Session 1: Modern C Chapters 1-3
May 16, 2023
/* This may look like nonsense, but really is -*- mode: C -*- */
#include <stdlib.h>
#include <stdio.h>
/* The main thing that this program does. */
int main(void) {
// Declarations
double A[5] = {
[0] = 9.0,
[1] = 2.9,
[4] = 3.E+25,
[3] = .00007,
};
// Doing some work
for (size_t i = 0; i < 5; ++i) {
printf("element %zu is %g, \tits square is %g\n",
i,
A[i],
A[i]*A[i]);
}
return EXIT_SUCCESS;
}Find which compilation command works for you.
element 0 is 9, its square is 81
element 1 is 2.9, its square is 8.41
element 2 is 0, its square is 0
element 3 is 7e-05, its square is 4.9e-09
element 4 is 3e+25, its square is 9e+50bad.c:4:1: warning: return type of 'main' is not 'int' [-Wmain-return-type]
void main() {
^
bad.c:4:1: note: change return type to 'int'
void main() {
^~~~
int
bad.c:16:6: error: call to undeclared library function 'printf' with type 'int (const char *, ...)'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
printf("element %d is %g, \tits square is %g\n", /*@\label{printf-start-badly}*/
^
bad.c:16:6: note: include the header <stdio.h> or explicitly provide a declaration for 'printf'
bad.c:22:3: error: void function 'main' should not return a value [-Wreturn-type]
return 0; /*@\label{main-return-badly}*/
^ ~/* This may look like nonsense, but really is -*- mode: C -*- */
#include <stdio.h>
/* The main thing that this program does. */
int main() {
// Declarations
int i;
double A[5] = {
9.0,
2.9,
3.E+25,
00007
};
// Doing some work
for (i = 0; i < 5; ++i) {
printf("element %d is %g, \tits square is %g\n",
i,
A[i],
A[i]*A[i]);
}
return 0;
}/* This may look like nonsense, but really is -*- mode: C -*- */
#include <stdio.h>
/* The main thing that this program does. */
int main() {
// Declarations
int i;
double A[5] = {
[0] = 9.0,
[1] = 2.9,
[4] = 3.E+25,
[3] = .00007
};
// Doing some work
for (i = 0; i < 5; ++i) {
printf("element %d is %g, \tits square is %g\n",
i,
A[i],
A[i]*A[i]);
}
return 0;
}-Werror to not create an executable even when there are just warnings#include, int, void, double, for, return
{}, (), [], /**/, and <>{}/* */ multiline, // one lineA, isize_t, specify sort of object
main
main must be present, it is the starting point of executionEXIT_SUCCESS= for initialization and assignment++ to increment<> only for specifying headers to include{} embracing body of function, embracing elements of array (vector), embracing multi-line conditional statement[] specifying length of array, specifying elements of array, indexing/* This may look like nonsense, but really is -*- mode: C -*- */
#include <stdlib.h>
#include <stdio.h>
/* The main thing that this program does. */
int main(void) {
// Declarations
double A[5] = {
[0] = 9.0,
[1] = 2.9,
[4] = 3.E+25,
[3] = .00007,
};
// Doing some work
for (size_t i = 0; i < 5; ++i) {
printf("element %zu is %g, \tits square is %g\n",
i,
A[i],
A[i]*A[i]);
}
return EXIT_SUCCESS;
}All identifiers have to be declared (maybe not directly in our program)
int main(void)
double A[5]
size_t i
printf declared in stdio.h
size_t and EXIT_SUCCESS come from stdlib.h
To look up something declared in a header file
{}, e.g. for loop{}, so limited by file -> globals[] to designate specific elements (e.g. in array)Instructions telling compiler what to do with declared identifiers
for
{}() or reuse for several loops(vs “call be reference” which allows value of variable to be changed)
Details about how objects are defined. Some main points
Recommends “One True Brace Style”
int main (int argc , char * argv [ argc +1]) {
puts (" Hello world !");
if ( argc > 1) {
while ( true ) {
puts (" some programs never stop ");
}
} else {
do {
puts ("but this one does ");
} while ( false );
}
return EXIT_SUCCESS ;
}There is no standard in base R (see R Dev Guide #133) but K&R is used more and just a minor variant (as above, but first { on new line).
As in R
0 represents logical false
Any value different from 0 represents logical true.
Don’t compare to 0, false, or true as redundant
if(i) found 5 times in base Rif (i != 0) found once/* This may look like nonsense, but really is -*- mode: C -*- */
#include <stdlib.h>
#include <stdio.h>
/* The main thing that this program does. */
int main(void) {
// Declarations
double A[5] = {
[0] = 9.0,
[1] = 2.9,
[4] = 3.E+25,
[3] = .00007,
};
// Doing some work
for (size_t i = 0; i < 5; ++i) {
if (i) {
printf("element %zu is %g, \tits square is %g\n",
i,
A[i],
A[i]*A[i]);
}
}
return EXIT_SUCCESS;
}$ ./getting-started-if
element 1 is 2.9, its square is 8.41
element 2 is 0, its square is 0
element 3 is 7e-05, its square is 4.9e-09
element 4 is 3e+25, its square is 9e+50for loopsWe can have multiple loops iterating on i as long as scopes don’t overlap. [?]
[Exs 2] Try to imagine what happens when i has value 0 and is decremented by means of the operator --
while and dowhile works as in R
do is similar to while but the condition after the dependent block:
do { // Iterates
x *= (2.0 - a*x); // Heron approximation
} while ( fabs (1.0 - a*x) >= eps ); // Iterates until close=> if condition false, will iterate once before closing.
Using {} recommended. Note ; after while statement after do {}.
break and continuebreak works as in R
continue is equivalent to next in R
for (;;) is equivalent to while(true) (former appears 23 times; latter just once in R sources)
#include <stdlib.h>
#include <stdio.h>
/* lower and upper iteration limits centered around 1.0 */
static double const eps1m01 = 1.0 - 0x1P-01;
static double const eps1p01 = 1.0 + 0x1P-01;
static double const eps1m24 = 1.0 - 0x1P-24;
static double const eps1p24 = 1.0 + 0x1P-24;
int main(int argc, char* argv[argc+1]) {
for (int i = 1; i < argc; ++i) { // process args
double const a = strtod(argv[i], 0); // arg -> double
double x = 1.0;
for (;;) { // by powers of 2
double prod = a*x;
if (prod < eps1m01) {
x *= 2.0;
} else if (eps1p01 < prod) {
x *= 0.5;
} else {
break;
}
}
for (;;) { // Heron approximation
double prod = a*x;
if ((prod < eps1m24) || (eps1p24 < prod)) {
x *= (2.0 - prod);
} else {
break;
}
}
printf("heron: a=%.5e,\tx=%.5e,\ta*x=%.12f\n",
a, x, a*x);
}
return EXIT_SUCCESS;
}heron.c using printfAdd condition print statement to loops, e.g.
$./heron 0.07 5 6E+23
i = 1, loop 1, x: 2.000000
i = 1, loop 1, x: 4.000000
i = 1, loop 1, x: 8.000000
i = 1, loop 1, x: 16.000000
i = 1, loop 2, x: 14.080000
i = 1, loop 2, x: 14.282752
i = 1, loop 2, x: 14.285714
heron: a=7.00000e-02, x=1.42857e+01, a*x=0.999999957002argc and argvint main(int argc, char* argv[argc+1]) {
printf("argc: %d\n", argc);
printf("argv[0]: %s\n", argv[0]);
... $./heron 0.07 5 6E+23
argc: 4
argv[0]: ./heron
argv[1]: 0.07
argv[2]: 5
argv[3]: 6E+23
argv[4]: (null)main receives an array of pointers to char: the program name, argc-1 program arguments, and one null pointer that terminates the array.
eps1m01Add condition print statement to loops, e.g.
$./heron 0.07 5 6E+23
eps1m01: 0.750000
i = 1, loop 1, x: 2.000000
i = 1, loop 1, x: 4.000000
i = 1, loop 1, x: 8.000000
i = 1, loop 1, x: 16.000000
i = 1, loop 2, x: 14.080000
i = 1, loop 2, x: 14.282752
i = 1, loop 2, x: 14.285714
heron: a=7.00000e-02, x=1.42857e+01, a*x=0.999999957002Originally eps1m01: 0.5.
switchIn R, switch only returns output for one matching case.
In C, switch jumps to matching case and continues to end or break.
With help from https://overiq.com/c-programming-101/the-switch-statement-in-c/ to get character input from scanf
#include <stdlib.h>
#include <stdio.h>
int main(void){
char arg;
printf("Enter arg: ");
scanf(" %c", &arg);
switch (arg) {
case 'm': puts (" this is a magpie ");
break ;
case 'r': puts (" this is a raven ");
break ;
case 'j': puts (" this is a jay ");
// break ;
case 'c': puts (" this is a chough ");
break ;
default : puts (" this is an unknown corvid ");
}
return EXIT_SUCCESS;
}$ ./switch
Enter arg: r
this is a raven
$ ./switch
Enter arg: j
this is a jay
this is a chough