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Creating cleanly formatted output is a common programming requirement--itimproves your user interface and makes it easier to read any debuggingmessages that you might print to the screen. In C, formatted output works viathe printf statement, but in C++, you can create nicely formatted output tostreams such as cout. This tutorial covers a set of basic I/O manipulationspossible in C++ from the iomanip header file. Note that all of the functionsin the iomanip header are inside the std namespace, soyou will need to either prefix your calls with 'std::' or put 'using namespacestd;' before using the functions.

Jul 08, 2018  code iomanip /codeis used to set up certain format parameters, so the question can actually mean two things: 1. Why do we set format parameters 2. We use iomanip istead of simple functions. Question 2 is a matter of history and habits: codec. C Programming Language Tutorial - C Manipulators (endl, setw, setprecision, setf). A humble request Our website is made possible by displaying online advertisements to our visitors. Please consider supporting us by disabling your ad blocker on our website. /.undefined./ setw (int n); Set field width. Sets the field width to be used on output operations. Behaves as if member width were called with n as argument on the stream on which it is inserted/extracted as a manipulator (it can be inserted/extracted on.

Dealing with Spacing Issues using iomanip

A principle aspect of nicely formatted output is that the spacing looks right.There aren't columns of text that are too long or too short, and everything isappropriately aligned. This section deals with ways of spacing outputcorrectly.

Setting the field width with setw

The std::setw function allows you to set the minimum width of the next outputvia the insertion operator. setw takes, one argument, the width of the nextoutput (insertion), an integer. if the next output is too short, then spaceswill be used for padding. There is no effect if the output is longer than thewidth--note that the output won't be truncated. The only strange thing aboutsetw is that its return value must be inserted into the stream. The setwfunction has no effect if it is called without reference to a stream.A simple example isThe output from the above would look like this:Note that since setw takes an argument, at runtime it would be possible tospecify the width of a column of output so that it is slightly wider than thelongest element of the column.
You might wonder whether it is possible to change the padding character. Itturns out that yes, you can, by using the setfill function, which takes acharacter to use for the padding. Note that setfill should also be used as astream manipulator only, so it must be inserted into the stream:The above code sets the padding character to a dash, the width of the nextoutput to be at least 80 characters, and then outputs a dash. This results inthe rest of the line being filled with dashes too. The output would look likethis:Note that the pad character is changed until the next time you call setfill tochange it again.

Aligning text with iomanip

It's possible to specify whether output is left or right aligned by using themanipulator flags that are part of ios_bas. In particular, it is possible tospecify that output should be either left or right aligned by passing inthe stream manipulators std::left and std::right.

Putting Your Knowledge of iomanip Together

Now that we know how to space and align text, we can correctly print formatteddata in columns. For instance, if you had a struct containing the names ofindividuals:If you then had a vector of persons, then you could output them in a nice waywith evenly spaced columns for the first and last name as follows:Note that the space output between the two fields wasn't strictly necessarybecause we could have added it by changing the first call to setw to set thewidth to one more than the longest first name (since it would use a space asthe padding for the extra character).

Printing Numbers

Another challenge in creating nice output is correctly formatting numbers; forinstance, when printing out a hexadecimal value, it would be nice if it werepreceded by the '0x' prefix. More generally, it's nice to correctly set thenumber of trailing zeros after a decimal place.

Setting the precision of numerical output with setprecision

The setprecision function can be used to set the maximum number of digits thatare displayed for a number. Like setw, it should be inserted into the stream.In fact, its usage is very similar to setw in all respects. For instance, toprint the number 2.71828 to 3 decimal places:Note that setprecision will change the precision until the next time it ispassed into a given stream. So changing the above example to also print out1.412 would result in the output of

C++ Setw Center

Output in different bases

In computer science, frequently numbers need to be printed in octal orhexadecimal. The setbase function returns a value that can be passed into astream to set the base of numbers to either base 8, 10, or 16. The inputnumber is still read as a number in base ten, but it is printed in the givenbase. For instance, will print out '20', which is 32 written in base 16. Note that you can usedec, oct, and hex as shorthand for setbase(10), setbase(8), and setbase(16)respectively when inserting into a stream. If you wish to include an indication of the base along with the printednumber, you can use the setiosflags function, again passed into a stream, withan input of ios_base::showbase.Using the ios_base::showbase flag will append a '0x' in front of hexadecimalnumbers and a 0 in front of octal numbers. Decimal numbers will be printed asnormal.This should get you started with the ability to create nicely formatted outputin C++ without having to resort to returning to printf!

C++ How To Use Setw

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