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Merge pull request #4097 from Naereen/patch-7

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[c/en] Just a few explanations, a couple of tiny changes
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Adam Bard 2021-02-24 09:35:00 -08:00 committed by GitHub
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1 changed files with 31 additions and 17 deletions
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@ -224,10 +224,18 @@ int main (int argc, char** argv)
(float)i1 / i2; // => 0.5f
i1 / (double)i2; // => 0.5 // Same with double
f1 / f2; // => 0.5, plus or minus epsilon
// Floating-point numbers and calculations are not exact
// for instance it is not giving mathematically correct results
(0.1 + 0.1 + 0.1) != 0.3; // => 1 (true)
// and it is NOT associative
1 + (1e123 - 1e123) != (1 + 1e123) - 1e123; // => 1 (true)
// this notation is scientific notations for numbers: 1e123 = 1*10^123
// Modulo is there as well
11 % 3; // => 2
// Modulo is there as well, but be careful if arguments are negative
11 % 3; // => 2 as 11 = 2 + 3*x (x=3)
(-11) % 3; // => -2, as one would expect
11 % (-3); // => 2 and not -2, and it's quite counter intuitive
// Comparison operators are probably familiar, but
// there is no Boolean type in C. We use ints instead.
@ -236,12 +244,12 @@ int main (int argc, char** argv)
// operators always yield 0 or 1.)
3 == 2; // => 0 (false)
3 != 2; // => 1 (true)
3 > 2; // => 1
3 < 2; // => 0
3 > 2; // => 1
3 < 2; // => 0
2 <= 2; // => 1
2 >= 2; // => 1
// C is not Python - comparisons don't chain.
// C is not Python - comparisons do NOT chain.
// Warning: The line below will compile, but it means `(0 < a) < 2`.
// This expression is always true, because (0 < a) could be either 1 or 0.
// In this case it's 1, because (0 < 1).
@ -349,25 +357,30 @@ int main (int argc, char** argv)
break;
}
/*
using "goto" in C
Using "goto" in C
*/
typedef enum { false, true } bool;
// for C don't have bool as data type before C99 :(
bool disaster = false;
int i, j;
for(i=0;i<100;++i)
for(j=0;j<100;++j)
for(i=0; i<100; ++i)
for(j=0; j<100; ++j)
{
if((i + j) >= 150)
disaster = true;
if(disaster)
goto error;
goto error; // exit both for loops
}
error :
error: // this is a label that you can "jump" to with "goto error;"
printf("Error occurred at i = %d & j = %d.\n", i, j);
/*
https://ideone.com/GuPhd6
this will print out "Error occurred at i = 51 & j = 99."
https://ideone.com/GuPhd6
this will print out "Error occurred at i = 51 & j = 99."
*/
/*
it is generally considered bad practice to do so, except if
you really know what you are doing. See
https://en.wikipedia.org/wiki/Spaghetti_code#Meaning
*/
///////////////////////////////////////
@ -741,11 +754,12 @@ typedef void (*my_fnp_type)(char *);
// Order of Evaluation
///////////////////////////////////////
// From top to bottom, top has higher precedence
//---------------------------------------------------//
// Operators | Associativity //
//---------------------------------------------------//
// () [] -> . | left to right //
// ! ~ ++ -- + = *(type)sizeof | right to left //
// ! ~ ++ -- + = *(type) sizeof | right to left //
// * / % | left to right //
// + - | left to right //
// << >> | left to right //
@ -783,8 +797,8 @@ as the C file.
/* included into files that include this header. */
#include <string.h>
/* Like c source files macros can be defined in headers and used in files */
/* that include this header file. */
/* Like for c source files, macros can be defined in headers */
/* and used in files that include this header file. */
#define EXAMPLE_NAME "Dennis Ritchie"
/* Function macros can also be defined. */
@ -823,7 +837,7 @@ Best to find yourself a copy of [K&R, aka "The C Programming Language"](https://
It is *the* book about C, written by Dennis Ritchie, the creator of C, and Brian Kernighan. Be careful, though - it's ancient and it contains some
inaccuracies (well, ideas that are not considered good anymore) or now-changed practices.
Another good resource is [Learn C The Hard Way](http://learncodethehardway.org/c/).
Another good resource is [Learn C The Hard Way](http://learncodethehardway.org/c/) (not free).
If you have a question, read the [compl.lang.c Frequently Asked Questions](http://c-faq.com).
@ -833,4 +847,4 @@ Readable code is better than clever code and fast code. For a good, sane coding
Other than that, Google is your friend.
[1] [Why isn't sizeof for a struct equal to the sum of sizeof of each member?](http://stackoverflow.com/questions/119123/why-isnt-sizeof-for-a-struct-equal-to-the-sum-of-sizeof-of-each-member)
[1] [Why isn't sizeof for a struct equal to the sum of sizeof of each member?](https://stackoverflow.com/questions/119123/why-isnt-sizeof-for-a-struct-equal-to-the-sum-of-sizeof-of-each-member)