language, which also has for, is totally different, even if the general idea of "control variable" is respected in 99% of the cases. The C for does not pre-compute limits (by design), can have more than one control variable, or even none at all:
for ( ; ; ) ...
is a valid for loop which does not have a control variable (and no meaning to break the loop!).
On the other hand, while, repeat, do ... while and so on are semantically clear (you can "talk" the code with your voice, and everything is self explaining), and do not imply variables (they imply nothing).
The above loops all have a common problem: they have a test which is performed at the beginning (or the end) of every iteration. Suppose you want to read characters from standard input, do something with them, and stop when this character is EOF. The C way is:
while ( ( ch=getchar() ) != EOF) ... ; // do something with ch
The C language can do this because an assignment is also an expression. If it was not so, one had to write:
ch=getchar();
while (ch != EOF) {
.... ; // do something with ch
ch = getchar();
}
// I must use two times the statement "ch = getchar();"
// or...
do {
ch = getchar();
if (ch == EOF) break;
... ; // do something with ch
} while (true)
// I use getchar() only once, but I end up with an awful "while (true)"
In your new language, you could invent a different cycle which goes like this:
cycle
// statements always executed at least once
when (condition)
// statement executed if the condition is true
// and, if true, the cycle restarts
end
With this syntax, the example about getchar() would become:
cycle ch=getchar(); when (ch != EOF) ... ; // do something bla bla bla...
The normal while, while (true) and do-while loops would become:
cycle when (condition); BODY // while
cycle BODY; // while (true)
cycle BODY; when (condition) // do-while
Think about it... :-)
Most languages have for and while for historical reasons and familiarity. Familiarity is important: if you reuse the same concepts and keywords as other languages, your language is easier to learn. Many languages use "!" for the negation for this reason (many other languages use "not"). Look at this page comparing syntax across languages, you'll see lots of similarities: http://rigaux.org/language-study/syntax-across-languages.html
Now, to answer your question. Although most languages have the while keyword, you really don't need it.
Best example I can think of is Go:
// C-like for
for i := 0; i < 10; i++ {
sum += i
}
// while
for sum < 1000 {
sum += sum
}
// infinite loop
for {
}
// do .. while
for ok := true; ok; ok = condition {
...
}
// range-for
for i, v := range pow {
fmt.Printf("2**%d = %d\n", i, v)
}
So, as you can see, it's perfectly possible to use a single keyword for all loops.
Main reason for having both for and while is familiarity and readability. But you can easily do without.
Is there a difference between for and while statements? Is it just syntax?
#include <stdio.h>
void main() {
int cent_temp = 0;
int fah_temp;
while (cent_temp <= 20) {
fah_temp = (9 * cent_temp) / 5 + 32;
printf("%d degrees C = %d degrees F\n", cent_temp, fah_temp);
cent_temp++;
}
}
This means to me....
While the value of cent_temp is less than 20 then calculate fah_temp. Then increase the value of cent_temp by 1 and check it is still less than 20; then go round the loop again.
Regarding the syntax:
printf("%d degrees C = %d degrees F\n", cent_temp, fah_temp);
This means %d means print to the screen, replace with a decimal number the value of cent_temp and %d means replace with a decimal number the value of fah_temp.
#include <stdio.h>
void main() {
int cent_temp;
int fah_temp;
for (cent_temp = 0; cent_temp <= 20; cent_temp++) {
fah_temp = (9 * cent_temp) / 5 + 32;
printf("%2d degrees C = %2d degrees F\n", cent_temp, fah_temp);
}
}
My interpretation of the above is:
for cent_temp = 0 repeat while cent_temp less than 20 and then execute cent_temp+1 at the end. So cent_temp 0 goes into the loop to calculate fah_temp and gets printed to the screen. Then cent_temp goes up by one then goes round the loop again. Here I've used %2d instead of %d to signify that it should have 2 spaces for a decimal number (and they line up when executed). Both codes will not execute if cent_temp > 20.
Similarly rearranging the statement in a do while loop has a similar effect and doesn't really have an impact on the result.
Does each type of loop have a different application?
Please correct me if I wrong!
Is there a difference between 'for' and 'while' statements? Is it just
syntax?
To me, it is just syntax.
From K&R section 3.5 Loops -- While and For, I quote:
The for statement
for (expr1; expr2; expr3)
statement
is equivalent to
expr1;
while (expr2) {
statement
expr3;
}
except for the behavior of continue.
Grammatically, the three components of a for loop are expressions.
Most commonly, expr1 and expr3 are assignments or function calls
and expr2 is a relational expression.
Notes
As user #chqrlie has mentioned in the comments, control statements like break and continue make the situation slightly murkier.
There are some situations where the modify statement is necessary in the loop body. For example Erase-remove idiom with std::set failing with constness-related error (in C++ though)
Example
As an example, let us write a loop to print all the odd numbers between 1 and 100.
int i = 1;
while (i <= 100) {
printf("%d\n", i);
i += 2;
}
for (int i = 1; i <= 100; i += 2) {
printf("%d\n", i);
}
Opinion
I am not a language expert, but in most situations in practice I find them transformable.
I personally prefer using for syntax because:
loop control structure is in one single place (the for header) making it easy to read, and
the loop variable (e.g. i) is not exposed to the outer scope.
for(cent_temp = 0; cent_temp <= 20; cent_temp++)
{ /* code */ }
is 100% equivalent to
cent_temp = 0;
while(cent_temp <= 20)
{
/* code */
cent_temp++;
}
But a do-while is different since it puts the condition check at the end.
As for when to use which loop, it is a matter of style and therefore a bit subjective. The industry de facto standard style, used by the majority of all C programmers, goes like this:
for loops should always be used when performing a known number of iterations. It is then considered the most readable form.
while loops should be used the the number of iterations is unknown in advance, or when the loop is turning complex for some reason. For example if you need to alter the loop iterator variable inside the loop body, then you should use a while loop instead of a for loop.
do while loops should be used for special cases where you need to skip the condition check the first lap of the loop, for example do { result = send(); } while(result == ok);.
I looked at my Code Complete by Steve McConnell (the bible).
Here is what you can read in chapter 16:
A for loop is a good choice when you need a loop that executes a specified number of times. [...]
Use for loops for simple activities that don't require internal loops controls. Use them when the loop involves simple increments or simple decrements, such as iterating through the elements in a container. The point of a for loop is that you set it up at the top of the loop and then forget about it. You don't have to do anything inside the loop to control it. If you have a condition under which execution has to jump out of a loop, use a while loop instead.
Likewise, don't explicitly change the index value of a for loop to force it to terminate. Use a while loop instead. The for loop is for simple uses. Most complicated looping tasks are better handled by a while loop.
In general, you would use a for loop to iterate over a finite set of values, whereas you'd use a while or do-while loop to iterate while a specific condition or set of conditions is true. In most of C's contemporaries (Basic, Pascal, Fortran, etc.), a for loop can only iterate over a scalar index:
Fortran:
DO 10 i=1,10
statements
10 CONTINUE
Pascal:
for i := 1 to 10 do
begin
statements
end;
Both of these snippets loop exactly 10 times. The index i is initialized and updated by the loop automagically. I'd have to go back and check, but I'm pretty sure you cannot write to i in the loop body.
C actually blurred the lines between a for and while loop by adding the control expression:
for ( init-expr ; control-expr ; update-expr )
statement
In C, a for loop can iterate over a scalar just like Fortran or Pascal:
for( i = 0; i < 10; i++ )
{
do_something_with( i );
}
Or it can iterate over multiple scalars:
for ( i = 0, j = 0; i < 10 && j < 10; i++, j++ )
{
do_something_with( i, j );
}
Or it can iterate over the contents of a file:
for( c = fgetc( in ); c != EOF; c = fgetc( in ) )
{
do_something_with( c );
}
Or it can iterate over a linked list:
for( cur = head; cur != NULL; cur = cur->next )
{
do_something_with( cur );
}
In Fortran and Pascal, those last three loops would have to be expressed as while loops (which I'm not going to do, because I've pretty much exhausted my Fortran and Pascal knowledge already).
The other big difference between a C for loop and those of Fortran or Pascal is that you can write to the loop index (i, j, c, or cur) in the loop body; it's not specially protected in any way.
A while or do-while loop is used to iterate as long as a specific condition or set of conditions is true:
while( control-expr )
statement
do
statement
while( control-expr );
In both a for and while loop, the condition is tested before the loop body executes; in a do-while loop, the condition is tested after the loop body executes, so a do-while loop will always execute at least once.
In C, you can use either a for loop or a while loop in many circumstances:
while ( ( c = fgetc( in ) ) != EOF )
do_something_with( c );
for ( c = fgetc( in ); c != EOF; c = fgetc( in ) )
do_something_with( c );
Both loops do exactly the same thing; it's just a matter of which one you think more clearly expresses your intent, or which you think would be easier for other people to understand.
From the point of view of algorithmic for and while are not the same. Shortly, in algorithmic, for should be used when bounds are known and while when you don't know if the condition can be met or when it can be. For is to repeat something n times (n known), which is exactly the case of your example computation; a for loop should be used (don't you think what the loop makes is more clearly stated in the for loop ?). If you want an example of a must be used while loop, look at something like Collatz sequence. From a point of view of computability, for loops can always be transformed in while loops but not the converse.
From the point of view of computer languages it is now common to fuse both, in C for example, it makes no difference, only syntactic. But remember that in some other language that could be very different, for example in Pascal for loops are very limited.
Source code is written not only to be compiled and executed by computers but also to be read and understood by humans.
A computer doesn't really mind whether a for loop, a while loop or a goto is used. On the other hand, a human expects different meanings for different structures.
computing values over a known range of inputs is best shown with a for loop;
reading a file up to its end is best shown with a while loop.
Choosing which structure to use is similar as choosing a variable name.
I have a doubt about the for loop of the following code in C:
main()
{
int i=1;
for(;;)
{
printf("%d",i++);
if(i>10)
break;
}
}
I saw this code in a question paper. I thought that the for loop won't work because it has no condition in it. But the answer says that the code has no error. Is it true ? If true, how ?
The regular for loop has three parts:
Initialization
Condition
Increment
Usually they are written like this:
for (initialization; condition; increment) { statements }
But all three parts are optional. In your case, all parts are indeed missing from the for loop, but are present elsewhere:
The initialization is int i=1
The condition is if (i>10) break
The increment is i++
The above code can be equivalently written as:
for (int i=1; i <= 10; i++) {
printf("%d", i);
}
So all the parts necessary for a for loop are present, except they are not inside the actual for construct. The loop would work, it's just not a very readable way to write it.
The for (;;) loop is an infinite loop, though in this case the body of the loop takes actions that ensure that it does not run forever. Each component of the control is optional. A missing condition is equivalent to 1 or true.
The loop would be more clearly written as:
for (int i = 1; i < 11; i++)
printf("%d", i);
We can still debate whether the output is sensible:
12345678910
could be produced more easily with:
puts("12345678910");
and you get a newline at the end. But these are meta-issues. As written, the loop 'works'. It is syntactically correct. It also terminates.
You are not specifying any parameters or conditions in your for loop, therefore, it would be an endless loop. Since there is a break condition based on another external variable, it would not be infinite.
This should be re-written as:
for (int i = 1; i <= 10; i++)
printf("%d",i++);
It's an infinite loop. When there is not a condition in for and we use ;; the statements in the body of for will be executed infinitely. However because there is a break statement inside it's body, if the variable i will be greater than 10, the execution will be stopped.
As it is stated in MSDN:
The statement for(;;) is the customary way to produce an infinite loop which can only be exited with a break, goto, or return statement.
For further documentation, please look here.
Even if a for loop is not having any condition in it, the needed conditions are specified inside the for loop.
The printf statement has i++ which keeps on increasing the value of i and next we have if statement which will check if value of i is less than 10. Once i is greater than 10 it will break the loop.
Small question just to clarify.
In ZX-Spectrum BASIC compiler there is FOR TO STEP NEXT looping, where TO is unconditional:
10 FOR i=1 TO 5 STEP 1
I've tried similar practice in C
for (i=1; i==5; i++);
and the of course loop does not work (== is never true here)... so the question is:
Is in C FOR loop we should always use a CONDITION to stop the loop (I mean CONDITION in parentheses of the FOR statement), like FOR (i=0; i<6; i++);
The correct translation of:
for i = 1 to 5 step 1
would be:
for (i = 1; i <= 5, i++)
In other words, a loop running five times with the control variable set to 1, 2, 3, 4 and 5 on sequential iterations.
There are other variations you could use such as different conditional operators and different termination values, but the one shown most matches the BASIC variant while still protecting you in the case where your step may be more than one (such as for i = 1 to 4 step 2).
Keep in mind that C arrays are 0-based so, if you're using that i to access an array, it needs to run from 0 to n-1, not 1 to n. If you just want the variable for other purposes (such as printing out the numbers one through five inclusive), the 1..n variant is okay.
You could use
for (i = 1; i != 6; i++);
But it would be better to use i = 0 ... i < 5, in case you alter i inside of the loop. It also communicates what you are doing better.
There is no any restriction that you must use the conditional statement in the for loop..
You also use this conditional statement in the body of for loop as follow....
for(int i=0;;i++)
{
if(i>=5)
{
break;// to break the loop
}
}
if you not use the conditional statement at in the for loop or in the for loop body then the loop goes into the infinite state as follow....
for(int i=0;;i++)
{
//any statements
}
Because In the for loop all three part are optional (initialization , conditional , incri/decri)
int i=0
for(;;)
{
if(i>=5)
{
//any statements
}
i++
}
Loop conditions are not necessary you can write loops like
for(int i=0;;i++)
{
//body
}
but note that body of the loop should contain some break statement else loop executes infinite time
There are two problems in your code, you shouldn't use a ; after your loop head, and you can use < or <= in your condition part like this:
for(int i=1; i<=5; i++){}
so it should be a condition but you do not have to use an inequality you can use any other condition but it should be finished a time