C Video Tutorial 4
Welcome to part 4 of my C Video Tutorial! Today I’m going to try something a little different to try and make the videos feel more like a classroom atmosphere.
We’ll be looking at pointers in c. I’ll cover what pointers are. What is the difference between * and &. How to use pointers and arrays. How to create an array of strings. We’ll also look at how to change the value of variables inside of functions and how to manipulate char arrays in functions.
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Code From the Video
CTutorial4_1.c
#include <stdio.h>
void main(){
// When you compile and run your program you are provided
// with a piece of memory in ram to store data like
// variables.
int rand1 = 12, rand2 = 15;
// Each time you create an int 4 bytes of data are saved
// and that part of memory has an address you can use
// to locate the data.
// You can return that address by placing a & before
// the variable name in c.
printf("rand1 = %p : rand2 = %p\n\n", &rand1, &rand2);
// Sometimes these variables are stored next to each other
// and at other times that isn't true
printf("Size of int %d\n\n", sizeof(int));
// To assign the address to another variable proceed it with
// an asterisk *
int * pRand1 = &rand1;
// If we use %p we get the hexadecimal version of the address
printf("Pointer %p\n\n", pRand1);
// If we use %d we get the decimal version of the address
printf("Value %d\n\n", pRand1);
// We have to use the * to get the value stored there
// This is known as dereferencing the pointer
// Dereferencing means to use the pointer to access the variable
printf("Value %d\n\n", *pRand1);
// ARRAYS AND POINTERS
// An array name is pretty much a pointer
int primeNumbers[] = {2,3,5,7};
// We can print the values by index
printf("First index : %d\n\n",
primeNumbers[0]);
// You can print the first value also with *
printf("First index with * : %d\n\n",
*primeNumbers);
// You can use pointer arithmetic to access the other values
// with *
// This is in essence adding 1 to the address and since
// it contains ints it jumps 4 bytes forward
// Yes you can use it to get other data stored in memory
printf("Second index with * : %d\n\n",
*(primeNumbers + 1));
// ARRAYS OF STRINGS
// To create an array of strings, you have to create an
// array of pointers
char * students[4] = {"Sally", "Mark", "Paul", "Sue"};
// You can retrieve them now like any other data in an array
for(int i = 0; i < 4; i++){
printf("%s : %d\n\n", students[i], &students[i]);
}
}
CTutorial4_2.c
#include <stdio.h>
#include <stdlib.h>
void generateTwoRandomNums(int rand1, int rand2){
// Generates a random number from 1 to 50
rand1 = rand() % 50 + 1;
rand2 = rand() % 50 + 1;
printf("New rand1 in function = %d\n\n", rand1);
printf("New rand2 in function = %d\n\n", rand2);
}
// Improved pointerRandomNumbers()
// If an argument is a pointer put a * after the datatype
void pointerRandomNumbers(int* rand1, int* rand2){
// You get the address of a variable by putting & in front
// You access the data at that address by putting * in front
// Saves these numbers to the space at the address
*rand1 = rand() % 50 + 1;
*rand2 = rand() % 50 + 1;
// You retrieve the value already stored in the address
// with star once again
printf("New rand1 in pointer function = %d\n\n", *rand1);
printf("New rand2 in pointer function = %d\n\n", *rand2);
}
// We refer to the array just like any other array
void editMessageSent(char* message, int size){
char newMessage[] = "New Message";
if(size > sizeof(newMessage)){
for(int i = 0; i < sizeof(newMessage); i++){
message[i] = newMessage[i];
}
} else {
printf("New Message is to big\n\n");
}
}
void main(){
int rand1 = 0, rand2 = 0;
generateTwoRandomNums(rand1, rand2);
// Why does the following not print the updated values?
// Because the variable wasn't sent, but instead the value
// stored in that variable was sent and then stored in
// the 2 new variables in the function named
// int rand1, int rand2
printf("rand1 = %d\n\n", rand1);
printf("rand2 = %d\n\n", rand2);
// Why not have everything set as a global variable?
// 1. Naming conflicts when using other functions
// 2. It can be hard to figure out where changes are coming from
// 3. It eliminates flexibility because you don't know if a
// change will effect another function
// Solving the problem with pointers
rand1 = 0, rand2 = 0;
printf("Main Before Function Call\n\n");
printf("rand1 = %d : rand2 = %d\n\n", rand1, rand2);
// To pass the address for the variable in memory use &
// You already did this above scanf(" %d", &whatToDo);
pointerRandomNumbers(&rand1, &rand2);
printf("Main After Function Call\n\n");
printf("rand1 = %d : rand2 = %d\n\n", rand1, rand2);
// Passing a String to a Function ----
char randomMessage[] = "Edit my function";
printf("Old Message: %s \n\n",
randomMessage);
editMessageSent(randomMessage,
sizeof(randomMessage));
printf("New Message: %s \n\n",
randomMessage);
}
So can I conclude that an array is treat exactly like a pointer to a sequence of elements with an offset?
//say I had the function
void incrementByOne(int input[]) //it takes an array as an input{
for(int i = 0; i < sizeof(input); i++){
input[i]++;
}
}
void main(){
int someArray[] = {1,4};
incrementByOne(someArray);
//is someArray[] equal to {2,5} ?
}
thanks
Yes you are correct. i’m going to revisit Arrays and pointers this weekend
Derek, Great videos, I am trying to find a way to reach out to you, but not finding to easy… Anyhow, ere is me email… I would like to have a direct Q&A with u by phone or skype… Maybe 15 minutes, and i would like to make a nice donation to your site for that help. Anyways, if u see this and would be interested, please feel free to reach out. The questions are short and easy, I just need to feel better about my thoughts and website.
Regards, John
Hi John, Please don’t make a donation. I’m very happy to answer any questions you have. If you want the message to be private send me a PM on YouTube. I check YouTube a couple times a day. I’m sorry, but I can’t talk to people over the phone. That would just get to crazy if people thought they could call me. I get a couple hundred comments a day as it is. I hope that makes sense.
Feel free to ask questions though and I’ll do my best to help if I can.
Derek