CODEWITHSUNDEEP
c++cprintfcoutunsigned-long-long-int
Marcus
Marcus

Reputation: 323

Why do cout and printf produce different results?

I'm learning C/C++ and started to try out programing with big numbers but when printing them I get a strange error.

Here is my example code:

unsigned long long shiftresult = 1ULL;
for (int i = 0; i < 64; i++) {
    shiftresult = (1ULL << i);
    printf("%lld << %d = %lld\n", 1ULL, i, shiftresult);
    cout << 1ULL << " << " << i << " = " << shiftresult << endl;
}

The problem being that when the loop reaches i = 63 printf prints -9223372036854775808 while cout prints 9223372036854775808 which is the "real" output.

Could someone explain why this happens?

Upvotes: -2

Views: 113

Answers (2)

hobbs
hobbs

Reputation: 239652

Because you told printf to print %lld (a signed long long), but your value is actually an unsigned long long, which should be printed with %llu. output streams can figure that out automatically, printf has to be told.

Upvotes: 3

dbush
dbush

Reputation: 223689

You're using the wrong format specifier for printf.

The %lld format specifier expects a long long argument, but you're passing an unsigned long long argument, so the unsigned number is being interpreted as a signed number.

You need to use %llu to print an unsigned long long.

Upvotes: 6

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