代做program、代写C/C++语言程序

日期：2024-11-18 09:44

Lab 4

Pointers

&

Structures

Part I: Nov. 4, 2024

Part II: Nov. 4, 2024

Faculty of Computing, HIT Weihai

Dept. of Computer Science & Technology

4-1/2-0

Pointers as Function Arguments

• Problem: how to exchange two variables using a function (p.95)

• Language points

➢Suppose main (the caller) calls swap (the callee)

➢Argument(s) passed from caller to callee by value

➢NOT vice versa

4-1-1

Faculty of Computing, HIT Weihai, 2024FA

Pointers as Function Arguments

• Problem: how to exchange two variables using a function (p.95)

• Language points

➢Suppose main (the caller) calls swap (the callee)

➢Argument(s) passed from caller to callee by value

➢NOT vice versa

• This program fails because of the wrong idea:

Since the value of a goes to x, afterwards the value of x will

also go back to a automatically; same is true for b and y

a and x are the same variable (not only equal, but they are

just one variable); b and y too

4-1-2

Faculty of Computing, HIT Weihai, 2024FA

Assignment 0: See How It is Wrong

• Use GDB to step through the following program

4-1-3

Faculty of Computing, HIT Weihai, 2024FA

void swap(int x, int y)

{

int temp;

temp = x;

x = y;

y = temp;

}

#include <stdio.h>

int main()

{

int x = 3, y = 4;

swap(x, y);

printf("%d %d", x, y);

}

Assignment 0: See How It is Wrong

• Use GDB to step through the following program

➢ Job 1:

✓ When in main, check out the address of x:

(gdb) p &x

✓ When in swap, check out the address of x

✓ Consider: are they the same variable?

➢ Job 2:

✓ Just before leaving swap, check out the value of x:

(gdb) p x

✓ Immediately after getting back to main, check out the value of x

➢ Carefully record the above results and give your observation in the

lab report.

4-1-4

Faculty of Computing, HIT Weihai, 2024FA

Assignment 1: Get the Task Done

• (p.96) By receiving the addresses of arguments from the caller,

the callee manages to reach the original arguments, i.e. a and b

• Step through this program

4-1-5

Faculty of Computing, HIT Weihai, 2024FA

int main()

{

int a = 3, b = 4;

swap(&a, &b);

printf("%d %d", a, b);

}

void swap(int *px, int *py)

{

int temp;

temp = *px;

*px = *py;

*py = temp;

}

Assignment 1: Get the Task Done

(Cont.)

• (p.96) By receiving the addresses of arguments from the caller,

the callee manages to reach the original arguments, i.e. a and b

• Step through this program

➢ When in main, check out the addresses of a and b

➢ When in swap, see if you could still visit a (or b) directly:

(gdb) p a

✓ The reason is because we are now beyond the scope of a and b (§4.4)

✓ Sounds weird, but you just have to remember this rule

➢ However, we are sure to be able to visit a through its address, which is

now held in pointer px:

✓ Try: (gdb) p px and (gdb) p *px

➢ Carefully record the above results and give your observation in the lab

report.

4-1-6

Faculty of Computing, HIT Weihai, 2024FA

Assignment 2: Further Thinking

• Run the following program and try to explain why

• To troubleshoot, step through it using proper GDB commands

• Report your work.

4-1-7

Faculty of Computing, HIT Weihai, 2024FA

int main()

{

int a = 3, b = 4;

swap(&a, &b);

printf("%d %d", a, b);

}

void swap(int *px, int *py)

{

int *temp;

temp = px;

px = py;

py = temp;

}

Arrays of Structures

• Consider the following array of structures:

athlete PRCplayers[] = {

{ "CHANG Yuan", 'F', 0x970624, "boxing", "100" },

{ "JI Bowen", 'M', 0x020222, "canoe", "100" },

{ "LIU Qingyi", 'F', 0x051019, "breaking", "001" },

{ "LIU Yang", 'M', 0x940910, "gymnastics", "110" },

{ "MA Long", 'M', 0x881020, "table tennis", "100" },

{ "PAN Zhanle", 'M', 0x040804, "swimming", "210" },

{ "SHENG Lihao", 'M', 0x041204, "shooting", "200" },

{ "WANG Zongyuan", 'M', 0x011024, "diving", "110" },

{ "WANG Chang", 'M', 0x010507, "badminton", "010" },

{ "ZHANG Yufei", 'F', 0x980419, "swimming", "015" },

};

4-2-1

Faculty of Computing, HIT Weihai, 2024FA

typedef struct {

char *name;

char gender;

unsigned birthday;

char *sport;

char *GSBmedals;

} athlete;

Arrays of Structures

– Comprehensive Problem Solving

• Assignment 3 (choose any ONE task):

➢ Calculate the average number of medals per capita

➢ Find out who has/have won the most gold medals

➢ Find out the youngest male athlete

➢ See if player SUN Yingsha is in this array, using binary search

➢ Rearrange the array into descending order by age, using any algorithm

➢ Show which athletes come from the same sport team, and which sport(s)

➢ Given the subscripts of any two players (0 for CHANG Yuan, …, 9 for

ZHANG Yufei), calculate how many days one is older than the other

➢ Change all surnames from capital letters to lower case except for the

initial letter; and show the full names of all players on the screen

• Report your work.

4-2-2

Faculty of Computing, HIT Weihai, 2024FA

Examples

• Insert the following element into the array:

{ "ZHENG Qinwen", 'F', 0x021008, "tennis", "100" }

➢ Step 1: find the proper position

➢ Step 2: move all elements behind it to make room for the insertion

➢ Step 3: put the new record into this vacated position

➢ Step 4: show the whole array (one more record than before)

4-2-3

Faculty of Computing, HIT Weihai, 2024FA

Examples – insert.c

• Language points

➢When applied to an array, sizeof() evaluates to the total

number of bytes in this array; when applied to a structure,

the result is the # of bytes in this type of structure

➢Structure variables CANNOT be compared directly

✓if (plyr == *pplyr) /* wrong! */

➢Bounds check: should loop condition be > or >=?

✓for (pplyr += n-1; n > 0; n--)

➢printf("%-15s\t%c\t%06x\t%-15s\t%s\n");

✓SHENG Lihao M 041204 badminton_______200

✓WANG Zongyuan M 011024 diving 110

4-2-4

Faculty of Computing, HIT Weihai, 2024FA

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