Week12 - Clark Science Center · 2019-11-25 · Exercise • Write a program that reads the ﬁle...

mith College

Computer Science

Dominique Thiébaut [email protected]

CSC231Week 12— Introduction to C

Fall 2019

Pointers

Computer Science Dominique Thiebaut

Concept


x6.5

0FFF1F0A

px

RAM

Concept


x6.5

0FFF1F0A

px

RAM

Example: Initialize an Array


Using

indexing

3 4 1 9 3 4 6 6 1 0

0 1 2 3 4 5 6 7 8 9

A



Using

indexing



3 4 1 9 3 4 6 6 1 0

0 1 2 3 4 5 6 7 8 9

A

Using

pointers


Using

pointers


./a.outp=0x7fff88d54560 A[0] = 0.00 *p = 0.00p=0x7fff88d54564 A[1] = 1.00 *p = 1.00p=0x7fff88d54568 A[2] = 2.00 *p = 2.00p=0x7fff88d5456c A[3] = 3.00 *p = 3.00p=0x7fff88d54570 A[4] = 4.00 *p = 4.00p=0x7fff88d54574 A[5] = 5.00 *p = 5.00p=0x7fff88d54578 A[6] = 6.00 *p = 6.00p=0x7fff88d5457c A[7] = 7.00 *p = 7.00p=0x7fff88d54580 A[8] = 8.00 *p = 8.00p=0x7fff88d54584 A[9] = 9.00 *p = 9.00


We Stopped HereLast Time

https://farm1.staticflickr.com/44/145211107_f2020ec804_z.jpg

We Stopped HereLast Time

https://farm1.staticflickr.com/44/145211107_f2020ec804_z.jpg

Why is fib3() soslow?

https://visualgo.net/en/recursion

(see

fib3.c

on marrax)



fib3 tree


Outline• Pointers

• Array

• Functions

• Input/Output (IO)

• Dynamic Variables

• File IO

• Multi-File Projects

Pointers: Quick Review


x6.5

0FFF1F0A

px

RAM

Arrays


v[0]

6.5

0FFF1F00

pv

pv+1

pv+2

pv+3

v[1]

v[2]

v[3]

0FFF1F04

0FFF1F08

0FFF1F0C

0FFF1F10

Arrays

• The name of an array is a pointer to the first cell of the array.

• name is the same as &(name[0])


* and &• * has two meanings, depending on context

• “Pointer to”

• “Contents of”

• Similarly, & means “the address of” or the “binary and”-operator


* and &



Exercise• Write a C program that copies Array A into Array B using indexing first, and then using pointers.

getcopy C/copyArrays.c

Exercise• Write a C program that finds the largest integer in Array A, using pointers.


Functions


Functions

• Functions are normally declared before they are used

• Functions can return values of simple types (int, char, floats), and even pointers.

• Functions get parameters of simple types, and pointers.

• Passing by value is automatic. Passing by reference requires passing a pointer.


Back to Assembly…x dd 3

push dword[x] call f1 …

f1: push ebb mov ebp, esp inc dword[ebp+8] … ret 4

z = 30z = 11sum( 11, 22) = 33


Example 1

z = 30x = 11

Example 2 (Incomplete code… Add missing elements!)


z = 30x = 11

Example 2 (Incomplete code… Add missing elements!)


Pass

by Reference!

Complete the code

shown here


getcopy C/decrementArray.c

Exercise

Exercise


• Write a C program that uses functions to find the largest integer in A. The result is passed back using a return statement.

Exercise

• Write another program that does the same thing but the max is passed back by reference.


Input/OutputIO

Input From User


(cont’d)

a.outEnter your name, please: MickeyEnter your age: 21Enter your version of pi: 3.14159Mickey is 21 years old, and thinks pi is 3.1415901184


Dynamic Variables


• Dynamic: think "new" in Java

• malloc() = Memore Allocation for new data structure


RAM


user

pro

g

malloc()

user

pro

g

malloc()

RAM


OperatingSystem

RAM


OperatingSystem

dyn

user

pro

g

malloc()

RAM


OperatingSystem

dyn

(void*) p

user

pro

g

malloc()

RAM


OperatingSystem

dyn

(void*) p

p

user

pro

g

malloc()


getcopy C/malloc1.c

Malloc

Exercise• Modify the previous program that computed the

max of an array, but this time you allocate the array dynamically, with numbers provided by the user.


sizeof()can be tricky…


sizeof ()


sizeof(A) = 20sizeof(A[0]) = 4sizeof(B) = 8sizeof(B[0]) = 4sizeof(p) = 8sizeof(*p) = 4sizeof(name) = 14strlen(name) = 13sizeof(a) = 4sizeof(x) = 4

sizeof ()


sizeof(A) = 20sizeof(A[0]) = 4sizeof(B) = 8sizeof(B[0]) = 4sizeof(p) = 8sizeof(*p) = 4sizeof(name) = 14strlen(name) = 13sizeof(a) = 4sizeof(x) = 4

Different!sizeof ()


File IO

File I/O: Output


C/fileIOOutput.c

File I/O: Input


C/fileIOInput.c


File I/O: Reading Ints [~handout] cat fileOfInts.txt 41 2 34 5 67 8 910 11 12


File I/O: Reading Ints

C/fileIOReadNumbers.c


[~handout] gcc fileIOReadNumbers.c[~handout] a.out1, 2, 34, 5, 67, 8, 910, 11, 12

File I/O: Reading Ints

Exercise• Write a program that reads the file data.txt, which

has the number of ints it contains on the first line, then one int per line for the rest of the file. Your program must use a dynamic array to store the numbers, and find the max of the array, and print it.


Date post:	31-Jul-2020
Category:	Documents
Upload:	others
View:	0 times
Download:	0 times

Week12 - Clark Science Center · 2019-11-25 · Exercise • Write a program that reads the ﬁle...

Documents