I am new to the objective c and i write the code according to a reference book.
but something went wrong and I don't know why.
#import <Foundation/Foundation.h>
int main (int argc, const char * argv[])
{
if (argc==1){
NSLog(#"you need to provide a file name");
return (1);
}
FILE *wordFile = fopen(argv[1], "r");
char word[100];
while(fgets(word , 100, wordFile)){
word[strlen(word)-1] = '\0';
NSLog(#"the length of the %s is %lu", word, strlen(word));
}
fclose(wordFile);
return 0;
}
the tool indicates that the while part went wrong, EXC_BAD_ACCESS.
Any idea?
It compiles and runs fine on my machine. But imagine you have an empty line in your file. Then strlen(word) will return zero. Hence word[strlen(word)-1] = '\0'; will try to set some memory which might not be valid since word[-1] might not be a valid memory cell, or a memory cell that you can legally access.
Oh, and by the way, it has nothing to do with objective-c. This is mostly (but for the NSLog call) pure ansi C.
Related
I have an extremely simple code snippet to get the application support directory of a use. Problem is, it doesn't work! I get the values in a CFArrayRef (because I want to use C code since I will use this snippet in a C++ application later) and try to get the first string in there using CFArrayGetValueAtIndex(). I convert the returned void* to a char* using a C-style cast and attempt to output it using printf(), but nothing gets outputted! Can anyone explain why? I've looked online through the documentation for CFArray and tried to google the issue, but came up with nothing. Any help would be really appreciated!
Code:
#include <Foundation/Foundation.h>
#include <typeinfo>
int main (int argc, const char * argv[])
{
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
CFArrayRef array = \
(CFArrayRef)NSSearchPathForDirectoriesInDomains(
NSApplicationSupportDirectory,
NSUserDirectory,
YES);
char* string = (char*)CFArrayGetValueAtIndex(array, 0);
printf("string: %s\n", string);
[pool drain];
}
Why are you converting it to a char*? The resulting array from NSSearchPathForDirectoriesInDomains() contains CFStringRefs, not char*s.
Try using CFShow((CFTypeRef)CFArrayGetValueAtIndex(array, 0));
And here I thought I was getting competent at ObjC, and this little C-type problem is giving me fits. :) This program is intended to read in a character from user input and print an expression that gives the character's decimal value. This program is generating duplicate NSLog() statements and I can't figure out why:
#import <Foundation/Foundation.h>
int main (int argc, const char * argv[]) {
NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
char a;
for (int i = 1; i <= 10; i++)
{
NSLog(#"Type in an ASCII character or type 'command-.' to exit.");
scanf("%c", &a);
NSLog(#"%c = %d", a, a);
}
[pool drain];
return 0;
}
Here's the output:
Type in an ASCII character or type 'command-.' to exit.
a
a = 97
Type in an ASCII character or type 'command-.' to exit.
= 10
Type in an ASCII character or type 'command-.' to exit.
When I change the read-in variable from a char to an int and make the according format specifier modification in the scanf(), the program runs in the console as intended. (Prompting the user for a character, printing out the value, and prompting again.) As soon as I go back to a char though, it does this. What am I doing wrong? Also, regardless of what type of char I enter, there's always the "= 10" output. What's the deal with that? Thanks in advance, guys.
"= 10" is the ascii code for the enter key.
So change your code into:
scanf("\n%c", &a);
I honestly did a) search using key words and b) read the 'questions with similar titles' before asking this.
Also I tried to make this question more concise, but I had a hard time doing that in this case. If you feel the question is too wordy, I get it. Just don't try to answer.
I'm trying to write very simple objective-C programs that mirror the basic assignments in my introductory java class. I worked through an objective-c book over the summer and now I want to do lots of practice problems in objective-c, at the same time as I do java practice problems. I'm avoiding the objective-c GUI environment and just want to focus on working with the language for awhile. I still have a lot to learn about how to figure things out.
The program I'm duplicating from my java homework, is a standard type. I ask the user for number input and string input via the console. I was able to get numeric input from the console using an example I found here using scan f. (I will put the couple code lines below). But I'm unsure on how to get console input and store it in a string (NSString). I'm trying to learn to use the apple documentation and found a reference to a scan type command, but I cannot figure out how to USE the command. The one that seems likely is
scanCharactersFromSet:(NSCharacterSet )scanSet intoString:(NSString *)name;
Here's what I understand and works
int age = 0;
NSLog (#"How old are y'all?");
scanf("%d", &age);
NSLog (#"\n Wow, you are %d !", age);
But I don't understand how to pickup an NSString called 'name'. I THINK I'm supposed to make my 'name'a pointer, because the class is NSString.
(BTW I did try using scanf to pickup the string, but the compiler doesn't like me trying to use scanf in conjunction with name. It says that I shouldn't be using 'scanf' because it's expecting a different kind of data. I'm not sure where I found the data type 'i'. I was looking through my text for different ideas. I'm guessing that scanf is related to 'scanfloat' which clearly deals with numeric data, so this is not a big surprise)
I realize that 'scanf' isn't the right command (and I don't really get why I can't even find scanf in the apple documentation - maybe it's C?)
I'm guessing that scanCharactersFromSet might be the right thing to use, but I just don't understand how you figure out what goes where in the command. I guess I tend to learn by example, and I haven't found an example. I'd like to figure out how to learn properly by reading the documentation. But I'm not there yet.
NSString* name ;
scanf("%i", &name);
//scanCharactersFromSet:(NSCharacterSet *)scanSet intoString:(NSString **)name;
...
My book is oriented towards moving me into a gui environment, so it doesn't deal with input.
Thank you for any pointers you can give me.
Laurel
I would recommend ramping up on C. Objective-c is a thin layer over C and that knowledge will pay for itself over and over.
There's multiple ways in C to read:
http://www.ehow.com/how_2086237_read-string-c.html
For example:
NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
char str[50] = {0}; // init all to 0
printf("Enter you Last name: ");
scanf("%s", str); // read and format into the str buffer
printf("Your name is %s\n", str); // print buffer
// you can create an NS foundation NSString object from the str buffer
NSString *lastName = [NSString stringWithUTF8String:str];
// %# calls description o object - in NSString case, prints the string
NSLog(#"lastName=%#", lastName);
[pool drain];
return 0;
NOTE: the simple scanf is succeptible to buffer overruns. There's multiple approaches around this. see:
How to prevent scanf causing a buffer overflow in C?
Here is what Objective C looks like:
NSString *FNgetInput() {
#autoreleasepool {
return [[[NSString alloc] initWithData:[[NSFileHandle fileHandleWithStandardInput] availableData] encoding:NSUTF8StringEncoding] stringByTrimmingCharactersInSet:[NSCharacterSet newlineCharacterSet]];
}
}
The way to get data from the standard input (or any other file handle) in cocoa is to use the NSFileHandle class. Check the docs for +fileHandleWithStandardInput
Here's how to get user input using Objective-C in 2020:
main.m
#import <Foundation/Foundation.h>
int main(int argc, const char * argv[]) {
#autoreleasepool {
// insert code here...
NSLog(#"Hello, World!");
char str[50] = {0}; // init all to 0
printf("Enter you Last name: ");
scanf("%s", str); // read and format into the str buffer
printf("Your name is %s\n", str); // print buffer
// you can create an NS foundation NSString object from the str buffer
NSString *lastName = [NSString stringWithUTF8String:str];
// %# calls description o object - in NSString case, prints the string
NSLog(#"lastName=%#", lastName);
return 0;
}
return 0;
}
Compile and run:
$ clang -framework Foundation main.m -o app
If I have an unsigned char *data pointer and I want to check whether size_t length of the data at that pointer is NULL, what would be the fastest way to do that? In other words, what's the fastest way to make sure a region of memory is blank?
I am implementing in iOS, so you can assume iOS frameworks are available, if that helps. On the other hand, simple C approaches (memcmp and the like) are also OK.
Note, I am not trying to clear the memory, but rather trying to confirm that it is already clear (I am trying to find out whether there is anything at all in some bitmap data, if that helps). For example, I think the following would work, though I have not tried it yet:
- BOOL data:(unsigned char *)data isNullToLength:(size_t)length {
unsigned char tester[length] = {};
memset(tester, 0, length);
if (memcmp(tester, data, length) != 0) {
return NO;
}
return YES;
}
I would rather not create a tester array, though, because the source data may be quite large and I'd rather avoid allocating memory for the test, even temporarily. But I may just being too conservative there.
UPDATE: Some Tests
Thanks to everyone for the great responses below. I decided to create a test app to see how these performed, the answers surprised me, so I thought I'd share them. First I'll show you the version of the algorithms I used (in some cases they differ slightly from those proposed) and then I'll share some results from the field.
The Tests
First I created some sample data:
size_t length = 1024 * 768;
unsigned char *data = (unsigned char *)calloc(sizeof(unsigned char), (unsigned long)length);
int i;
int count;
long check;
int loop = 5000;
Each test consisted of a loop run loop times. During the loop some random data was added to and removed from the data byte stream. Note that half the time there was actually no data added, so half the time the test should not find any non-zero data. Note the testZeros call is a placeholder for calls to the test routines below. A timer was started before the loop and stopped after the loop.
count = 0;
for (i=0; i<loop; i++) {
int r = random() % length;
if (random() % 2) { data[r] = 1; }
if (! testZeros(data, length)) {
count++;
}
data[r] = 0;
}
Test A: nullToLength. This was more or less my original formulation above, debugged and simplified a bit.
- (BOOL)data:(void *)data isNullToLength:(size_t)length {
void *tester = (void *)calloc(sizeof(void), (unsigned long)length);
int test = memcmp(tester, data, length);
free(tester);
return (! test);
}
Test B: allZero. Proposal by Carrotman.
BOOL allZero (unsigned char *data, size_t length) {
bool allZero = true;
for (int i = 0; i < length; i++){
if (*data++){
allZero = false;
break;
}
}
return allZero;
}
Test C: is_all_zero. Proposed by Lundin.
BOOL is_all_zero (unsigned char *data, size_t length)
{
BOOL result = TRUE;
unsigned char* end = data + length;
unsigned char* i;
for(i=data; i<end; i++) {
if(*i > 0) {
result = FALSE;
break;
}
}
return result;
}
Test D: sumArray. This is the top answer from the nearly duplicate question, proposed by vladr.
BOOL sumArray (unsigned char *data, size_t length) {
int sum = 0;
for (int i = 0; i < length; ++i) {
sum |= data[i];
}
return (sum == 0);
}
Test E: lulz. Proposed by Steve Jessop.
BOOL lulz (unsigned char *data, size_t length) {
if (length == 0) return 1;
if (*data) return 0;
return memcmp(data, data+1, length-1) == 0;
}
Test F: NSData. This is a test using NSData object I discovered in the iOS SDK while working on all of these. It turns out Apple does have an idea of how to compare byte streams that is designed to be hardware independent.
- (BOOL)nsdTestData: (NSData *)nsdData length: (NSUInteger)length {
void *tester = (void *)calloc(sizeof(void), (unsigned long)length);
NSData *nsdTester = [NSData dataWithBytesNoCopy:tester length:(NSUInteger)length freeWhenDone:NO];
int test = [nsdData isEqualToData:nsdTester];
free(tester);
return (test);
}
Results
So how did these approaches compare? Here are two sets of data, each representing 5000 loops through the check. First I tried this on the iPhone Simulator running on a relatively old iMac, then I tried this running on a first generation iPad.
On the iPhone 4.3 Simulator running on an iMac:
// Test A, nullToLength: 0.727 seconds
// Test F, NSData: 0.727
// Test E, lulz: 0.735
// Test C, is_all_zero: 7.340
// Test B, allZero: 8.736
// Test D, sumArray: 13.995
On a first generation iPad:
// Test A, nullToLength: 21.770 seconds
// Test F, NSData: 22.184
// Test E, lulz: 26.036
// Test C, is_all_zero: 54.747
// Test B, allZero: 63.185
// Test D, sumArray: 84.014
These are just two samples, I ran the test many times with only slightly varying results. The order of performance was always the same: A & F very close, E just behind, C, B, and D. I'd say that A, F, and E are virtual ties, on iOS I'd prefer F because it takes advantage of Apple's protection from processor change issues, but A & E are very close. The memcmp approach clearly wins over the simple loop approach, close to ten times faster in the simulator and twice as fast on the device itself. Oddly enough, D, the winning answer from the other thread performed very poorly in this test, probably because it does not break out of the loop when it hits the first difference.
I think you should do it with an explicit loop, but just for lulz:
if (length == 0) return 1;
if (*pdata) return 0;
return memcmp(pdata, pdata+1, length-1) == 0;
Unlike memcpy, memcmp does not require that the two data sections don't overlap.
It may well be slower than the loop, though, because the un-alignedness of the input pointers means there probably isn't much the implementation of memcmp can do to optimize, plus it's comparing memory with memory rather than memory with a constant. Easy enough to profile it and find out.
Not sure if it's the best, but I probably would do something like this:
bool allZero = true;
for (int i = 0; i < size_t; i++){
if (*data++){
//Roll back so data points to the non-zero char
data--;
//Do whatever is needed if it isn't zero.
allZero = false;
break;
}
}
If you've just allocated this memory, you can always call calloc rather than malloc (calloc requires that all the data is zeroed out). (Edit: reading your comment on the first post, you don't really need this. I'll just leave it just in case)
If you're allocating the memory yourself, I'd suggest using the calloc() function. It's just like malloc(), except it zeros out the buffer first. It's what's used to allocate memory for Objective-C objects and is the reason that all ivars default to 0.
On the other hand, if this is a statically declared buffer, or a buffer you're not allocating yourself, memset() is the easy way to do this.
Logic to get a value, check it, and set it will be at least as expensive as just setting it. You want it to be null, so just set it to null using memset().
This would be the preferred way to do it in C:
BOOL is_all_zero (const unsigned char* data, size_t length)
{
BOOL result = TRUE;
const unsigned char* end = data + length;
const unsigned char* i;
for(i=data; i<end; i++)
{
if(*i > 0)
{
result = FALSE;
break;
}
}
return result;
}
(Though note that strictly and formally speaking, a memory cell containing a NULL pointer mustn't necessarily be 0, as long as a null pointer cast results in the value zero, and a cast of a zero to a pointer results in a NULL pointer. In practice, this shouldn't matter as all known compilers use 0 or (void*) 0 for NULL.)
Note the edit to the initial question above. I did some tests and it is clear that the memcmp approach or using Apple's NSData object and its isEqualToData: method are the best approaches for speed. The simple loops are clearer to me, but slower on the device.
Why 'exactly' does this code loop endlessly if you enter a non number character?
The first question comes about because I want to learn good defensive coding. Does anyone know a good way to check user input? My google-fu failed me. Some people seemed to be of the opinion that if I specify %f in scanf that I am 'demanding' a float; I verified this, in a way, by printing the value of userInput. In fact, if I comment out the do while loop, there is 'no problem' with the execution of the code. It assigns a 0 to userInput and goes about its business.
#import <Foundation/Foundation.h>
int main (int argc, const char * argv[]) {
NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
float userInput;
float result;
NSLog(#"3X^3 -5x^2 + 6");
do {
NSLog(#"What is x?");
scanf("%f", &userInput);
NSLog(#"userInput = %f", userInput);
} while(userInput == 0);
result = 3 * (userInput * userInput * userInput) - 5 * (userInput * userInput) + 6;
NSLog(#"the result is: %f", result);
[pool drain];
return 0;
}
This is really nothing to do with Objective-C or Cocoa. The issue is simply to do with the use of the standard C library function scanf, and handling the error condition. From the scanf manpage, describing the return code:
Zero indicates that, although there was input available, no conversions were assigned; typically this is due to an invalid input character, such as an alphabetic character for a `%d' conversion.
A valid numeric input can be parsed by scanf with the %f specifier, so that obviously works as expected. But if you enter in a non-numeric character, scanf cannot convert this to a float, and leaves the text in the buffer of stdin. Since the code is not checking the return code from scanf, and only testing if userInput is non-zero, the loop will never exit, as userInput happens to start at 0.0, and will never be updated as scanf will not pull the non-numeric characters out of the stdin buffer. So that is why your code runs in an infinite loop.
If you had initialized userInput to a non-zero value, that would fix the problem one way, as non-numeric input would cause scanf to fail and the while condition would be triggered. But a better fix would be to check the return code of scanf. If it is zero, print an error message, and do a fpurge(stdin) to clear out the invalid input before you loop around again, like this:
int rc = scanf("%f", &userInput);
if (rc == 0)
{
NSLog(#"Invalid input, try again.");
fpurge(stdin);
}
So this is the plain C approach to input and parsing. The bottom line for defensive coding is that you should always check the return code!
As Chris mentions, for an actual Cocoa application, you would want to look at NSNumberFormatter and the like, but then you would presumably be taking input from widgets rather than file streams, so the code would be quite different to the above.
The proper way to validate user input in Cocoa is to use an instance of an appropriate subclass of NSFormatter, in this case something like NSNumberFormatter.