I used fgets() to read input of a file into a
char buf[10];
the input of the file is
10,10,4,10
I want to iterate through the line and store each number in the char array as its own individual integer value but I am a little lost on how to do that. If someone could point me in the right direction I'd really appreciate it, thanks!
There are different approaches to do this. If you are learning C, I would recommend writing your own by-hand solution and after understanding how it works, use some standard library functions like strtok as #Jeff Holt said.
The easiest approach is to create a variable which will be the current number which we are reading and iterate over the buf array and at each step check if the current character is a number or not.
See this question about converting characters to integers.
So the result will be something like that:
const int size = 10;
char buf[size];
// fill buf, but be sure that input is less than 10 characters
//create array for result
char result[size];
int result_size = 0;
//can also be char, but if input numbers are big enough, it might overflow
int current_value = 0;
for (int i = 0; i < size; i++) {
if (buf[i] >= '0' && buf[i] <= '9') {
//convert to int and add to current_value
} else {
//store parsed integer
result[current_size] = current_value;
current_size++;
current_value = 0;
}
}
Related
I'm not familiar with C at all so this might be a simple problem to solve. I'm trying to take an input char* array of binary character sequences, ex. "0100100001101001", and output its relative string ("Hi"). The problem I'm having is coming up with a way to split the input into seperate strings of length 8 and then convert them individually to ulimately get the full output string.
char* binaryToString(char* b){
char binary[8];
for(int i=0; i<8; ++i){
binary[i] = b[i];
}
printf("%s", binary);
}
I'm aware of how to convert 8-bit into its character, I just need a way to split the input string in a way that will allow me to convert massive inputs of 8-bit binary characters.
Any help is appreciated... thanks!
From what I can tell, your binaryToString() function does not do what you'd want it to. The print statement just prints the first eight characters from the address pointed to by char* b.
Instead, you can convert the string of 8 bits to an integer, utilizing a standard C function strtol(). There's no need to convert any further, because binary, hex, decimal, etc, are all just representations of the same data! So once the string is converted to a long, you can use that value to represent an ASCII character.
Updating the implementation (as below), you can then leverage it to print a whole sequence.
#include <stdio.h>
#include <string.h>
void binaryToString(char* input, char* output){
char binary[9] = {0}; // initialize string to 0's
// copy 8 bits from input string
for (int i = 0; i < 8; i ++){
binary[i] = input[i];
}
*output = strtol(binary,NULL,2); // convert the byte to a long, using base 2
}
int main()
{
char inputStr[] = "01100001011100110110010001100110"; // "asdf" in ascii
char outputStr[20] = {0}; // initialize string to 0's
size_t iterations = strlen(inputStr) / 8; // get the # of bytes
// convert each byte into an ascii value
for (int i = 0; i < iterations; i++){
binaryToString(&inputStr[i*8], &outputStr[i]);
}
printf("%s", outputStr); // print the resulting string
return 0;
}
I compiled this and it seems to work fine. Of course, this can be done cleaner and safer, but this should help you get started.
I just need a way to split the input string in a way that will allow me to convert massive inputs of 8-bit binary characters.
You can use strncpy() to copy the sequence of '0' and '1' in a chunk of 8 characters at a time from the input string, something like this:
//get the size of input string
size_t len = strlen(b);
//Your input array of '0' and '1' and every sequence of 8 bytes represents a character
unsigned int num_chars = len/8;
//Take a temporary pointer and point it to input string
const char *tmp = b;
//Now copy the 8 chars from input string to buffer "binary"
for(int i=0; i<num_chars; ++i){
strncpy(binary, tmp+(i*8), 8);
//do your stuff with the 8 chars copied from input string to "binary" buffer
}
Maybe this can help. I didnt compile it but there is the idea. You can loop every 8 bit separately with while loop. And assign 8 bit to binary array with for loop. After that send this binary array to convert8BitToChar function to get letter equivalent of 8 bit. Then append the letter to result array. I'm not writing c for 3 year if there is mistakes sorry about that. Here pseudo code.
char* binaryToString(char* b){
char* result = malloc(sizeof(256*char));
char binary[8];
int nextLetter = 0;
while (b[nextLetter*8] != NULL) { // loop every 8 bit
for(int i=0; i<8; ++i){
binary[i] = b[nextLetter*8+i];
}
result[nextLetter] = 8bitToChar(binary));// convert 8bitToChar and append yo result
nextLetter++;
}
result[nextLetter] = '\0';
return result;
}
I am attempting to put a list of characters AND integers into an array of just integers. The file.txt looks like:
a 5 4 10
4 10 a 4
In the array I want the values to come out as {97,5,4,10,4,10,97,4}
This is part of my code:
int * array = malloc(100 * sizeof(int));
FILE* file;
int i=0;
int integer = 1;
file=fopen(filename,"r");
while (fscanf(file,"%d",&integer) > 0)
{
array[i] = integer;
i++;
}
Your problem is that, at first read, your while condition will exit because first element in the file is a char and fscanf won't interpret it as an integer, returning 0. I would suggest, if you are sure that your separator is a space, reading a string (it will automatically stop at space) and convert read value to int with strtol.
Something like:
int * array = malloc(100 * sizeof(int));
FILE* file;
int i=0;
char tmp[2], *pEnd;
file=fopen("./test.txt","r");
while (fscanf(file,"%s",tmp) > 0)
{
if( !(array[i] = strtol(tmp, &pEnd,10)))
array[i] = tmp[0];
i++;
}
Note that I assumed that you'll have no integer bigger than one digit (tmp array size) and that I check strtol response for detecting non integer chars.
It seems to me that what you want to do is use fscanf("%s", some_string) since numerics can be received as strings but strings cannot be received as numerics. Then with each input, you need to decide if the string is actually numeric or not, and then derive the value you want to place into the array accordingly.
I have a string that has ints and I'm trying to get all the ints into another array. When sscanf fails to find an int I want the loop to stop. So, I did the following:
int i;
int getout = 0;
for (i = 0; i < bsize && !getout; i++) {
if (!sscanf(startbuffer, "%d", &startarray[i])) {
getout = 1;
}
}
//startbuffer is a string, startarray is an int array.
This results in having all the elements of startarray to be the first char in startbuffer.
sscanf works fine but it doesn't move onto the next int it just stays at the first position.
Any idea what's wrong? Thanks.
The same string pointer is passed each time you call sscanf. If it were to "move" the input, it would have to move all the bytes of the string each time which would be slow for long strings. Furthermore, it would be moving the bytes that weren't scanned.
Instead, you need to implement this yourself by querying it for the number of bytes consumed and the number of values read. Use that information to adjust the pointers yourself.
int nums_now, bytes_now;
int bytes_consumed = 0, nums_read = 0;
while ( ( nums_now =
sscanf( string + bytes_consumed, "%d%n", arr + nums_read, & bytes_now )
) > 0 ) {
bytes_consumed += bytes_now;
nums_read += nums_now;
}
Convert the string to a stream, then you can use fscanf to get the integers.
Try this.
http://www.gnu.org/software/libc/manual/html_node/String-Streams.html
You are correct: sscanf indeed does not "move", because there is nothing to move. If you need to scan a bunch of ints, you can use strtol - it tells you how much it read, so you can feed the next pointer back to the function on the next iteration.
char str[] = "10 21 32 43 54";
char *p = str;
int i;
for (i = 0 ; i != 5 ; i++) {
int n = strtol(p, &p, 10);
printf("%d\n", n);
}
This is the correct behavior of sscanf. sscanf operates on a const char*, not an input stream from a file, so it will not store any information about what it has consumed.
As for the solution, you can use %n in the format string to obtain the number of characters that it has consumed so far (this is defined in C89 standard).
e.g. sscanf("This is a string", "%10s%10s%n", tok1, tok2, &numChar); numChar will contain the number of characters consumed so far. You can use this as an offset to continue scanning the string.
If the string only contains integers that doesn't exceed the maximum value of long type (or long long type), use strtol or strtoll. Beware that long type can be 32-bit or 64-bit, depending on the system.
I am new to the C programming language and trying to improve by solving problems from the Project Euler website using only C and its standard libraires. I have covered basic C fundamentals(I think), functions, pointers, and some basic file IO but now am running into some issues.
The question is about reading a text file of first names and calculating a "name score" blah blah, I know the algorithm I am going to use and have most of the program setup but just cannot figure out how to read the file correctly.
The file is in the format
"Nameone","Nametwo","billy","bobby","frank"...
I have searched and searched and tried countless things but cannot seem to read these as individual names into an array of strings(I think thats the right way to store them individually?) I have tried using sscanf/fscanf with %[^\",]. I have tried different combos of those functions and fgets, but my understanding of fgets is everytime I call it it will get a new line, and this is a text file with over 45,000 characters all on the same line.
I am unsure if I am running into problems with my misunderstanding of the scanf functions, or my misunderstanding with storing an array of strings. As far as the array of strings goes, I (think) I have realized that when I declare an array of strings it does not allocate memory for the strings themselves, something that I need to do. But I still cannot get anything to work.
Here is the code I have now to try to just read in some names I enter from the command line to test my methods.
This code works to input any string up to buffer size(100):
int main(void)
{
int i;
char input[100];
char* names[10];
printf("\nEnter up to 10 names\nEnter an empty string to terminate input: \n");
for(int i = 0; i < 10; i++)
{
int length = 0;
printf("%d: ", i);
fgets(input, 100, stdin);
length = (int)strlen(input);
input[length-1] = 0; // Delete newline character
length--;
if(length < 1)
{
break;
}
names[i] = malloc(length+1);
assert(names[i] != NULL);
strcpy(names[i], input);
}
}
However, I simply cannot make this work for reading in the formatted strings.
PLEASE advise me as to how to read it in with format. I have previously used sscanf on the input buffer and that has worked fine, but I dont feel like I can do that on a 45000+ char line? Am I correct in assuming this? Is this even an acceptable way to read strings into an array?
I apologize if this is long and/or not clear, it is very late and I am very frustrated.
Thank anyone and everyone for helping, and I am looking forward to finally becoming an active member on this site!
There are really two basic issues here:
Whether scanning string input is the proper strategy here. I would argue not because while it might work on this task you are going to run into more complicated scenarios where it too easily breaks.
How to handle a 45k string.
In reality you won't run into too many string of this size but it is nothing that a modern computer of any capacity can't easily handle. Insofar as this is for learning purposes then learn iteratively.
The easiest first approach is to fread() the entire line/file into an appropriately sized buffer and parse it yourself. You can use strtok() to break up the comma-delimited tokens and then pass the tokens to a function that strips the quotes and returns the word. Add the word to your array.
For a second pass you can do away with strtok() and just parse the string yourself by iterating over the buffer and breaking up the comma tokens yourself.
Last but not least you can write a version that reads smaller chunks of the file into a smaller buffer and parses them. This has the added complexity of handling multiple reads and managing the buffers to account for half-read tokens at the end of a buffer and so on.
In any case, break the problem into chunks and learn with each refinement.
EDIT
#define MAX_STRINGS 5000
#define MAX_NAME_LENGTH 30
char* stripQuotes(char *str, char *newstr)
{
char *temp = newstr;
while (*str)
{
if (*str != '"')
{
*temp = *str;
temp++;
}
str++;
}
return(newstr);
}
int main(int argc, char *argv[])
{
char fakeline[] = "\"Nameone\",\"Nametwo\",\"billy\",\"bobby\",\"frank\"";
char *token;
char namebuffer[MAX_NAME_LENGTH] = {'\0'};
char *name;
int index = 0;
char nameArray[MAX_STRINGS][MAX_NAME_LENGTH];
token = strtok(fakeline, ",");
if (token)
{
name = stripQuotes(token, namebuffer);
strcpy(nameArray[index++], name);
}
while (token != NULL)
{
token = strtok(NULL, ",");
if (token)
{
memset(namebuffer, '\0', sizeof(namebuffer));
name = stripQuotes(token, namebuffer);
strcpy(nameArray[index++], name);
}
}
return(0);
}
fscanf("%s", input) reads one token (a string surrounded by spaces) at a time. You can either scan the input until you encounter a specific "end-of-input" string, such as "!", or you can wait for the end-of-file signal, which is achieved by pressing "Ctrl+D" on a Unix console or by pressing "Ctrl+Z" on a Windows console.
The first option:
fscanf("%s", input);
if (input[0] == '!') {
break;
}
// Put input on the array...
The second option:
result = fscanf("%s", input);
if (result == EOF) {
break;
}
// Put input on the array...
Either way, as you read one token at a time, there are no limits on the size of the input.
Why not search the giant string for quote characters instead? Something like this:
#include <stdio.h>
#include <string.h>
int main(void)
{
char mydata[] = "\"John\",\"Smith\",\"Foo\",\"Bar\"";
char namebuffer[20];
unsigned int i, j;
int begin = 1;
unsigned int beginName, endName;
for (i = 0; i < sizeof(mydata); i++)
{
if (mydata[i] == '"')
{
if (begin)
{
beginName = i;
}
else
{
endName = i;
for (j = beginName + 1; j < endName; j++)
{
namebuffer[j-beginName-1] = mydata[j];
}
namebuffer[endName-beginName-1] = '\0';
printf("%s\n", namebuffer);
}
begin = !begin;
}
}
}
You find the first double quote, then the second, and then read out the characters in between to your name string. Then you process those characters as needed for the problem in question.
I am processing an input string, which consists of a process name, followed by an arbitrary amount of arguments.
I need the process name , along with all of the arguments, in one string.
I thought I could use strcat in a loop, so that it cycles through all of the args and each time appends the arg to the string, but I am having problems with getting a string that in empty to begin the loop.
Can anyone help me out with some basic code?
Thanks.
EDIT:
I'm posting my code for clarity. Mike's post is closest to what I have now:
char * temp;
strcpy(temp,"");
for (i = 4; i < argc-1; i++) // last arg is null, so we need argc-1
{
strcat(temp,argv[i]);
strcat(temp," ");
}
ignore the 4 in my for loop for the moment (magic number, i know.)
I am getting a segfault with this code. Is it because of my string assignment? I assume that is the case and hence I asked the question of how i could combine the strings.
Let's say your input strings are in an array of char pointers, suggestively called argv, of length argc.
We first need to determine how much space is needed for the output:
int length = 0;
for (int i = 0; i < argc; ++i)
length += strlen(argv[i]);
Then we allocate it, adding an extra char for the '\0' terminator:
char *output = (char*)malloc(length + 1);
Finally, the concatenation:
char *dest = output;
for (int i = 0; i < argc; ++i) {
char *src = argv[i];
while (*src)
*dest++ = *src++;
}
*dest = '\0';
Note that I don't use strcat here. Reason is that this sets us up for a Schlemiel the Painter's algorithm: for each iteration, the entire output string would be scanned to find its end, resulting in quadratic running time.
Don't forget to free the output string when you're done:
free(output);
I'm a bit tired so I may be overlooking something here. A better solution, using standard library functions, is welcome. It would be convenient if strcat returned a pointer to the terminator byte in dest, but alas.
You want an empty C string? Is this what you are looking for: char p[] = "";?
UPDATE
After you posted some code it is clear that you have forgotten to allocate the buffer temp. Simply run around the arguments first, counting up the length required (using strlen), and then allocate temp. Don't forget space for the zero terminator!
You could provide the "arbitrary amount of arguments" as one argument, ie an array/list, then do this pseudocode:
str = "";
i = 0;
while i < length of input
{
str = strcat ( str , input[i]);
i++;
}
#include<stdio.h>
#include<stdarg.h>
int main(int argc, char** argv) {
// the main parameters are the same situation you described
// calling this program with main.exe asdf 123 fdsa, the program prints out: asdf123fdsa
int lengths[argc];
int sum =0;
int i;
for(i=1; i<argc; i++) { // starting with 1 because first arg is program-path
int len = strlen(argv[i]);
lengths[i] = len;
sum+=len;
}
char* all = malloc(sum+1);
char* writer = all;
for(i=1; i<argc; i++) {
memcpy(writer, argv[i], lengths[i]);
writer+=lengths[i];
}
*writer = '\0';
printf("%s\n", all);
system("pause");
return 0;
}
A string in C is represented by an array of characters that is terminated by an "null" character, '\0' which has the value 0. This lets all string functions know where the end of a string is. Here's an exploration of different ways to declare an empty string, and what they mean.
The usual way of getting an empty string would be
char* emptyString = "";
However, emptyString now points to a string literal, which cannot be modified. If you then want to concatenate to an empty string in your loop, you have to declare it as an array when you initialize.
char buffer[] = "";
This gives you an array of size one. I.e. buffer[0] is 0. But you want an array to concatenate to- it has to be large enough to accomodate the strings. So if you have a string buffer of certain size, you can initialize it to be empty like so:
char buffer[256] = "";
The string at buffer is now "an empty string". What it contains, is buffer[0] is 0 and the rest of the entries of the buffer might be garbage, but those will be filled once you concatenate your other strings.
Unfortunately, the problem with C, is you can never have an "infinite" string, where you are safe to keep concatenating to, you have to know it's definite size from the start. If your array of arguments are also strings, you can find their length using strlen. This gives you the length of a string, without the null character. Once you know the lengths of all your sub-strings, you will now know how long your final buffer will be.
int totalSize; // assume this holds the size of your final concatenated string
// Allocate enough memory for the string. the +1 is for the null terminator
char* buffer = malloc(sizeof(char) * (totalSize + 1));
buffer[0] = 0; // The string is now seen as empty.
After this, you are free to concatenate your strings using strcat.