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Feature/add c section #89

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chore(layouts): update default.html layout
FrancescoPaoloL Oct 18, 2025
7187498
feat(languages): add C language section
FrancescoPaoloL Oct 18, 2025
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79 changes: 79 additions & 0 deletions content/languages/c.html
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---
title: Floating-point cheat sheet for C
description: tips for using floating-point, arbitrary-precision, and fixed-point numbers in C
---

Floating-Point Types
---
C has [IEEE 754](https://en.wikipedia.org/wiki/IEEE_754) single and double precision types supported by keywords:

float f = 0.1f;
double d = 0.1;
long double ld = 0.1L;


Because of binary floating-point representation, some decimal fractions cannot be represented exactly (e.g., 1/10 → 0.1 or π), so small rounding errors can occur. In fact, in this naive example we have:

#include <stdio.h>

int main() {
double pi = 3.1415926535897932384626433832795028841971;
double y = 0.1;
printf("%.30f\n", pi + y);
// may show small error
// input: 3.1415926535897932384626433832795028841971
// output: 3.241592653589793204815805438557
}

So we perform rounding.

Decimal Types
-------------
C does not have a built-in decimal type, but exact decimal arithmetic can be done using libraries like [GMP/MPFR](https://gmplib.org/).

#include <gmp.h>

int main(void) {
mpf_set_default_prec(256);
mpf_t pi;
mpf_init_set_str(pi, "3.1415926535897932384626433832795028841971", 10);
// output: 3.14159265358979323846264338327950288419710000000000
gmp_printf("%.50Ff\n", pi);
mpf_clear(pi);
return 0;
}

Otherwise, arbitrary-precision libraries allow you exact storage up to the specified precision but rounding is done manually when needed.

Fixed-Point Types
-----------------
C does not have built-in fixed-point types, but you can implement them using integers with a fixed scaling factor.

#include <stdio.h>

#define SCALE 65536 // 2^16 scaling factor

int main() {
double pi = 3.1415926535897932384626433832795028841971;
// output: 3.1415863037109375 (limited by 16-bit fractional precision

int pi_fixed = (int)(pi * SCALE);
double pi_value = (double)pi_fixed / SCALE;
printf("%.16f\n", pi_value);

return 0;
}


Resources
---------
* [C Standard Library Reference](https://en.cppreference.com/w/c)
* [IEEE 754 Standard](https://en.wikipedia.org/wiki/IEEE_754)
* [float type](https://en.cppreference.com/w/c/language/arithmetic_types#Real_floating_types)
* [double type](https://en.cppreference.com/w/c/language/arithmetic_types#Real_floating_types)
* [long double type](https://en.cppreference.com/w/c/language/arithmetic_types#Real_floating_types)
* [GMP library (GNU Multiple Precision)](https://gmplib.org/)
* [MPFR library (Multiple Precision Floating-Point)](https://www.mpfr.org/)
* [Fixed-point arithmetic](https://en.wikipedia.org/wiki/Fixed-point_arithmetic)
* [Math functions (math.h)](https://en.cppreference.com/w/c/numeric/math)

17 changes: 9 additions & 8 deletions layouts/default.html
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Expand Up @@ -13,13 +13,13 @@
<link rel="stylesheet" type="text/css" href="/style.css?v1" media="screen">
<link rel="shortcut icon" href="/favicon.ico" type="image/vnd.microsoft.icon">
</head>
<body>
<body>
<div id="main">
<h1><%= @item[:title] %></h1>
<%= yield %>

<div id="license">
<p>&copy; Published at <a href="https://floating-point-gui.de/">floating-point-gui.de</a> under the
<p>&copy; Published at <a href="https://floating-point-gui.de/">floating-point-gui.de</a> under the
<a href="http://creativecommons.org/licenses/by/3.0/">Creative Commons Attribution License (BY)</a></p>
</div>
</div>
Expand All @@ -33,22 +33,22 @@ <h2>The Floating-Point Guide</h2>
<li><a href="/references/">References</a></li>
<li><a href="/xkcd/">xkcd</a></li>
</ul>
<h2>Number Formats</h2>

<h2>Number Formats</h2>
<ul>
<li><a href="/formats/binary/">Binary Fractions</a></li>
<li><a href="/formats/fp/">Floating-Point</a></li>
<li><a href="/formats/exact/">Exact Types</a></li>
<li><a href="/formats/integer/">On Using Integers</a></li>
</ul>
<h2>Errors</h2>

<h2>Errors</h2>
<ul>
<li><a href="/errors/rounding/">Rounding</a></li>
<li><a href="/errors/comparison/">Comparison</a></li>
<li><a href="/errors/propagation/">Propagation</a></li>
</ul>

<h2>Language<br>cheat sheets</h2>
<ul>
<li><a href="/languages/csharp/">C#</a></li>
Expand All @@ -60,6 +60,7 @@ <h2>Language<br>cheat sheets</h2>
<li><a href="/languages/ruby/">Ruby</a></li>
<li><a href="/languages/rust/">Rust</a></li>
<li><a href="/languages/sql/">SQL</a></li>
<li><a href="/languages/c/">C</a></li>
</ul>
</div>
<a href="http://github.com/brazzy/floating-point-gui.de"><img style="position: absolute; top: 0; right: 0; border: 0;" src="/forkme_right_white_ffffff.png" alt="Fork me on GitHub" /></a>
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