function to make upper case alphabet not working

Still trying to play around with pointers, thought I would make this function. For some reason, it's not working and it's giving me this error.
I made it in a seperate file though.

Code:

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int strlen(const char* string){
    int n = 0;
    for(;;)
    {
        if(string[n] == '\0') break;
        n++;
    }
    return n;
}
void to_upper(char *string)
{
    for(int i = 0, n = strlen(string); i < n; i++)
    {
        if( string[i] > 122 || string[i] < 97 ) continue;
        (int) string[i] += 32;
    }
}

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now I'll call it in main

Code:

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int main(){
    char *name = "Aahmed";
    to_upper(name);
    printf("%s\n", name);
    return 0;
}

---

giving me this error on compiling
error: lvalue required as left operand of assignment

You should not name your function strlen as that is a name from the standard library. You can use a name prefix to differentiate it from the standard library function.

The error is because you're casting string[i], and you cannot assign to the result of a cast. Get rid of that cast.

You should also avoid magic numbers (in this case by using character constants), and where you have to have a magic number, document what it means (either by naming it with a macro, or by a comment).

In main, you have another problem: name is a pointer to a string literal, therefore it cannot be modified. Change name into an array.

EDIT:
Also, it looks like you have a logic error: you probably want to subtract 32 rather than add.

Quote:

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Originally Posted by laserlight

You should not name your function strlen as that is a name from the standard library. You can use a name prefix to differentiate it from the standard library function.

The error is because you're casting string[i], and you cannot assign to the result of a cast. Get rid of that cast.

You should also avoid magic numbers (in this case by using character constants), and where you have to have a magic number, document what it means (either by naming it with a macro, or by a comment).

In main, you have another problem: name is a pointer to a string literal, therefore it cannot be modified. Change name into an array.

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I have done what you said in green and totally understood it.

What you have said in blue, is it really impossible for me to change a string literal in C?

I didn't understand what's in red, what do you mean by magic numbers? It's the ASCII code for the characters.

Quote:

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Originally Posted by 7usseiny

What you have said in blue, is it really impossible for me to change a string literal in C?

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Yes and no. Yes, in that it results in undefined behaviour, so it might or might not work, and could well result in a crash when it doesn't work. No, because it might work... only for your program to crash when you demonstrate your program to your teacher who will then award you a zero, or to the big boss who will then fire you.

Quote:

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Originally Posted by 7usseiny

I didn't understand what's in red, what do you mean by magic numbers? It's the ASCII code for the characters.

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Quick, without referring to the ASCII table, what is the character corresponding to the ASCII code 35? If you took more than 1 ms to come up with the correct answer, you're too slow, and so this is a "magic number": readers are unlikely to immediately know what 35 means, even if they might know that it is the value of a character in ASCII. Therefore, you should prefer to use '#' instead of 35. Similiarly, what's 32, and are you sure it is correct? You could #define LOWERCASE_UPPERCASE_DIFF ('a' - 'A') and then use LOWERCASE_UPPERCASE_DIFF instead.

Quote:

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Originally Posted by laserlight

Yes and no. Yes, in that it results in undefined behaviour, so it might or might not work, and could well result in a crash when it doesn't work. No, because it might work... only for your program to crash when you demonstrate your program to your teacher who will then award you a zero, or to the big boss who will then fire you.


Quick, without referring to the ASCII table, what is the character corresponding to the ASCII code 35? If you took more than 1 ms to come up with the correct answer, you're too slow, and so this is a "magic number": readers are unlikely to immediately know what 35 means, even if they might know that it is the value of a character in ASCII. Therefore, you should prefer to use '#' instead of 35. Similiarly, what's 32, and are you sure it is correct? You could #define LOWERCASE_UPPERCASE_DIFF ('a' - 'A') and then use LOWERCASE_UPPERCASE_DIFF instead.

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Thanks a lot!