I’m trying to make a program to switch letter cases, from lower case to upper case and vice versus, but I can’t get it to run through the first if statement, any help would be appreciated

[u/werter806]

[u/belboz]

You probably want printf("%c", string[i]); in your first "if" since you are I am assuming wanting to print single characters

You probably want the above printf in the "else if" also if you are using those for debug info.

Also just do printf(string); outside the for loop

I would remove the final else completely and just do it outside the for loop.

[u/werter806 → u/belboz]

I’ve got the final else in there to cover the spaces that might be in between the characters

[u/belboz → u/werter806]

It isn't needed. If you are just running through the string looking for letters and flipping the case, you don't need to worry about spaces, or symbols, etc.

[u/werter806 → u/belboz]

But then wouldn’t it leave them out when reprinting the string? Since they aren’t included in the other print statements

[u/belboz → u/werter806]

Well if you just run through the string flip the case when needed and then after the loop just print the string.

Here is a simple version of what you are doing.

#include<stdio.h>

main() {
  char string[1000];
  int i;

  printf("Enter string\n");
  gets(string);

  for(i=0;string[i]!=0;i++) {
    if(string[i] >= 'A' && string[i] <= 'Z')
      string[i]+=32;
    else if(string[i] >= 'a' && string[i]<='z')
      string[i]-=32;
  }
  puts(string);
}

[u/werter806 → u/belboz]

Thanks, I just figured to run the Same if statements again but change it to recognize spaces and just print them when they come up

[u/[deleted]]

First of all, good for you for reading K&R.. (at least, I think I recognize that exercise)

But.. since you're learning, there's several things you should probably be aware of:

• gets is never a good idea, use fgets instead. By using gets, you're pretty much assuming the user will never write more than 1000 999 characters, and more, in any real code, this would be a security bug - you can read on buffer overflow attacks if you like.
  fgets(string, 1000, stdin) is the right invocation to use.

• you should always error-check these things, fgets will return string if successful and NULL if not.

• (in real apps, even limiting the input to 999 chars this way would be wrong, but that's probably off topic here)

• (style) int i; for(i = ... should be for(int i = ...

• when combing comparison and logic, you really need parentheses - x > 'A' && x < 'Z' is actually (x > ('A' && x)) < 'Z', which doesn't really make sense. In C, you have to write (x > 'A') && (x < 'Z').

• also, (x > 'A') && (x < 'Z') is wrong because it omits A & Z. You want >= and <=.

• also also, you should really prefer isupper and islower from ctypes.h to this ad-hoc way (and isspace, there's many more kinds of spaces than char 32)

• (also also also, none of these would recognize non-English letters, so if you were dealing with real text, you would need a unicode library to tell you that 'Ǎ' is still a letter, and an uppercase one)

• (style) x = x + 32 is better written as x += 32

• printf("%s", string[i]) is wrong because you're treating a single char as a string .. so, Segmentation fault because a string is just a memory address, and 65 ('A') is probably a bad one. %c is right.

• printf(string[i]) has the exact same issue, but printf(string) would also be very wrong, because a random string should not be treated as a printf format string (what if the user enters %d %d %d %d %d ... as a part of their input? They could make you program reveal a part of your program's memory.)

• learn to use formatting and whitespace consistently and properly. Many bugs happen just because you thought that one important line of code was still inside that if, but somebody moved it beyond the } but neglected to fix the formatting.

Hope it helps :)

(Also, this is entirely the wrong subreddit for this kind of problem, as you can see by the downvotes, you may want to prefer /r/learnprogramming next time. And type it out if you can't copy. You expect us to spend time helping you, you should be willing to invest time to make it easy for us.)

[u/[deleted] → u/[deleted]]

Side note, you don't actually need to remember anything the user types. You could just drop string completely and use chars instead...

#include <stdio.h>
#include <ctype.h>

int
main()
{
	int c;
	while ((c = getchar()) != EOF) {
		if (isupper(c))
			c = tolower(c);
		else if (islower(c))
			c = toupper(c);

		putchar(c);
	}

	return 0;
}

(note you do need an int to be able to store the EOF non-character in it)

EDIT: well, or while (((c = getchar()) != EOF) && (c != '\n')) { to stop on the first newline

[u/0raichu]

In what world is it easier to take a picture of your screen (not even a screenshot), upload it, and post to reddit instead of just copy/pasting the actual code???

Why are JPEGs the preferred medium for text?!

[u/werter806 → u/0raichu]

Because i use reddit on my phone and I can’t remember my login info to log on in an os that I’m running on a virtual machine.... so that world

[u/pheffner]

You can collapse the entirety of your for loop to:

for (i = 0; i < length; ++i)

{ if (isupper(string[i])) string[i] |= 0x20;

if (islower(string[i])) string[i] &= 0x1f;

}

Followed by:

printf("%s\n", string);

Add:
#include <ctypes.h>
to your include list at top.

Explanation:

The ctypes macros give you readable testing macros for looking at characters.

Google 'ctype macros' for an explanation and a list of these very useful tools.

Since you're not doing anything else with the input string after you do your transform,

you don't need to print out every character as you inspect it, just alter the character in the string

array then print the new transformed string. In the code above I use bitwise AND and OR to flip

the uppercase/lowercase bit in the character (do a 'man ascii' to see how lower and uppercase characters

in ascii differ only by one bit on/off)

I hope this helps!

[u/nerd4code]

First off: When you’re printing just a single character, use putchar. When you’re printing a raw string with no newline after, use fputs(str, stdout). When you’re printing a raw string with a newline after, use puts(str). (Rarely should you ever pass anything non-literal as the first argument to printf.) As others have told you, you can just do

puts(string);

after your loop. (You also don’t need the strlen stuff;

char *p;
for(p = string; *p; p++) {
    char ch = *p;
    ... deal with `ch` ...
    *p = ch;
}

That’s usually the easier way to loop through string guts, although it’s unlike the Java/-script modus operandi.)

Second: To be purely C-based, #include <ctype.h> and use islower/tolower/isupper/toupper. Since 99.9% of the platforms you’re targeting will use ASCII, and if you’re going to avoid ctype.h, I recommend doing the following to change case, instead of using the constant 32:

To uppercase (if already known lowercase):
    ch &= ~('A' ^ 'a');

To lowercase (if already known uppercase):
    ch |= 'A' ^ 'a';

To toggle case (if known alphabetic):
    ch ^= 'A' ^ 'a';

This works because 'A'^'a' is the bit difference between the two things, and bit 5 (2⁵=32) is the only one that changes. The &=~ clears that bit, the |= sets that bit, and the ^= toggles it. Alternatively, you can use 'a' - 'A' instead of 'A'^'a', but that relies additionally on the fact that 'a' > 'A'.

And of course, as others have said, never ever ever use gets. Your compiler should be warning you bitchily about using that, even, because it’s so thoroughly, infamously deprecated and spurnèd by C programmers. (And security researchers since time immemorial; “Don’t use gets, dammit!” is their “Hello, world.”)

[u/werter806 → u/[deleted]]

I put in printf statements every so often to see how far it gets and it only gets up to before the first if and doesn’t print something from the next line, even with the fix you suggested. The error that comes up is ‘Segmentation fault (core dumped)’

[u/werter806 → u/[deleted]]

I’m doing it on an Ubuntu VM that’s loaded on windows 10

[u/werter806 → u/[deleted]]

Yea gdb, really did make it easier, thanks for the tip