English is not my first language, but since the keywords in programming languages are English words, I usually find it easy to read source code as English sentences:

  • if (x > 10) f(); => "If variable x is greater than 10, then call function f."
  • while (i < 10) ++i; => "While variable i is less than 10, increase i by 1."

But how a for loop is supposed to be read?

  • for (i = 0; i < 10; ++i) f(i); => ???

I mean, I know what a for loop is and how it works. My problem is only that I don't know what the English word "for" exactly means in for loops.

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    It is code. It isn't English. So... they don't directly compare. Don't try to find a direct meaning. The creators of a language had a limited vocabulary to choose from and went for terse. – Oded Oct 28 '13 at 11:34
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    You may read it as for each. So it would become for each i, till i is less than 10, perform the following steps. – Aziz Shaikh Oct 28 '13 at 11:36
  • @Oded OK, but why do you think they chose this word, and not another? OK, it's short, but I don't think this explains it. – kol Oct 28 '13 at 11:39
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    @kol for each i pretty much explains everything. – vallentin Oct 28 '13 at 11:42
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    It is confusing in C, and the keyword is there for historical reasons. In languages like Algol it reads much more naturally: "for an index variable blah-blah-blah in range blah-blah-blah do blah-blah-blah". In C it can be "for nothing do unconditionally something" which is confusing indeed. – SK-logic Oct 28 '13 at 11:46

For this particular case, it would be something like:

"For every i starting from 0 up to (but not including) 10, do f(i)"

It would have to be worded a bit differently if the numbers were doubles or the looping conditions were more complicated, but you shouldn't really worry if you can't find a really natural sounding translation to English because programming languages are only based on natural languages, or mathematical notation: for i = 1,... 10 is something you write in math, which is much older than programming.

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    +1 for the math reference. in math you define a function for values in a domain. the part between the parentheses is the domain of the function defined between the braces. an empty for has an infinite domain, thus running indefinitely. – devnull Oct 28 '13 at 12:08
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    @devnull I like the domain explanation as an answer – Wim Ombelets Oct 28 '13 at 20:16

It's a lot more confusing because you've written out a C-style FOR loop, which isn't really a FOR loop; it's a WHILE loop with weird syntax. The FOR loop was defined by ALGOL. Pascal and BASIC picked up the concept; C, unfortunately, borrowed the name but not the semantics. (The best indication of this is to look at the for (;;) construct; it's not possible to define a true FOR loop that has no range to iterate over.)

The idea is that you define a range of elements, and an operation to perform for each element. For example, in Pascal you would write your example loop as:

for i := 0 to 9 do

In BASIC it's similar:

for i = 0 to 9
next i

This is a lot more readable: for each element i from 0 to 9, do something with the value. Note that there is no loop-ending condition involving i being < 10; instead, the last value of the range (9) is specified.

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For loops in C/C++ are very flexible, and as such, they don't translate directly to English very well. The idea behind the for loop is iteration, and thus you would start out describing a simple initializer/condition/increment expression list with for each value of i.

This is my flexible, very verbose English representation of your for loop expressions:

for (i = 0; i < 10; ++i)

For each value of i (initially 0), and while i is less than 10, iterate by incrementing i.

This representation allows you to adapt to more complicated expressions, such as:

int count = 10;
for (i = 0, j = count - 1; i < count; i++, j--) { //... }

For each value of i (initially 0) and j (initially count - 1), and while i is less than count, iterate by incrementing i and decrementing j.

Or even:

vector<T> v;
for(vector<T>::iterator it = v.begin(); it != v.end(); ++it) { //... }

For each value of it (initially the first element of v), and while it is not the last element of v, iterate by incrementing it to the next element of v.

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  • +1 Thank you, very clear explanations. I especially like the way you use the word "initially". – kol Oct 28 '13 at 20:36

In English we use for in contexts like "This is for children under 6 year of age". The for statement was adapted in structured programming in this context.

Read for (i = 0; i < 10; ++i) f(i); as for (; i < 10;) f(i); and it might be clearer. The for statement has three parameters: initialization, condition, update. In some languages, the parameters can be lists.

For make the looping conditions clearer than the equivalent while loop. If the body is long the increment may not be obvious.

 i = 0;
 while ( i < 10 ) {

In the following version the increment is misplaced. The for construct doesn't allow the increment (update) to be misplaced in this manner.

 i = 0;
 while ( i < 10 ) {

Some languages provide a for each statement which is used to iterate over sets.

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Meriam Webster definition

For - "used to indicate ... something is going to or toward "

so your statement is i (starting at 0) going to 9.

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    That is the meanng in a sentence such as "I bought this book for my sister". In this phrase, for is indeed used to indidcate that the book is going to(ward) my sister. There is no range here, certainly no loop. I don't think it is this meaning that is referred to in programming languages. – Kris Vandermotten Oct 28 '13 at 12:04
  • The OP was looking at how the code statement could be interpreted. Most of the answers were you can't. I provided an alternative, a way it could be(loosely). – dbasnett Oct 28 '13 at 12:51
  • I think the question was meaning to ask "Why is this word used", or "how should the word be interpretted so that it is consistant with English". You gave a literal interprettation that didn't answer the intent of the question. – user69037 Oct 29 '13 at 22:53

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