I want to check if sq. root of a number is int

Question

this is not working :

for(int i=a;i<=b;i++)  
{      
    if(sqrt(i)==int) count++;  
}

What should I use?

of course it doesn't. Trying to guess the syntax of a language rarely does. — Jean-François Fabre, Sep 02 '17 at 17:47
and this is completely backwards way to do this. Instead use a loop variable that *is* the square root, and **square** it and check if it is still within the limits. — Antti Haapala -- Слава Україні, Sep 02 '17 at 17:49
Step 1. Take the square root. Step 2. Extract the fractional part of the square root. Step 3. See if the fractional part is 0 or not. [Steps 2 and 3 might be tricky. See also alternative method in next comment.] — Steve Summit, Sep 02 '17 at 17:55
Step 1. Take the square root. Step 2. Round off to the nearest int. Step 3. Multiply that int by itself (square it). Step 4. See if you get the original number or not. — Steve Summit, Sep 02 '17 at 17:56
@SteveSummit "Extract the fractional part of the square root." and "See if the fractional part is 0 or not." is easy with `modf()`. — chux - Reinstate Monica, Sep 02 '17 at 20:30

sjsam · Accepted Answer · 2017-09-02T18:11:31.047

1

Do something like below :

double a = 5, b = 25;
short count = 0;
for (double i = a; i <= b; i++) {
    double sqrut=sqrt(i); 
    if ((int)(sqrut) == sqrut) {
        printf("Perfect square %.0f found\n", i);
        count++;
    }
}
printf("Total number of perfect squares is %d\n", count);

or like below :

double a = 5, b = 25;
short count = 0;
for (double i = a; i <= b; i++) {
    int sqrut = sqrt(i); // An implicit casting happens here.
    if ((sqrut * sqrut) == i) {
        printf("Perfect square %.0f found\n", i);
        count++;
    }
}
printf("Total number of perfect squares is %d\n", count);

edited Sep 02 '17 at 18:11

answered Sep 02 '17 at 17:56

sjsam

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there's still the accuracy issue. – Jean-François Fabre Sep 02 '17 at 17:58
Indeed, I was making that change. – sjsam Sep 02 '17 at 17:58
@Jean-FrançoisFabre Indeed there is, and for the same reason, this is not a general solution to the problem. – sjsam Sep 02 '17 at 18:00
@sjsam your method is wrong it is better to make int value squared and then compared – 0___________ Sep 02 '17 at 18:02

score 0 · Answer 2 · answered Sep 02 '17 at 17:50

0

One way to approach this is to truncate the root to an int and check if this affects the value:

double root = sqrt(i);
if ((int) root == root) {
    count++;
}

answered Sep 02 '17 at 17:50

Mureinik

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I wouldn't rely on that (accuracy issues). must be a duplicate, I found this for C++ (should work for C as well): https://stackoverflow.com/questions/23558888/checking-if-a-number-is-perfect-square – Jean-François Fabre Sep 02 '17 at 17:53

score 0 · Answer 3 · answered Sep 02 '17 at 20:26

To see if a double has a fraction, use modf()

The modf functions break the argument value into integral and fractional parts, C11dr §7.12.6.12 2

double root = sqrt(x);
double int_part;
if (modf(root, &int_part) == 0.0) puts("root is a whole number");

Note that sqrt(x) only provides the nearest representable double to the mathematical square root of x and so may give approximately correct results as root.

score -1 · Answer 4 · answered Sep 02 '17 at 17:59

-1

double iii = 4.0000000000000006;

double eee = sqrt(iii);

int fff = eee;

printf("%d\n", (int)eee == eee);    // returns 1 - which is incorrect

printf("%d\n", fff * fff == eee);   // returns 0  -- correct answer

answered Sep 02 '17 at 17:59

0___________

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okay Peter, but don't forget that the input is an integer in the current case. – Jean-François Fabre Sep 02 '17 at 18:02
1

@Jean-François Fabre but squaring the integer result is much more accurate in more general case – 0___________ Sep 02 '17 at 18:03
Agreed. And doesn't need using floating point... Noone proposed a correct solution, you could [edit] your answer to propose one (there are some answers, but they're for the c++ tag) – Jean-François Fabre Sep 02 '17 at 18:04
@Jean-François Fabre where is the c++ tag? – 0___________ Sep 02 '17 at 18:05
here: https://stackoverflow.com/questions/23558888/checking-if-a-number-is-perfect-square. That said, this question has 7 downvotes so far. I wouldn't invest in an answer... – Jean-François Fabre Sep 02 '17 at 18:05
@Jean-François Fabre but not in this question or maybe I am blind i see many questions with the C++ tag https://stackoverflow.com/questions/46016646/how-to-enable-c17-support-in-code-blocks but not this one – 0___________ Sep 02 '17 at 18:07
and how about rounding issues? truncating numbers? the question was about "should use" – Antti Haapala -- Слава Україні Sep 02 '17 at 18:07
every float method has an accuracy issue. Larger or smaller. – 0___________ Sep 02 '17 at 18:16

I want to check if sq. root of a number is int

4 Answers4