Bert's blog

Input and output with strings, part 1: output

If you ever wrote a piece of code - proper Fortran or C(++) code, or a script in some other language - you almost certainly at some point got confronted with input and output, either from/to text or binary files stored on the local file system, or from/to the command line terminal. If this is the case, then you probably also got confronted with the need to do conversions: numerical values that are written out or read in as plain text need to be converted from or to strings; for binary in/output individual bytes need to be converted into a variable of the write byte size.

The latter of these is quite straightforward, albeit generally system-dependent as well. If the value you are trying to read/write is a 32-bit (4 byte) integer number, then the conversion simply consists of writing out the individual 4 bytes of this number, or reading them in. The only thing that could go wrong is the order: some systems store integer values from large to small byte components (small endian) while others use the opposite approach. To avoid inter-system confusion, it is therefore a good idea to use standardised binary formats for binary in/output (e.g. HDF5), which means you will need to use an external library for all your input and output. This might be a bit annoying, but also means all conversions will be done for you.

String input and output is - from a programmer’s point of view - much more annoying. Different (scripting) languages use different mechanisms to support it, and while these mechanisms are definitely powerful enough to achieve everything you might want to achieve, it takes some time to master them. In all cases, string output requires you to specify some kind of output format for all numerical values that determines how they are put in a string and with what precision they are displayed. String input requires parsing the input string and retrieving numerical values by applying some regular expressions to it.

In this and a next post, I will discuss these two topics in more detail. This post will focus on string output, i.e. ways of formatting numerical values so that they can be written out.

The standard way

Before I start exploring the complicated ways of formatting output, it is worth pointing out that most programming/scripting languages ship with some form of default formatting, that in many cases is sufficient for your basic output needs.


In Python, terminal and file output is provided by the functions print() and write() (in the soon to be deprecated Python 2, print was a statement instead of a function, so the brackets could be omitted). The print() function support strings:


multiple strings (concatenated with a ` ` separator):

print("Hello", "there!")

And also output values of other types:

var = 42
print("Number:", var)

For numerical types containing a single value, print() will just print the value itself. For arrays, it will print the entire array. If your variable is a Python object, it will either print the internal reference to that object (which looks a bit like the linker symbols I described in a post a while back), or a custom output string provided by the optional __str__() member function for the class. The write() function is unfortunately not as flexible.


I am generally not a big fan of Fortran, but in this case, the default Fortran output mechanisms are actually very good. Fortran has a print statement that is very similar to the Python print() (although it does not add a space in between the different parts of the output string):

var = 42
print *, "Hello ", "there! Var:", var

Even better is that this also works for output to a file using the write() function:

var = 42
open(unit = 1, file = "test.txt")
write(1, *) "Hello, var:", var


In C++, the recommended mechanism for all input and output are streams. For terminal output, this functionality is part of the <iostream> header, and looks like this:

const double var = 42.;
std::cout << "Hello " << "there! Var: " << var << std::endl;

For file output (the <fstream> header), this becomes

const double var = 42.;
std::ofstream ofile("test.txt");
ofile << "Hello, var: " << var << std::endl;

These are very similar to the Python and Fortran output, except that now you explicitly need to iterate for arrays:

const double array[4] = {0., 4., 2., 3.};
std::cout << array[0];
for(int i = 1; i < 4; ++i){
  std::cout << " " << array[i];
std::cout << std::endl;

You can immediately see that this is somewhat annoying if you want to include some separator (there are ways around this, but they are quite complex themselves).


As far as I am aware, C does not have a default way of formatting values for output. This means that all C output always requires format specifiers.

The hard way


Since C does not have a default way to output numerical values, its standard output mechanism is in fact immediately the hard way. So let’s start there.

To output to the terminal, C provides the printf() function (contained in the stdio.h header):


Unlike the print functions we encountered above, this function only takes a single string argument, plus an unspecified number of additional arguments that matches the number of format specifiers contained in this string. These format specifiers are essentially placeholders for these additional arguments in the string, and their type needs to match the type of the corresponding argument. For e.g. a string, the format specifier is %s:

printf("Hello %s!", "there");

For numerical values, the format specifiers depend on the type, and sometimes also on the byte size of the variable. For floating point values, the easiest format specifier is %g:

const double var = 42.;
printf("Var: %g", var);

This will use either exponential notation (format specifier %e or %E depending on whether you want the exponential sign to be small or capital; the latter requires %G) or floating point notation (%f), depending on which one results in the shortest output. This works for both single and double precision values (but not for long double precision values).

For integers, the format specifier %i or %u can be used, depending on whether the integer is signed or not. If you want to output in hexadecimal notation, you can use %x or %X, and for pointer variables there is the specifier %p. This only works for integer values with a size of 32 bits or less. For larger values (long integer), an additional l needs to be added to the format specifier (e.g. %lu), and for even longer values this could sometimes be two (the compiler will tell you if you didn’t add enough).

These basic format specifiers can be extended with additional parts, like this:


The length part is the l already mentioned before. The flags part gives additional information about justification, padding and whether or not a sign character/hexadecimal prefix is written out. The width part specifies the minimum width of the output string in number of characters (shorter output is padded until this width is reached), and the .precision (always preceded by the .) gives the precision as number of digits behind the decimal point (%e/E or %f) or as total number of significant digits (%g/G). Note that the width and precision parts also accept the value *, which means the value is provided as an additional argument to the printf function that precedes the argument that is printed.

For file output, the format specifiers are the same, but the fprintf function needs to be used. This function takes an additional first argument that points to a file to write to.


The format specifiers described above also apply to C++, where the printf function is also available. An additional complication can arise when the values to be printed are the more flexible C++ integer types contained in the header <cinttypes>, since these types can have different byte sizes on different systems. For convenience, <cinttypes> defines macros that contain the correct format specifiers for these cases:

const uint_fast32_t var = 42;
printf("Var: %" PRIuFAST32, var);

Note the space in between %" and PRIuFAST32 that is obligatory in this case. Also note the absence of commas in the first argument.

Alternatively, the output streams that are more recommended in C++ also have features to specify output precision and formatting. To change from the default decimal output to hexadecimal or octal for example, you can use

const uint_fast32_t var = 42;
std::cout << std::hex;
std::cout << "Hexadecimal var: " << var << std::endl;
std::cout << std::oct;
std::cout << "Octal var: " << var << std::endl;

To change the width or precision, you can use setw and setprecision:

std::cout << setw(10);
std::cout << setprecision(8);

The default precision is 6.


In Fortran, format specifiers can be provided as follows:

var = 42
print "Var: I3", var

Generally, Fortran only has two important formats: I for integer values and F (or E) for floating point values. I takes one required and one optional additional argument:

I[width].[minimum width]

width is the maximum width of the string in characters (values that are longer than this will not be displayed properly), minimum width the mimimal width (shorter values will be padded).

For F and E, the additional arguments are



E[width][.precision]E[exponent width]

Note that all format specifiers can optionally be preceded by a counter that duplicates the given format specifier, separated by spaces:

var1 = 42
var2 = 43
print "2I3", var1, var2


Python - being Python - offers a range of different format specifiers. My personal favourite is the format() function for strings:

var = 42
print("Var: {0}".format(var))

Here, every occurrence of curly brackets is replaced with an argument to the format() function. The curly brackets either contain the index (starting from 0) of the argument in the list of arguments (arguments can be used multiple times or not be used at all), or a label that identifies an argument:

var = 42
print("Var: {var}".format(var = var))

If arguments are simply used in the order they are given, the indices and labels can also be omitted:

var1 = 42
var2 = 43
print("{} {}".format(var1, var2))

Type specifiers can also be provided as an additional argument to a value placeholder, separated by a colon (:):

var = 42.
print("Var: {0:.2f}".format(var))

The format specifiers are reasonably straightforward: d, b, o and x/X for respectively decimal, binary, octal and hexadecimal integers, f for floating point values and e for exponential notation. Additional flags, width and precision arguments can be specified as well, similar to C(++). For a zero-padded 3 digit counter for example, you can use

var = 42
print("Var: {0:03d}".format(var))

Note that it is possible to unpack arrays or lists as arguments for format(). Values will however only be displayed if they also have a placeholder in the string:

a = [42., 32., 12.]
print("Format: {} {} {}".format(*a))

Alternatively, Python also supports an old syntax, that is very similar but less flexible, and makes use of %:

var = 42
print("Var: %i" % (var))

Python 3 has an even more flexible way of formatting strings, called f strings. They allow you to include variable names, function calls and even code statements in a string:

def add(a, b):
  return a + b

var1 = 42
var2 = 43
print(f"Var1: {var1}")
print(f"var1 + var2: {var1 + var2}")
print(f"var1 + var2: {add(var1, var2)}")

Apart from this, the syntax is pretty much the same as for the format() function. F strings are supposedly faster than their % and format() counterparts, although I am not really familiar with their usage (yet).

Professional astronomer.