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copied!<p>The code:</p> <pre><code> char *s = "hello ppl."; </code></pre> <p>gives you a pointer to what it <em>probably</em> read-only memory. That's because string constants in C are non-modifiable. When you try to write to that memory, you'll most likely get a segmentation violation. The relevant part of the standard (<code>C99 6.4.5/6</code> on <code>String literals</code>) states:</p> <blockquote> <p>It is unspecified whether these arrays are distinct provided their elements have the appropriate values. If the program attempts to modify such an array, the behavior is undefined.</p> </blockquote> <p>So, while the memory doesn't <em>have</em> to be read-only, you're still breaking the rules by trying to modify it.</p> <p>Try this instead:</p> <pre><code>char s[] = "hello ppl."; </code></pre> <p>which is conceptually the same as:</p> <pre><code>char s[11]; // for the whole string plus null terminator strcpy (s, "hello ppl."); </code></pre> <p>In other words, it puts the string you want to change into <em>writable</em> memory. The following code:</p> <pre><code>#include <stdio.h> #include <string.h> int main(void) { int i; char s[] = "hello ppl."; for (i = 0; i < strlen(s); i++) { char c = s[i]; if (c >= 97 && c <= 122) { c += 2; s[i] = c; } } printf("%s\n",s); return 0; } </code></pre> <p>gives you <code>"jgnnq rrn."</code> as you desired.</p> <p>A few other things I'd like to point out which are not fatal:</p> <ul> <li>It's not usually a good idea to use 'magic' numbers like <code>97</code> and <code>122</code>. It's just as easy, and clearer in intent, to use 'a' and 'z'.</li> <li>If you really want to rotate, you can't blindly add 2 to 'y' and 'z'. You have to treat them specially (subtract 24) so that they map correctly to 'a' and 'b'.</li> <li>The C standard doesn't guarantee that alpha characters are contiguous. If you know you're using ASCII, you're probably okay but I thought I'd just mention that. As an aside, it <em>does</em> guarantee that for the numeric characters.</li> </ul> <p>Having said that, I'd rather use a mapping table as follows:</p> <pre><code>#include <stdio.h> #include <string.h> int main (void) { char *lkupPtr, *strPtr; char str[] = "hello ppl."; const char * const from = "abcdefghijklmnopqrstuvwzyz"; const char * const to = "cdefghijklmnopqrstuvwzyzab"; for (strPtr = str; *strPtr != '\0'; strPtr++) if (lkupPtr = strchr (from, *strPtr)) != NULL) *strPtr = to[(int)(lkupPtr - from)]; printf("%s\n",str); return 0; } </code></pre> <p>This takes care of all the points I raised above and you can, if necessary, add more mappings if you're in an internationalized environment (rather than just plain ASCII or EDCDIC).</p> <p>This should be, in my opinion, fast enough for all but the most demanding of requirements (I clocked it at over 3 million characters per second on my PC). If you have a near-insatiable need for performance over and above that, yet don't want to opt for hand-crafted assembly targeted to your specific CPU, you could <em>try</em> something like the following.</p> <p>It's still fully compliant with the C standard but may deliver better performance by virtue of the fact all heavy calculation work is done once at the start. It creates a table holding all possible character values, initializes it so that every character translates to itself by default, then changes the specific characters you're interested in.</p> <p>That removes any checking for characters from the translation itself.</p> <pre><code>#include <stdio.h> #include <string.h> #include <limits.h> static char table[CHAR_MAX + 1]; static void xlatInit (void) { int i; char * from = "abcdefghijklmnopqrstuvwzyz"; char * to = "cdefghijklmnopqrstuvwzyzab"; for (i = 0; i <= CHAR_MAX; i++) table[i] = i; while (*from != '\0') table[*from++] = *to++; } int main (void) { char *strPtr; char str[] = "hello ppl."; xlatInit(); // Do this once only, amortize the cost. for (strPtr = str; *strPtr != '\0'; strPtr++) *strPtr = table[*strPtr]; printf("%s\n",str); return 0; } </code></pre>

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