Field Width and Decimal Precision (sec 3-5)
By default, C++ prints real numbers with zero to six digits after the decimal point. This is
not necessarily what we want, e.g. dollars and cents.
I'll just give you the code to specify the decimal precision, the number of digits after the
decimal point, without any explanation. Add these two lines to a program in which you
want to print real values with two decimal places:
cout.setf(ios::fixed,ios::floatfield);
cout.precision(2);
The first line tells the computer that we want the line cout.precision() to specify the number
of decimal places. After this is done, the line cout.precision(2) specifies that we want
floating point numbers to print with two decimal places.
Notes:
 If you omit the call to cout.setf(), the call to cout.precision() will specify the number
of significant digits.
 A call to cout.precision() and cout.setf() stays in effect for the rest of the program or
until the next call that overrides it. This is called persistent.
double cost = 43.67645;
cout << cost;
The output from this (in at least one compiler) will be 43.6765, with a total of six positions.
We can print a different number of digits after the decimal point:
cout.setf(ios::fixed,ios::floatfield);
cout.precision(2);
cout << cost << endl;
cout.precision(6);
cout << cost << endl;
The output is the following:
43.68
43.676450
Note that the values are rounded off: 43.6735 becomes 43.67
Setting the Field Width: width()
To specify the exact number of character positions a value should occupy when it is
printed, use a call to cout.width().
EXAMPLE 3-8a:
In Program 2 in the text, they wanted to print the values of gpa and result neatly aligned in
columns; here are two of those sets of values:
1.5
2
154.5
12.6
In Chapter 2, we created columns by using tabs, but a nicer way is to use calls to
cout.width() inside the loop:
cout.width(10);
cout << gpa;
cout.width(10);
cout << result;
Output: The values would print right-aligned in columns 10 spaces wide:
1.5
2
154.5
12.6
WARNING,Warning,Warning
A call to cout.width() applies only to the very next value to be printed. It doesn't even apply
to two values printed in the same cout. This is different from calls to cout.precision() and
cout.setf().
Left and Right Alignment
Once you have specified a field width using cout.width(), you can also specify whether you
want the value left or right aligned within its field. Left aligned means starting in the far left
position in the field; right aligned means ending in the far right position. Right alignment is
the default for numbers.
Default is right alignment. To set alignment, use a call to cout.setf(), specifying left or right
alignment:
cout.setf(ios::left);
cout.setf(ios::right);
// aligns left
// aligns right
The value set in this way remains in effect for the rest of the program or until the next call
that overrides it. What do we call that? Persistent.
You must know above for the final.
SEE try_i_o.cpp for a better way. Not in the text. Must know for my exam.
There are additional I/O stream manipulators defined in iomanip. They ease formatting.
It is a common practice for C++ programmers to #include<iomanip> along with
<iostream>.
Need fixed for setprecision to be digits after decimal point, rather than just digits.
setw() is not persistent. It applies only to the next item.
cout << left
inserts a left justification flag in the output stream.
You can embed the modifier in the output stream. You can put multiple values in one
cout statement:
cout << fixed << setprecision(2)
setw(10) << result;
If-else SEE my handwritten notes.
<<
setw(10)
<<
gpa
<<