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See this page as a slide show

CS253 Manipulators

I/O Manipulators

As you recall, there are a number of I/O manipulators:

<ios> or <iostream>	<iomanip>
These don’t take an argument [no]boolalpha [no]showbase [no]showpoint [no]showpos [no]skipws [no]uppercase left / right / internal dec / hex / oct fixed / scientific	These do take an argument resetiosflags setiosflags setbase setfill setprecision setw (non-sticky)

[no]boolalpha

cout                << false << ' ' << true << '\n'
     << boolalpha   << false << ' ' << true << '\n'
     << noboolalpha << false << ' ' << true << '\n';

0 1
false true
0 1

[no]showbase

cout               << dec << 65 << ' ' << hex << 65 << ' ' << oct << 65 << '\n'
     << showbase   << dec << 65 << ' ' << hex << 65 << ' ' << oct << 65 << '\n'
     << noshowbase << dec << 65 << ' ' << hex << 65 << ' ' << oct << 65 << '\n';

65 41 101
65 0x41 0101
65 41 101

[no]showpoint

cout                << 123.0 << ' ' << 456.789 << '\n'
     << showpoint   << 123.0 << ' ' << 456.789 << '\n'
     << noshowpoint << 123.0 << ' ' << 456.789 << '\n';

123 456.789
123.000 456.789
123 456.789

[no]showpos

cout              << 123 << ' ' << 0 << ' ' << -456 << '\n'
     << showpos   << 123 << ' ' << 0 << ' ' << -456 << '\n'
     << noshowpos << 123 << ' ' << 0 << ' ' << -456 << '\n';

123 0 -456
+123 +0 -456
123 0 -456

[no]skipws

istringstream iss;
int a;

iss.str("\n\r\v\f 123 ");
iss >> a;
cout << (iss?"good":"bad") << ' ' << a << '\n';

iss.str("\n\r\v\f 456 ");
iss >> skipws >> a;
cout << (iss?"good":"bad") << ' ' << a << '\n';

iss.str("\n\r\v\f 789 ");
iss >> noskipws >> a;
cout << (iss?"good":"bad") << ' ' << a << '\n';

good 123
good 456
bad 0

[no]uppercase

cout                << hex << 64206 << ' ' << 1e99 << " end\n"
     << uppercase   << hex << 64206 << ' ' << 1e99 << " end\n"
     << nouppercase << hex << 64206 << ' ' << 1e99 << " end\n";

face 1e+99 end
FACE 1E+99 end
face 1e+99 end

left / right / internal

cout                         << 123 << "★\n"
     << setw(10) <<     left << 123 << "★\n"
     << setw(10) <<    right << 123 << "★\n"
     << setw(10) << internal << 123 << "★\n";

123★
123       ★
       123★
       123★

cout                         << -123 << "★\n"
     << setw(10) <<     left << -123 << "★\n"
     << setw(10) <<    right << -123 << "★\n"
     << setw(10) << internal << -123 << "★\n";

-123★
-123      ★
      -123★
-      123★

cout                                         << -123 << "★\n"
     << setw(10) << setfill('x') <<     left << -123 << "★\n"
     << setw(10) << setfill('x') <<    right << -123 << "★\n"
     << setw(10) << setfill('x') << internal << -123 << "★\n";

-123★
-123xxxxxx★
xxxxxx-123★
-xxxxxx123★

dec / oct / hex

cout        << 10 << ' ' << 30 << ' ' << 50 << '\n'
     << dec << 10 << ' ' << 30 << ' ' << 50 << '\n'
     << oct << 10 << ' ' << 30 << ' ' << 50 << '\n'
     << hex << 10 << ' ' << 30 << ' ' << 50 << '\n';

10 30 50
10 30 50
12 36 62
a 1e 32

cout << bin << 10;

c.cc:1: error: 'bin' was not declared in this scope

fixed / scientific

cout               << 1.2e12 << ' ' << 3.14159 << ' ' << 4.5e-12 << '\n'
     << fixed      << 1.2e12 << ' ' << 3.14159 << ' ' << 4.5e-12 << '\n'
     << scientific << 1.2e12 << ' ' << 3.14159 << ' ' << 4.5e-12 << '\n';

1.2e+12 3.14159 4.5e-12
1200000000000.000000 3.141590 0.000000
1.200000e+12 3.141590e+00 4.500000e-12

setfill

cout                             << 123 << '\n'
                     << setw(10) << 456 << '\n'
     << setfill('*') << setw(10) << 789 << '\n';

123
       456
*******789

setprecision

http://www.cplusplus.com/reference/ios/ios_base/precision/ says:

• Using the default floating-point notation, the precision field specifies the maximum number of meaningful digits to display in total counting both those before and those after the decimal point. Notice that it is not a minimum, and therefore it does not pad the displayed number with trailing zeros if the number can be displayed with less digits than the precision.
• In both the fixed and scientific notations, the precision field specifies exactly how many digits to display after the decimal point, even if this includes trailing decimal zeros. The digits before the decimal point are not relevant for the precision in this case.

setprecision

cout << 123.45 << '\n'
     << setprecision(1) << 123.45 << '\n'
     << setprecision(9) << 123.45 << '\n';

123.45
1e+02
123.45

cout << fixed
     << 123.45 << '\n'
     << setprecision(1) << 123.45 << '\n'
     << setprecision(9) << 123.45 << '\n';

123.450000
123.5
123.450000000

cout << scientific
     << 123.45 << '\n'
     << setprecision(1) << 123.45 << '\n'
     << setprecision(9) << 123.45 << '\n';

1.234500e+02
1.2e+02
1.234500000e+02

setw

setw is not sticky—it is reset upon formatted output:

cout << setw(5) << 1 << 2 << 3 << 4 << '\n';

    1234

It affects more than just numbers:

cout << setw(9) <<   6.7 << '\n'
     << setw(9) <<   'x' << '\n'
     << setw(9) << "foo" << '\n';

      6.7
        x
      foo

It pads with the fill character:

cout << setfill('*') << setw(4) <<     8.9 << '\n'
                     << setw(4) <<     'y' << '\n'
                     << setw(4) <<   "bar" << '\n'
                     << setw(4) << "alpha" << '\n';

*8.9
***y
*bar
alpha

Defining Your Own

It’s easy to define your own I/O manipulators:

ostream &dog(ostream &os) {
    return os << "Kokopelli";
}

int main() {
    cout << "My dog is " << dog << ".\n";
    return 0;
}

My dog is Kokopelli.

Of course, if that’s all you want, just say: const string dog = "Kokopelli";

Better Example

ostream &bucks(ostream &os) {
    return os << fixed << setprecision(2);
}

int main() {
    cout << 34.5 << ' ' << bucks << 78.9 << '\n';
    return 0;
}

34.5 78.90

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