How do i read some 5 to 10 characters from a sample txt file using an fread funtion.
I have the following code:
#include <stdio.h>
main()
{
char ch,fname[20];
FILE *fp;
printf("enter the name of the file:\t");
gets(fname);
fp=fopen(fname,"r");
while(fread(&ch,1,1,fp)!=0)
fwrite(&ch,1,1,stdout);
fclose(fp);
}
when i enter any sample filename..it prints all the data of the file.
my question is how to print only the first 5 to 10 characters from the sample file.
Your while loop runs until read reaches the end of the file (reads 0 bytes for the first time).
You will want to change the condition by using a for loop or a counter.
i.e. (these are suggestions, not the full working code):
int counter = 10;
while(fread(&ch,1,1,fp)!=0 && --counter)
fwrite(&ch,1,1,stdout);
or
int i;
for(i=0; i < 10 && fread(&ch,1,1,fp) > 0 ; i++)
fwrite(&ch,1,1,stdout);
Good luck!
P.S.
To answer your question in the comments, fread allows us to read the data in "atomic units", so that if a whole unit isn't available, no data will be read.
A single byte is the smallest unit (1), and you are reading one unite (of a single byte), this is the 1,1 part in the fread(&ch,1,1,fp).
You could read 10 units using fread(&ch,1,10,fp) or read all the bytes unrequited for a single binary int (this won't be portable - it's just a demo) using int i; fread(&i,sizeof(int),1,fp);
read more here.
Here is a modified version of your code. Check the comments at the lines that are modified
#include <stdio.h>
#define N_CHARS 10 // define the desired buffer size once for code maintenability
int main() // main function should return int
{
char ch[N_CHARS + 1], fname[20]; // create a buffer with enough size for N_CHARS chars and the null terminating char
FILE *fp;
printf("enter the name of the file:\t");
scanf("%20s", fname); // get a string with max 20 chars from stdin
fp=fopen(fname,"r");
if (fread(ch,1,N_CHARS,fp)==N_CHARS) { // check that the desired number of chars was read
ch[N_CHARS] = '\0'; // null terminate before printing
puts(ch); // print a string to stdout and a line feed after
}
fclose(fp);
}
Related
I understand how to read in a text file and scan/print the entire file, but how can a line be split into several strings? Also, can variables be assigned to those strings to be called later?
My code so far:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main()
{
FILE *fPointer;
fPointer = fopen("p1customer.txt", "r");
char singleLine[150];
int id;
while (!feof(fPointer)){
fscanf(fPointer, "%d", &id);
printf("%d",id);
}
fclose(fPointer);
return 0;
}
Example Text File to be read:
99999 John Doe Basketball
Example Output:
John Doe has ID number 99999 and plays Basketball
I am attempting to split/tokenize those strings and assign them variables (IDnumber, Name, Sport) and print the output in a new file.
you can use a library function strtok(str,chrs) function.
A sequence of calls of strtok(str,chrs) splits str into tokens, each delimited by a character from chrs.
The first call in a sequence is a non Null str.It finds the first token in str consisting of chars not int chrs;it terminates that by overwrtting the next characters of str by \0 and return pointer to token. Each subsequent call,indicated by a NULL value of str,retuens a pointer to next such token, searching from just past the end of privious one.
You should post an example of the input file so that you can help in more detail.
I've seen you've also entered a string, I guess you want to fill in with something but you did not specify that.
If you wanted to treat the file as a list of numbers, the sample of the code might be the following.
#include <stdio.h>
int main() {
FILE *infile;
char buf[100];
int len_file=0;
if(!(infile = fopen("p1customer.txt", "r"))) { /*checks the correct opening of the file*/
printf("Error in open p1customer.txt\n");
return 1;
}
while(fgets(buf,sizeof(buf),infile)!=NULL) /*check the lenght of the file (number of row) */
len_file++;
int id[len_file];
int i=0;
rewind(infile);
while(fgets(buf,sizeof(buf),infile)!=NULL) {
sscanf(buf,"%i",&id[i]);
i++;
}
for(i=0;i<len_file;i++)
printf("%i\n",id[i]);
fclose(infile);
return 0;
}
If you want to treat the file as an indefinite list of numbers on each row separated by a space, you can use the parsing of the string by using in the sscanf formatting %31[^ ]which has the task of reading the number until it encounters a space, also you can add a variable that is incremented for each char/number read.
Then you can refine the code by checking if there are any characters in the line using the isalpha function in the ctype.h library to see if there are any characters and then insert them into a string until you find the termination character '\ 0'.
The possibilities are infinite so it would useful have the input file, when you provided it, i'll update the answer.
Here's the program:
#include <stdio.h>
#define BUF_LEN 200
#define LINE_NUMBER 3
int line_len(char* filename, int n)
{
FILE* f;
char buf[BUF_LEN];
int j, i = 0;
if ((f = fopen(filename, "r")))
{
for (j = 0; j < n; j++)
fgets(buf, BUF_LEN, f);
for (i = 0; buf[i]; i++) /* find end of buf */ ;
fclose(f);
}
return i;
}
int main()
{
printf("%d\n", line_len("test.txt", LINE_NUMBER));
return 0;
}
From what I understand, the function line_len receives the name of the file and the number of line we are interested in. It then opens the file in a read only mode and iterates till reaching the line n, through each iteration reading BUF_LEN-1 characters from the file f and storing these characters in buf. So when the first for loop breaks, buf will contain all the characters of the first n lines.
I do not understand the need for the second loop. When does it terminate?
How does this function work? If at the end of the first for loop buf will contain the characters of the first n lines, then how come this function returns the length of the line n?
Thanks in advance!
I do not understand the need for the second loop. When does it terminate?
The second loop has buf[i] as its loop condition. It will keep executing as long as buf[i] is true i.e. non zero. So when it gets to the nul character at the end of the line (added by fgets()) the loop will terminate.
How does this function work?
Simplistically, it reads n lines. Each line is put into the buffer overwiriting the previous line. After it's read n lines, it counts the characters in the buffer from when it read the nth line.
It's a poor piece of code though. There's no error checking on the fgets call and if the nth line has more than 199 characters in it, it will give the wrong answer. In fact, if you consider the length of a line to exclude the line feed, it always gets the wrong answer.
It also returns zero if the file has fewer than n lines and if it was unable to open the file. If an error occurs reading a line, it returns an indeterminate number and if it runs off the end of the file, the length of the last line will be returned
An error result would be better in those cases.
I have a text file which I have filled a number lines from different texts, having different line length.
What I want to do is calculate the average number characters per line which matters to me in my job. I wrote the following code in C to achieve this. However I cannot run the program once it is compiled.
#include<stdio.h>
#include<stdlib.h>
#define LENGTH 10000000
int main()
{
char c;
int i;
int line_length;
int j;
int char_count;
char char_a[LENGTH];
int line_a[LENGTH];
int line_count;
long sum;
float avg_char_count;
FILE *fp=fopen("input.txt","r");
if(!fp){
fprintf(stderr,"cannot open file");
exit(1);
}
/*read into file*/
i=0;
sum=0;
while(char_a[i++]=fgetc(fp))
sum++;
printf("chars count: %d \n",sum);
/*process array*/
char_count=i;
j=0;
line_count=0;
while(j++<char_count){
if(char_a[j]=='\n'){
sum--;
line_count++;
}
}
/* calculate the average*/
avg_char_count=sum/(float)line_count;
printf("\naverage # of chars in a line is: %f\n ",avg_char_count);
return EXIT_SUCCESS;
}
By the way I am using Borland C++ command-line tool BCC32, running on Windows 7 SP1.
What's wrong with my code?
Try declaring char_a and line_a as pointers to char and int as:
char *char_a;
int *line_a;
And then allocate memory dynamically using malloc.
char_a=(char*)malloc(10000000*sizeof(char));
line_a=(int*)malloc(10000000*sizeof(int));
Secondly, your while loop should end when you reach end of file, i.e. EOF.
while(char_a[i]=fgetc(fp)){
if(char_a[i++]==EOF)
break;
sum++;
}
And, you should initialize the line_count to 1 instead of 0, because when there is no '\n' in the text file, there can still be one line. If there is one '\n' in the text file, it means there are two lines (say, you are in line 1, and then you hit enter, which is '\n', and then you get to the new line, so for one '\n', there are 2 lines).
/*process array*/
char_count=sum;
j=0;
line_count=1;
while(j++<char_count){
if(char_a[j]=='\n'){
sum--;
line_count++;
}
}
NOTE-Currently your char_count include newlines ('\n') when it is printed. Print the statement in the end, because in the end of your program, you have already excluded the newlines by decrementing the sum in the if statement of the second while loop.
The most probable cause is that you allocate 20 Mb of variables on the stack.
I would change the program so that it reads the file on line at a time (or even one character at a time).
That way you only need to allocate space for one line and not for the entire file.
code doesnt accept values from file since when i print some garbage pops up
the file is a textfile with a letter representing a status, and theres a number. there are 5 sets of these , each on new line the letter and the number is seperated with a space, i need to get the numbers in one array
#include <stdio.h>
#include <stdlib.h>
typedef struct {
char status;
int number;
} information;
int main() {
int array[5] = {0};
int i;
FILE *pointer = fopen("transaction22.txt", "r");
information information1;
for (i = 0; i < 5; i++) {
fread(&information1, sizeof(information1), 1, pointer);
array[i] = information1.number;
printf("%d", information1.number);
}
return 0;
}
You can use fscanf instead of fread as:
fscanf(pointer, "%s %d", info1.status, &info1.number);
where status will be defined as char status[2]; inside struct information.
fread is used for reading raw bytes (blocks of data) from a file.
Now, you are using a text file and trying to read sizeof(info) amount of data which is 5 bytes of data (assuming 32-bit int), therefore what you get after first fread is:
info1.status (1 byte) info1.number (4 bytes)
Byte 1 SPACE + number + NEWLINE + [One more byte]
read by fread (next four bytes read by fread)
Thus info1.number is storing a garbage value.
Also the successive fread call starts reading after the data read by previous fread.
Better to use a character array like c[5]. and replace this is with struct. variable in fread ..like in for loop
fread(c,sizeof(c),1,pointer)
than use
print("%s\n",c);
Let's say I'm reading in numbers from a text file. The text file consists of one-hundred fifty digit numbers (those are separate, there are 100 instances of 50 digit numbers).
I wanted to save each number as a row of a 2D array. To do this, I declared an array
char input[99][50] //50 columns to utilize the newlines in the text file
But it wouldn't read in the entire text file, even though, it seemed to me, it was the right size. It read in through the 99th number. For the 100th line, it printed a newline then a bunch of garbage symbols, etc. Please see the following:
#include <stdio.h>
#include <stdlib.h>
int main(void)
{
//Set up the use of the input text file
FILE * ifp;
ifp = fopen("input.txt", "r");
//Declare variables
char input[100][50]; //Array to hold the input numbers
int i, j; //Variables to work with loops
//Begin by reading in the input file as characters,
//otherwise fscanf will take each entire line as
//a single number
for (i = 0; i <= 99; i++)
{
printf("%d)", i);
for (j = 0; j <= 50; j++)
{
fscanf(ifp, "%c", &input[i][j]);
printf("%c", input[i][j]);
}
}
system("pause");
return 0;
}
This will print it correctly. The thing that seems weird to me, is that it doesn't actually need to use that extra row that solves the problem...fscanf still only functions for the same range as before (0-99).
So...why does the array need to be [100]x[50]? Why does [99]x[50] cause so many issues?
Also, I added a newline to the last line of the text file because if I didn't, instead of a newline it printed an apostrophe ' to the screen at the end of the last line. Is this the symbol for some sort of end of file character?
Thanks in advance!
By the way, if you're interested in compiling this and seeing it happen, here's the text file input.txt:
37107287533902102798797998220837590246510135740250
46376937677490009712648124896970078050417018260538
74324986199524741059474233309513058123726617309629
91942213363574161572522430563301811072406154908250
23067588207539346171171980310421047513778063246676
89261670696623633820136378418383684178734361726757
28112879812849979408065481931592621691275889832738
44274228917432520321923589422876796487670272189318
47451445736001306439091167216856844588711603153276
70386486105843025439939619828917593665686757934951
62176457141856560629502157223196586755079324193331
64906352462741904929101432445813822663347944758178
92575867718337217661963751590579239728245598838407
58203565325359399008402633568948830189458628227828
80181199384826282014278194139940567587151170094390
35398664372827112653829987240784473053190104293586
86515506006295864861532075273371959191420517255829
71693888707715466499115593487603532921714970056938
54370070576826684624621495650076471787294438377604
53282654108756828443191190634694037855217779295145
36123272525000296071075082563815656710885258350721
45876576172410976447339110607218265236877223636045
17423706905851860660448207621209813287860733969412
81142660418086830619328460811191061556940512689692
51934325451728388641918047049293215058642563049483
62467221648435076201727918039944693004732956340691
15732444386908125794514089057706229429197107928209
55037687525678773091862540744969844508330393682126
18336384825330154686196124348767681297534375946515
80386287592878490201521685554828717201219257766954
78182833757993103614740356856449095527097864797581
16726320100436897842553539920931837441497806860984
48403098129077791799088218795327364475675590848030
87086987551392711854517078544161852424320693150332
59959406895756536782107074926966537676326235447210
69793950679652694742597709739166693763042633987085
41052684708299085211399427365734116182760315001271
65378607361501080857009149939512557028198746004375
35829035317434717326932123578154982629742552737307
94953759765105305946966067683156574377167401875275
88902802571733229619176668713819931811048770190271
25267680276078003013678680992525463401061632866526
36270218540497705585629946580636237993140746255962
24074486908231174977792365466257246923322810917141
91430288197103288597806669760892938638285025333403
34413065578016127815921815005561868836468420090470
23053081172816430487623791969842487255036638784583
11487696932154902810424020138335124462181441773470
63783299490636259666498587618221225225512486764533
67720186971698544312419572409913959008952310058822
95548255300263520781532296796249481641953868218774
76085327132285723110424803456124867697064507995236
37774242535411291684276865538926205024910326572967
23701913275725675285653248258265463092207058596522
29798860272258331913126375147341994889534765745501
18495701454879288984856827726077713721403798879715
38298203783031473527721580348144513491373226651381
34829543829199918180278916522431027392251122869539
40957953066405232632538044100059654939159879593635
29746152185502371307642255121183693803580388584903
41698116222072977186158236678424689157993532961922
62467957194401269043877107275048102390895523597457
23189706772547915061505504953922979530901129967519
86188088225875314529584099251203829009407770775672
11306739708304724483816533873502340845647058077308
82959174767140363198008187129011875491310547126581
97623331044818386269515456334926366572897563400500
42846280183517070527831839425882145521227251250327
55121603546981200581762165212827652751691296897789
32238195734329339946437501907836945765883352399886
75506164965184775180738168837861091527357929701337
62177842752192623401942399639168044983993173312731
32924185707147349566916674687634660915035914677504
99518671430235219628894890102423325116913619626622
73267460800591547471830798392868535206946944540724
76841822524674417161514036427982273348055556214818
97142617910342598647204516893989422179826088076852
87783646182799346313767754307809363333018982642090
10848802521674670883215120185883543223812876952786
71329612474782464538636993009049310363619763878039
62184073572399794223406235393808339651327408011116
66627891981488087797941876876144230030984490851411
60661826293682836764744779239180335110989069790714
85786944089552990653640447425576083659976645795096
66024396409905389607120198219976047599490197230297
64913982680032973156037120041377903785566085089252
16730939319872750275468906903707539413042652315011
94809377245048795150954100921645863754710598436791
78639167021187492431995700641917969777599028300699
15368713711936614952811305876380278410754449733078
40789923115535562561142322423255033685442488917353
44889911501440648020369068063960672322193204149535
41503128880339536053299340368006977710650566631954
81234880673210146739058568557934581403627822703280
82616570773948327592232845941706525094512325230608
22918802058777319719839450180888072429661980811197
77158542502016545090413245809786882778948721859617
72107838435069186155435662884062257473692284509516
20849603980134001723930671666823555245252804609722
53503534226472524250874054075591789781264330331690
So...why does the array need to be [100]x[50]? Why does [99]x[50]
cause so many issues?
100 is the size you allocate, the indices go from 0 to 99 (99 + 1 = 100 ).
When you only allocate 99 you're missing the last line.
Since you're using a C string to handle the fifty digits, you have to get 1 extra char to terminate the string.
char input[100][51];
for (int i = 0; i != 100; ++i) /* 100 entries */
{
/* read your 50 digits normally here */
input[i][50] = '\0'; /* remember to terminate the string */
}