[Tutorial] Introduction to programming using C (VII)

[Wdingdong]

@waratte: Thanks for the explanation man.

Well I see you already know functions and flow control, that's nice Although I will recover this exercises soon enough

I thought you want us to learn it by ourselves. So,I've gained some basic knowledge about it. But, whatever I know, I lay no claim on it, it's all because of you and this forum

Here are my exercise answers:
[spoiler]

Code: Select all

#include <stdio.h>
#include <stdlib.h>

void exercise1And2()	//first and second exercise combined 
{
	int* a = malloc(50*sizeof(int));
	*(a+4) = 1;
	*(a+5) = 2;
	*(a+7) = 3;
	printf("%d %d %d\n",*(a+4),*(a+5),*(a+7));
}

void exercise3()	//Using only pointers: given a random string, print from the 5th character onwards.
{
	char* s2 = "Not galaxy s2";
	printf("%s\n",(s2+4));
}//Is it this much only?

void exercise4()	//XOR operation
{
	char s[5] = "Uncle";
	char s1[5] = "Jessy";
	int a = s[0] ^ s1[0];
	int b = s[1] ^ s1[1];
	int c = s[2] ^ s1[2];
	int d = s[3] ^ s1[3];
	int e = s[4] ^ s1[4];
	printf("%d %d %d %d %d",a,b,c,d,e);
}

int main()
{
	exercise1And2();
	exercise3();
	exercise4();
	return 0;
}

[/spoiler]
Corrections accepted

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[snailface]

Here is my submission, Prof. Moskito. I hope I pass.

[spoiler]

Code: Select all

    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>

    int main()
    {
//Using only pointers: declare an array of signed integers and assing it a size of 50 integers.
printf("Ex1\n\n");

    int *array=(int *)malloc(50);

//Same as the last exercise but also assign positions 5, 6, and 8, and then print them.\n\n");
printf("Ex2\n\n");

    int *array2=(int *)malloc(50);
    *(array2+5)=76;
    *(array2+6)=79;
    *(array2+8)=76;
    printf("%c %c %c\n\n",*(array2+5),*(array2+6),*(array2+8));

//Using only pointers: given a random string, print from the 5th character onwards.\n\n");
printf("Ex3\n\n");

    char *wololo="Skip A string.\n\n";
    printf(wololo+5);

//Given 2 strings of length 5, encrypt the first one using the second one using an XOR operation.\n\n");
printf("Ex4\n\n");

    char *str=(char*)malloc(6);
    char *str2=(char*)malloc(6);
    memcpy(str,"Snail",6);
    memcpy(str2,"faces",6);
    int x=2;

    while(x--)
    {

       do
       {
           *str^= *str2;
           str++;
           str2++;

       }while(*str);

           str-=5;
           str2-=5;
           printf(str);
           printf("\n");
    }

getch();
return 0;
}

[/spoiler]

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[Wdingdong]

What's the difference between these two:

Code: Select all

int* a = (int*) malloc(sizeof(int));

and

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int* a = malloc(sizeof(int));

[m0skit0]

Corrections accepted

Sure I will

void exercise1And2()

I said positions 5, 6, and 8, not 5th, 6th and 8th You have a memory leak here (you didn't free the memory).

void exercise4()

Sorry, my bad...The result of the encryption must be another string. You can print the resulting string in hexadecimal representation since maybe some (most) of resulting the characters are not printable.

What's the difference between these two

If you don't use the first one, any decent compiler will issue a warning, since malloc() returns void* and not int*.

printf("Ex1\n\n");

Memory leak!

printf("Ex2\n\n");

Memory leak!

printf(wololo+5);

Huh? Does not compile. This is not Python!

char *str=(char*)malloc(6);
char *str2=(char*)malloc(6);
memcpy(str,"Snail",6);
memcpy(str2,"faces",6);

You don't need to malloc() (you also have 2 memory leaks here as well). To copy strings, use strcpy() (or strncpy()).

while(x--)
{

do
{
*str^= *str2;
str++;
str2++;

}while(*str);

str-=5;
str2-=5;
printf(str);
printf("\n");
}

This is way too much complicated for such a simple thing... You don't even need to use loops. Also, why you initialize x to 2? Makes no sense to me.

getch();

Please avoid this, dear Windows user If you don't want your console to close, use another IDE.

[Wdingdong]

I said positions 5, 6, and 8, not 5th, 6th and 8th You have a memory leak here (you didn't free the memory).

My English problem I think and added free();

Sorry, my bad...The result of the encryption must be another string. You can print the resulting string in hexadecimal representation since maybe some (most) of resulting the characters are not printable.

Fixed(It's still wrong I think).

while(x--)
{

do
{
*str^= *str2;
str++;
str2++;

}while(*str);

str-=5;
str2-=5;
printf(str);
printf("\n");
}

I had a nightmare due to this

Revision0.1 :
[spoiler]

Code: Select all

#include <stdio.h>
#include <stdlib.h>

void exercise1And2()	//first and second exercise combined
{
	int* a = malloc(50*sizeof(int));
	*(a+5) = 1;
	*(a+6) = 2;
	*(a+8) = 3;
	printf("%d %d %d\n",*(a+5),*(a+6),*(a+8));
	free(a);
}

void exercise3()	//Using only pointers: given a random string, print from the 5th character onwards.
{
	char* s2 = "Not galaxy s2";
	printf("%s\n",(s2+4));
}//Is it this much only?

void exercise4()	//XOR operation
{
	char* s = "Uncle";
	char* s1 = "Jessy";
	char x[5];
	x[0] = s[0] ^ s1[0];
	x[1] = s[1] ^ s1[1];
	x[2] = s[2] ^ s1[2];
	x[3] = s[3] ^ s1[3];
	x[4] = s[4] ^ s1[4];
	printf("%x%x%x%x%x",x[0],x[1],x[2],x[3],x[4]);
}

int main()
{
	exercise1And2();
	exercise3();
	exercise4();
	return 0;
}

[/spoiler]

[m0skit0]

exercise1And2()

Yeah!

exercise4()

Correct!

[Salahkun]

Mr moskito would you like to make more tuts of psp programming when you have some free time

[m0skit0]

Nope, I won't. Anyway, this is valid for any platform, it is standard C.

[snailface]

Code: Select all

char *str=(char*)malloc(5+1);    //I can't initialize as a const literal because I can't modify the string that way
strcpy(str,"Snail");
char *str2="Faces";                 //I can here because it the string doesn't need modification

   int x=0;

       for(x=0;x<10+1;x++){
           if( !(x%5) )printf("%s\n",str);   //prints unencrypted 'Snail' at x=0, encrypted x=5, decrypted x=10
           *(str + x%5) ^= *(str2 + x%5);    //xors 'Snail' character for character with 'faces'
       }

Here is a rewrite of Ex4. It encrypts, decrypts, and prints at each stage. It's a lot more compact than my first attempt and hopefully, you'll like it better.

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int *array=(int *)malloc(50);
free(array);   //thx FrEdDy :)

Memory leak!

Can you please explain why? I've seen that (int*) typecast convention with malloc() used in several books including K&R.

Code: Select all

char *wololo="Skip A string.\n\n";
    printf(wololo+5);

Huh? Does not compile. This is not Python!

I know this is unorthodox code, but it is perfectly valid code and I've tested it in GCC and VS and it compiles and works with no warnings. The first arg in printf is looking for a const char* format --which is what wololo+5 points to in this example.

Still, I should probably avoid using this weird method since it confuses and frightens people.

[FrEdDy]

Repeat after me: always free your memory,always free your memory...