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
Related
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.
In this for loop statement
#include<stdio.h>
int main()
{
static int i;
for(++i;++i;++i) {
printf("%d ",i);
if(i==4)
break;
}
return 0;
}
Variable i is at first 0. The arguments in the for-loop at 1st round are 1st ++i: i = 0 + 1 = 1 2nd ++i: i=1+1=2 So, in first loop I have this for(i=1; i<2; ++i); or for(i=1; i<=2; ++i);?EDIT I found this example online in a test about C. I run this (inside the for-loop , I have a break point so after some loops it breaks) but I was just guessing the behavior of that so I asked it here to be sure. I am learning now C so stupid questions exists for me. Its better to ask, than not.
In the second argument it is actually ++i!=0, The loop is interpreted as
for(++i;++i!=0;++i)
If you start with a positive i or 0, it will be an infinite loop and will invoke undefined behavior when i reaches INT_MAX.
If i was -Ve initially the loop may stop at a defined run.
EDIT: As you changed your question, Your code will not crash, but you can clearly understand the dry-run by replacing the second ++i with ++i!=0.
So the 1st iteration becomes:
(i=1;2!=0;++i/*this will execute later*/)
2nd iteration becomes:
i=3 //this is from the 1st iteration last part.
(/*initialization is done 1st time only*/;4!=0;++i/*again, this will execute after this iteration*/)
It will print 2 4.
Before the for loop, i will be 0. It hasn't been assigned anything yet, and static variables are guaranteed to be zero initialized before they are first used.
It will execute the first ++i in the for loop, since that expression is evaluated once at the beginning of the loop. i will be 1.
It will execute the second ++i, because that is evaluated BEFORE every loop to see if it should run an iteration of the loop. i will be 2.
It will run the loop body. This will print 2.
The if condition won't be true so it won't break.
It will execute the third ++i in the for loop statement, since it evaluates that AFTER every iteration. i will be 3.
It will execute the second ++i again, since it needs to see if it needs to run another loop. It will be nonzero, so it will run another loop. i will be 4.
It will print 4.
The if condition will be true, it will break out of the loop.
However, it is a nonsense way to do it. This is a more appropriate way to do that:
int i;
for (i = 2; i <= 4; i += 2)
printf("%d ", i);
or better yet:
printf("2 4 ");
static int i;
While the C standard guarantees that variables with static storage duration are initialized to 0, you should not abuse that. Always initialize your variables, either at the line where they are declared or in runtime. So change this to static int i=0;
The first ++i is indeed equivalent to having i=1 there. Esentially your loop does this:
for(i=1; loop_until_program_crash; i++)
If you have a break inside the loop, then the loop is likely poorly written. If you know in advance when the loop should end, then that condition should be inside the for loop condition. If you don't know in advance, then use a while loop instead.
It should be :
for(i=1; i<=2; ++i);
static int i=0;
for(++i;++i<=2;++i)
{
printf("4rth :%d\n",i);
}
see : http://ideone.com/TGLYlL
I was just wondering why this :
int i;
for (i=0; i<5; i++){
printf("%d\n",i)
}
printf("Here i get the result that misleads me : %d\n",i)
The last value is 5.
My logic is :
From 0 to 4 -> printf
If i > 4 (since we are dealing with integers) stop the loop.
But the loop stopped at 4 not 5 ! Why do I get 5 after the loop ? Why does it ever increment ?
Arbitrary ?
Thanks,
There are three clauses in the for statement.
The init-stmt statement is done before the loop is started, usually
to initialize an iteration variable.
The condition expression is tested before each time the loop is
done. The loop isn't executed if the boolean expression is false
(the same as the while loop).
The next-stmt statement is done after the body is executed. It
typically increments an iteration variable.
So, end of each for loop execution, increment operation executed, and, in the 4th iteration, value of i is 5 and the for loop was broke as the value is 5 in 5th iteration.
unroll what's happening
int i = 0;
while( i < 5 )
{
// body of for loop
i++;
}
// i == 5 here as i must be greater than or equal to 5 to break out of while loop
How does for loop works:
for (initialization; condition; increment-decrement)
Statement
Run initialization (i=0)
Check condition (i<5), if true then jump to 3 else jump to 5
Do statement ({ printf("%d\n",i) })
Do increment-decrement (i++), jump to 2
Exit loop
Before last iteration i == 4, then it prints 4, increments i. Therefore after last iteration i == 5, !(5 < 5), i.e. condition is false, exit from loop.
At the end of each iteration, the loop index is incremented. Then the program checks whether to run another loop. It finds "no the index is too big and fails the condition", so it doesn't execute and exits.
Until the index gets too big, the condition doesn't fail. It HAS to get too big (so the loop doesn't execute).
And that is why...
Because after the 4th loop:
i++ is performed, i becomes 5
i<5 is evaluated as false
Thus the loop exits
And then you get i==5 after the end of loop.
Indeed, after each loop, i++ is evaluated before checking for the exit condition, i<5.
for (int i = 0; i < 5; ++i) goes like this:
i <- 0
check whether i < 5, if yes, loop, otherwise stop
do the loop
i <- i + 1
goes to step 2
Let's see this loop:
for (i = 0; 0; i++)
do_smth();
You'll see that this loop doesn't execute at all. Because the condition is false even from the beginning.
Now
for (i = 0; i < 4; i++)
i++;
Clearly increases i twice for every run (once inside the loop and then because of the third part of the for). In the end, i will have the first even value which make the condition false. That is 6. Thus, there is no link between the step and the value of the variable outside the loop.
guys i'm new at programming and i was surprised by the result of post increment value, now i'm bound by confusion after i found out and executed the code below, if for loop says
1. initialize
2. check for condition if false terminate
3. incrementation.
where does i++ happens? where does i value is equal to 1?
int main()
{
int i, j;
for (int i =0; i<1; i++)
{
printf("Value of 'i' in inner loo[ is %d \n", i);
j=i;
printf("Value of 'i' in outter loop is %d \n", j);
// the value of j=i is equals to 0, why variable i didn't increment here?
}
//note if i increments after the statement inside for loop runs, then why j=i is equals to 4226400? isn't spose to be 1 already? bcause the inside statements were done, then the incrementation process? where does i increments and become equals 1?
//if we have j=; and print j here
//j=i; //the ouput of j in console is 4226400
//when does i++ executes? or when does it becomes to i=1?
return 0;
}
if Post increment uses the value and add 1? i'm lost... Please explain... thank you very much.
I'm not really sure what you're asking, but sometimes it's easier for beginners to understand if rewritten as a while loop:
int i = 0;
while (i < 1)
{
...
i++; // equivalent to "i = i + 1", in this case.
}
Your loop declares new variable i and it shadows the i declared earlier in main(). So if you assign i to j outside of the loop, you are invoking undefined behaviour because i is not initialised in that context.
Prior to the first iteration, i is initialised to 0. This is the "initialize" phase, as you called it.
The loop condition is then evaluated. The loop continues on a true value.
The loop body is then executed. If there's a continue; statement, that will cause execution to jump to the end of the loop, just before the }.
The increment operator is then evaluated for it's side-effects.
Hence, it is after the first iteration that i changes to 1. i keeps the value 1 for the entirety of the second iteration.
It looks like you have a clash of variable names: i is declared before the loop, and also inside the loop.
The i that is declared in the for statement is the only one that will ever be 1. It will be one just after the body of the loop is executed.
Try setting a breakpoint and using the debugger to step through the loop whilst you watch the value of the variable (here's a video of what I mean by stepping with the debugger).
To remove the ambiguitity of having two variables called i you could change the for loop to:
for (i = 0; i < 1; i++) // remove the `int`
this will ensure that there is only one i in your code.
A comment on #CarlNorum's answer which wouldn't look good as a comment:
The C Standard defines
for ( A; B; C ) STATEMENT
as meaning almost the same thing as
{
A;
while (B) {
STATEMENT
C;
}
}
(where a {} block containing any number of statements is itself a kind of statement). But a continue; statement within the for loop will jump to just before the next statement C;, not to the next test of expression B.
for (i=0; i<x; i++)
Is equivalent to
i=0;
while(i<x) {
// body
i = i + 1;
}