Cryptic for-loop statement

Question

I am looking at some open source software and they use a for loop/operators in a different way then I have seen and I was wondering if someone can tell me what it is doing in English. I believe the open source is in C, maybe C++, not sure if it makes a difference but I am working in C++.

The for loop given is, TSTEP=60, tt and t are just double variables

for (tt=t<0.0?-TSTEP:TSTEP;fabs(t)>1E-9;t-=tt)

I’m not sure what caused the downvotes. The title isn’t very descriptive though. — Konrad Rudolph, Dec 19 '13 at 23:49
I think the part you wouldnt understand is this : http://en.wikipedia.org/wiki/%3F: — Chris Condy, Dec 19 '13 at 23:50

score 5 · Accepted Answer · answered Dec 19 '13 at 23:45

5

    if(t < 0)
        tt = -TSTEP;
    else
        tt = TSTEP;

    for(; fabs(t) > 1E-9; t -= tt)

Hopefully this is deciphered enough

answered Dec 19 '13 at 23:45

SirGuy

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score 2 · Answer 2 · edited May 23 '17 at 12:28

2

It's certainly ugly code, but really the only confusing part is the first part of the for statement:

tt=t<0.0?-TSTEP:TSTEP;

It might be easier to read with brackets:

tt = (t < 0.0 ? -TSTEP : TSTEP);

In English, this is "if t is less than 0.0, assign -TSTEP to tt, otherwise assign TSTEP to tt". If you haven't seen this syntax before, look up the ternary operator.

edited May 23 '17 at 12:28

Community

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answered Dec 19 '13 at 23:46

Joseph Mansfield

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score 1 · Answer 3 · answered Dec 19 '13 at 23:50

I guess the ?: operator is what puzzles you. The loop itself changes t by tt (60) steps in the direction towards 0, until t is almost 0, independent of if t was positive or negative from the start. lvalue = (expr ? a : b) is a common shorthand for

if (expr) {
  lvalue = a;
} else {
  lvalue = b;
}

Cryptic for-loop statement

3 Answers3