[Tutorial] Introduction to programming using C (VI)

[ChemDog]

Wrong, the average is not correctly calculated and shown. Average could be a decimal. You're rounding it down using int.

Whoops, rounded means aren't very useful
[spoiler]

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

int main()
{
     char* s1 = "New\nLine\n";
     char* s2 = "Another string!";
     char* s4 = "And another string.";
     char* s5 = "Yet another string.";
     char s3[100];
     strcpy(s3, s1);
     strcat(s3, s2);
     strcat(s3, s4);
     strcat(s3, s5);
     unsigned int len_s3 = strlen(s3);
     float x = len_s3;
     float y = 4;
     float z = x / y;
     printf("%s and the length is: %u\n",s3 , len_s3);
     printf("The average length of these four strings is: %f\n", z);

     return 0;
}

[/spoiler]

Wrong. The program should be able to handle any string.

Right, I was thinking that seemed too easy to be right. This took me a while, but it seems good now.
[spoiler]

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

int main()
{
     char* s1 = ("I am me, you are you.");
     char s2[20];
     int s1len = strlen(s1);  		                //string 1 length
     int ssmelen = strlen(strstr(s1, "me"));     //sub string "me" length
     strncpy(s2, s1, s1len - ssmelen);	        //copy characters equal to the difference of s1len and ssmelen 
     strcat(s2, "me");
     printf("%s\n",s2);

     return 0;
}

[/spoiler]

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[m0skit0]

Whoops, rounded means aren't very useful

Still rounded down. Keep in mind that if you perform an operation with 2 given types, the operation will be done using that type. In this case, you're dividing 2 integers, thus the result will be integer. The you assign it to a float type, which will convert the integer into float, which will result in printing the same integer but all zeroes decimals.

Right, I was thinking that seemed too easy to be right. This took me a while, but it seems good now.

I don't see it right. This is the output of your program:

Code: Select all

I am +@me

which is definitely wrong.

PS: please format your code

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[ChemDog]

Still rounded down. Keep in mind that if you perform an operation with 2 given types, the operation will be done using that type. In this case, you're dividing 2 integers, thus the result will be integer. The you assign it to a float type, which will convert the integer into float, which will result in printing the same integer but all zeroes decimals.

That makes a lot of sense, thank you for clarifying that.

I don't see it right...

That does seem to be wrong, but I am not sure why. When I run the program I am left with just "I am me". Is it just luck that I am getting the right outcome on my computer?

PS: please format your code

Sorry about that. From now on it will be formatted.

[m0skit0]

When I run the program I am left with just "I am me". Is it just luck that I am getting the right outcome on my computer?

It is definitely luck, or your compiler initializes data for you, but not all compilers do this. Since this can be hard to spot, I'll explain why this is happening:

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char s2[20]

Here you define s2 as a string, but you don't initialize it to anything. This means this 20 bytes buffer can contain any values. For example, here's what it looks like on my computer:

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"BN\\300w\\000\\032@\\000X\\377\\"\\000f\\032@\\000\\000\\032@"

It will be different on each different computer, even each time you run it on the same computer.

Then you do:

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strncpy(s2, s1, s1len - ssmelen);

You copy 5 bytes from s1 to s2. This will only modify the first 5 bytes of s2 buffer, leaving the rest unmodified:

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"I am \\032@\\000X\\377\\"\\000f\\032@\\000\\000\\032@"

Then you do

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strcat(s2, "me");

Which concatenates "me" at the end of s2. But where s2 ends? At first NUL character (0x00 byte) from start. Taking s2, the s2 string is actually:

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"I am \\032@\\00"

Thus "me" will be appended to this string, resulting in:

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"I am \\032@me\\00"

Hope this clears things up. Now it's up to you to fix it!

[ChemDog]

Thank you for breaking it down like that, this helped a whole lot.
My first thought to fix this was:

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strncpy(s2, s1, s1len - ssmelen + 2);           //copy characters equal to the difference of s1len and ssmelen and add two to account for "me"

basically just because it wasn't adding "me" to the end, but I thought this might still show random characters from the buffer. Then I started wondering why this hasn't happened when I concatenated other strings(probably my compiler like you said). Rereading your post:

Which concatenates "me" at the end of s2. But where s2 ends? At first NUL character (0x00 byte) from start. Taking s2, the s2 string is actually:

made me remember your string examples when you showed:

// Setting the buffer to all zeroes (ASCII code 0, NUL character)
memset(string2, 0, 100);

So this way there are no junk characters and they are NUL. So here is my fix:
[spoiler]

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

int main()
{
	char* s1 = ("I am me, you are you.");
	char s2[20];
	memset(s2, 0 , 20);                   
	int s1len = strlen(s1);                //string 1 length
	int ssmelen = strlen(strstr(s1, "me"));  //sub string "me" length
	strncpy(s2, s1, s1len - ssmelen + 2);    //copy characters equal to the difference of s1len and ssmelen and add two to account for "me"
	printf("%s\n",s2);

return 0;
}

[/spoiler]
So I'm thinking this should be done for any uninitialized char?

[Wdingdong]

made me remember your string examples when you showed:

// Setting the buffer to all zeroes (ASCII code 0, NUL character)
memset(string2, 0, 100);

So this way there are no junk characters and they are NUL.

Very good idea

[ChemDog]

Very good idea

Thank you! I was pretty happy when it popped into my head

[m0skit0]

So I'm thinking this should be done for any uninitialized char?

Depends on what you want to do with it, but it's always good to initialize memory (this goes for variables as well) to be sure what values it holds. Also you can do it in a different way. If you want, I'll challenge you to find another solution Anyway such exercises are very good to get a good grasp of C and fully understanding how stuff works

PS: also C has a good benefit compared to other languages: it actually shows you how the computer works internally. I know other languages like LUA, Python, Java and the like are way more "friendly" and hide all this stuff, but internally they're still doing it the way C does That's why it is commonly said that if you don't know C (or assembly) then you can't call yourself a programmer

[Wdingdong]

Anyway such exercises are very good to get a good grasp of C and fully understanding how stuff works

Yes, I totally agree.
So, please, can you give us atleast exercise questions for the succeeding topics of this tutorial? So that we can practise and you can solve our doubts. So, that some good C programmers will be added to this world(by you)

[m0skit0]

Sure, but for now there are no more lessons I'm looking forward to make more now that there's some activity