Ëåêöèÿ 3 (20.09.2021)

Èíäóêòèâíûå ôóíêöèè -

íàä ïîñëåäîâàòåëüíîñòÿìè ÷èñåë â ôàéëå.

Çàäà÷à 1. ßâëÿåòñÿ ëè ïîñëåäîâàòåëüíîñòü ïîñòîÿííîé?

Íå èñïîëüçîâàòü feof()!!!

#include <stdio.h>

int main(void) {

FILE *IN;

int first, num;

IN = fopen("tmp.dat", "r");

if (IN == NULL) {

printf("File not opened\n");

return -1;

}

if (fscanf(IN, "%d", &first) != 1) {

printf("File empty\n");

fclose(IN);

return -1;

}

while (fscanf(IN, "%d", &num) == 1) {

if (num != first) {

printf("No\n");

fclose(IN);

return 0;

}

}

printf("Yes\n");

fclose(IN);

return 0

}

Âîïðîñ. Çà÷åì çàêðûâàòü ôàéëû?

Îòâåò.

1. Â ôàéëàõ, îòêðûòûõ íà çàïèñü, íå ñîõðàíÿòñÿ èçìåíåíèÿ.

2. Âñå îòêðûòûå ôàéëû çàíîñÿòñÿ â òàáëèöó îòêðûòûõ ôàéëîâ

â îïåðàöèîííîé ñèñòåìå, îáùàÿ äëÿ âñåõ ïðîöåññîâ

(òàáëèöà äåñêðèïòîðîâ).

Ðàçìåð òàáëèöû îãðàíè÷åí, ïîýòîìó

íåíóæíûå ôàéëû íàäî çàêðûâàòü, ÷òîáû òàáëèöà íå

ïåðåïîëíèëàñü.

Ïðèìåð 1. Íàéòè ìàêñèìàëüíûé ðàçìåð òàáëèöû äåñêðèïòîðîâ

(â ôàéëàõ)

#include <stdio.h>

int main(void) {

FILE *IN;

int i = 0;

while (1) { /* Inifinite loop */

IN = fopen("tmp.dat", "r");

if (IN == NULL)

break;

++i;

}

printf("Files opened: %d\n", i);

return 0;

}

Âûïîëíåíèå ýòîé ïðîãðàììû:

Ðåçóëüòàò 3197 îòêðûòûõ ôàéëîâ.

Çàäà÷à 2. Íàéòè çíà÷åíèå ìíîãî÷ëåíà, êîýôôèöèåíòû

êîòîðîãî íàõîäÿòñÿ â ôàéëå "tmp.dat" â ïîðÿäêå âîçðàñòàíèÿ

ñòåïåíåé, â òî÷êå x, çàäàâàåìîé ñ êëàâèàòóðû.

Ïðèìåð 2.  ôàéëå - ÷èñëà:

1

0

-1

Çíà÷åíèå ýòèõ ÷èñåë:

1 - ýòî êîýôôèöèåíò ïðè íóëåâîé ñòåïåíè ìíîãî÷ëåíà

0 - ïðè ïåðâîé

-1 - ïðè âòîðîé ñòåïåíè

Ïîëó÷àåòñÿ, ÷òî ôàéëå çàïèñàí ìíîãî÷ëåí:

y = -x*x + 0 * x +1

#include <stdio.h>

int main(void) {

double res = 0;

double a;

double x;

double xn = 1; /* This is x in degree of current n */

FILE *IN;

IN = fopen("tmp.dat", "r");

if (IN == NULL) {

printf("File not opened\n");

return -1;

}

printf("Input x\n");

scanf("%lf", &x);

while (fscanf(IN, "%lf", &a) == 1) {

res += a * xn;

xn *= x;

}

fclose(IN);

printf("Res: %lf\n", res);

return 0;

}

Ñîìåðñåò Ìîýì: "Ãëàâíîå â ïðîãðàììå - ÷òîáû îíà áûëà âûïîëíåíà"

Âûïîëíåíèÿ ïðîãðàììû-ðåøåíèÿ çàäà÷è 2:

Ôîðìàòû (îòâåò íà âîïðîñ èç çàëà):

%lf – ôîðìàò ÷òåíèÿ/çàïèñè âåùåñòâåííîãî ÷èñëà äâîéíîé òî÷íîñòè (Long Float)

%d - ôîðìàò ÷òåíèÿ/çàïèñè äåñÿòè÷íîãî öåëîãî ÷èñëà (Decimal)

man printf – ìîæíî ïîñìîòðåòü âñå ôîðìàòû (êîìàíäà man – êîìàíäà Ëèíóêñà, êîòîðàÿ âûäàåò êîðîòêóþ äîêóìåíòàöèþ ïî ôóíêöèÿì). Âûõîä — êîìàíäîé «q”.

info printf – áîëåå ïîëíàÿ äîêóìåíòàöèÿ. Âûõîä — êîìàíäîé «:q”.

Âû÷èñëåíèå ïðîèçâîäíîé äëÿ çàäà÷è 2:

#include <stdio.h>

int main(void) {

double val = 0, proizv = 0;

double a;

double x;

double xn = 1; /* This is x in degree of current n */

int n = 0;

FILE *IN;

IN = fopen("tmp.dat", "r");

if (IN == NULL) {

printf("File not opened\n");

return -1;

}

printf("Input x\n");

scanf("%lf", &x);

if (fscanf(IN, "%lf", &a) != 1) {

printf(“File empty\n”);

fclose(IN);

return -1;

}

val +=a;

++n;

while (fscanf(IN, "%lf", &a) == 1) {

proizv += n * a * xn; /* xn == x^(n-1) */

xn *= x;

val += a * xn; /* xn == x^n because already multiplied by x in the line above */

++n;

}

fclose(IN);

printf("Val: %lf, proizv: %lf\n", val, proizv);

return 0;

}

Ðåçóëüòàò âûïîëíåíèÿ ýòîé ïðîãðàììû:

Çàäà÷à 3. Íàéòè çíà÷åíèå ìíîãî÷ëåíà, êîýôôèöèåíòû

êîòîðîãî íàõîäÿòñÿ â ôàéëå "tmp.dat" â ïîðÿäêå óáûâàíèÿ

ñòåïåíåé, â òî÷êå x, çàäàâàåìîé ñ êëàâèàòóðû

(çà îäíî ïðî÷òåíèå ôàéëà, íåëüçÿ ïîëüçîâàòüñÿ äîïîëíèòåëüíîé

ïàìÿòüþ)

Ïðèìåð 2.  ôàéëå - ÷èñëà:

1

0

-1

1 - ýòî êîýôôèöèåíò ïðè âòîðîé ñòåïåíè ìíîãî÷ëåíà

0 - ïðè ïåðâîé

-1 - ïðè íóëåâîé ñòåïåíè

Ïîëó÷àåòñÿ ìíîãî÷ëåí:

y = 1*x*x + 0 * x - 1

y = a_n * xⁿ + a_(n-1) * x^(n-1) + ... + a₁ * x¹ + a₀ * x⁰

Ñõåìà Ãîðíåðà:

y = ((...(((a_n)*x + a_(n-1))*x + a_(n-2))*x + ...)*x+a1)*x+a0

Ðåøåíèå çàäà÷è 3:

#include <stdio.h>

int main(void) {

double res = 0;

double a;

double x;

FILE *IN;

IN = fopen("tmp.dat", "r");

if (IN == NULL) {

printf("File not opened\n");

return -1;

}

printf("Input x\n");

scanf("%lf", &x);

while (fscanf(IN, "%lf", &a) == 1) {

res = res*x + a;

}

fclose(IN);

printf("Res: %lf\n", res);

return 0;

}

Âûïîëíåíèå ýòîé ïðîãðàììû:

Óïðàæíåíèå: íàéòè çíà÷åíèå ìíîãî÷ëåíà è åãî ïðîèçâîäíîé, åñëè ìíîãî÷ëåí çàäàí, êàê â çàäà÷å 2 (êîýôôèöèåíòàìè ïî óáûâàíèþ ñòåïåíåé)

Çàäà÷à 4. Ñ÷èòàòü öåëîå ÷èñëî ñ êëàâèàòóðû è íàéòè

÷èñëî ñ òåìè æå öèôðàìè, ÷òî è èñõîäíîãî, íî

èäóùèìè â îáðàòíîì ïîðÿäêå, è âûâåñòè åãî

Ïðèìåð 3. 1234 -> 4321

(((4)*10 + 3)*10 + 2)*10 + 1 == 4321 (êàæäàÿ öèôðà — êîýôôèöèåíò ïðè 10ⁿ)

Ïîäñêàçêà: íàéòè ïîñëåäíþþ öèôðó öåëîãî ÷èñëà ìîæíî ïðè ïîìîùè îïåðàöèè «%» - îñòàòîê îò äåëåíèÿ. Íàïðèìåð, 21%6 == 3

1234%10 → 4, 1234/10 ->123

123%10 → 3, 123/10 →12

12%10 → 2, 12/10 → 1

1%10 → 1, 1/10 → 0 - âûõîä

Ðåçóëüòàò - 4321

#include <stdio.h>

int main(void) {

int n, last_digit, res = 0;

printf("Input integer number\n");

scanf("%d", &n);

while(n > 0) {

last_digit = n % 10;

res = res * 10 + last_digit;

n /= 10;

}

printf("Res: %d\n", res);

return 0;

}

Ïðèâåäåíèå òèïîâ:

Íàïðèìåð, åñëè âñå îïåðàíäû â âûðàæåíèè öåëûå,

òî ðåçóëüòàò òîæå áóäåò öåëûé (íåÿâíîå ïðèâåäåíèå òèïîâ â ÿçûêå C).

while:

last_digit res n

1234

1. 4 4 123

2. 3 43 12

3. 2 432 1

4. 1 4321 0

while(n) - n==0 => óñëîâèå öèêëà íå âûïîëíåíî => âûõîä

èç öèêëà

Âûïîëíåíèå ýòîé ïðîãðàììû:

Ïðèìåð 4. Êàê â ÿçûêå C ïîäåëèòü öåëûå ÷èñëà, ÷òîáû ðåçóëüòàò áûë âåùåñòâåííûé?

Ðåçóëüòàò âûïîëíåíèÿ ïðîãðàììû ñ ÿâíûì ïðåîáðàçîâàíèåì òèïîâ:

Çàäà÷à 5.  ïîñëåäîâàòåëüíîñòè ÷èñåë, çàïèñàííûõ â ôàéëå

"tmp.dat", íàéòè êîëè÷åñòâî ëîêàëüíûõ ìàêñèìóìîâ

(çà îäíî ïðî÷òåíèå ôàéëà).

Îïðåäåëåíèå. Ëîêàëüíûé ìàêñèìóì â ïîñëåäîâàòåëüíîñòè

÷èñåë - ýòî ÷èñëî, êîòîðîå íå ìåíüøå ñâîèõ ñîñåäåé.

Ïðèìåð:

5

0

5

-1

2

5

5

Âîïðîñ: Ñêîëüêî ëîêàëüíûõ ìàêñèìóìîâ? Îòâåò: 4

#include <stdio.h>

int main(void) {

int res = 0;

int a, b, c;

FILE *IN;

IN = fopen("tmp.dat", "r");

if (IN == NULL) {

printf("File not opened\n");

return -1;

}

if (fscanf(IN, "%d", &a) != 1) {

printf("File empty\n");

fclose(IN);

return -1;

}

if (fscanf(IN, "%d", &b) == 1) {

if (a >= b)

++ res;

while (fscanf(IN, "%d", &c) == 1) {

if ((b >= a) && (b >= c))

++res;

a = b;

b = c;

}

if (b >= a)

++res;

} else {

res = 1;

}

fclose(IN);

printf("Res: %d\n", res);

return 0;

}

tmp.dat:

5

0

5

-1

2

5

5

Ïðîòîêîë âûïîëíåíèÿ ïðîãðàììû ïî øàãàì:

Ðåçóëüòàò âûïîëíåíèÿ ïðîãðàììû 5: