Let's say I am creating a 3*4 matrix (or a 2D array of 12 elements). So I want user to enter values of elements one by one as a sequence, divided by either spaces/tabs or enter-key. Also, if a value in a sequence is bad (in my case, any non-integer values are bad), I want to ignore it and read next values until I have all 12 of them.
int fill(Matrix * mtx)
{
puts("Start entering numbers. Please note that only integer values will be recorded.");
int temp = 0;
for (int i = 1; i <= mtx -> rows; i++)
{
for (int j = 1; j <= mtx -> cols; j++)
{
scanf(" %d", &temp);
setElement(mtx, i, j, temp);
}
}
return 0;
}
This is my most basic vision of the algorithm; I was wondering about the implementation of this "skip input if bad" condition.
Just started learning C btw, so any kind of advice is hugely appreciated!
You have to check the return value of scanf to be sure whether it scanned the integer input correctly or not. In case it fails due to some bad input - scanf can't take that as input and then you have to make sure that you clear the stdin so that the next calls of scanf don't fail. What you can do is, when scanf returns 0 - consume all characters (using getchar or similar) and do this until you get \n (Considering that user inputs the number each in a line). Illustration would be:
int n; //total number inputs to take.
n = 10;
int num, num_inputs = 0;
while( 1 )
{
while(scanf("%d", &num) != 1)
{
int c;
while ((c = getchar()) != EOF && c != '\n')
;
if(c == EOF){ fprintf(stderr,"Error in input\n"); exit(1)}
}
num_inputs++; // one more int correctly input.
...
if( num_inputs == n )
break;
}
An alternative and better way would be to use strto* functions and then considering the return value of it to understand whether there is any error or not. Illustration would be: (Here we have shown just the case where there is single int input in each line - this can be extended to process multiple int inputs in a line).
char buf[MAXLEN];
int num_inputs = 0;
while(fgets(buf,MAXLEN,stdin)){
char *en;
errno = 0;
long n = strtol(line, &en, 10);
if (errno == 0 && *en== '\0' && n >= INT_MIN && n < INT_MAX)
{
// n is an correctly inputted int
num_inputs++;
}
}
Check the man page for strtol - there is a detailed listing of the erros one might get. Check it.
Check what scanf returns. If not 1 means entered value is not a digit in this case. So discard the data from the stream first before entering next data.
int fill(Matrix * mtx) {
puts("Start entering numbers. Please note that only integer values will be recorded.");
int temp = 0, v;
for (int i = 1; i <= mtx->rows; i++) {
for (int j = 1; j <= mtx->cols; j++) {
v = scanf(" %d", &temp);
if (v != 1) { //if invalid input.
while ( (v = getchar()) != EOF && v != '\n' ); // discard invalid input.
j--; //don't forget to `j--` or else you will skip one position.
}
else { // if correct input.
setElement(mtx, i, j, temp);
}
}
}
return 0;
}
Related
So I'm trying to do a program that reads a sequence of numbers separated by spaces and new lines. The output should be the same sequence, but erasing unnecessary zeros(The sequence of charachters 'EOF' ends the program). Per example
01492 102934 should come out as 1492 102934
9312 0 01923 should come out as 9312 0 1923
0001249 0000 should come out as 1249 0
Well I've achieved that purpose but have come across a roadblock. The program doesn't exit unless I type the EOF sequence. Maybe it's because I have a while(1) running that gives an infinite loop. But when I try to delete it the program doesn't even print at all. I'm still learning this is for a school project.
Any help would be apreciated!
Here's the code:
#include <stdio.h>
int main(){
char c;
int i=0;
while(1){
c=getchar();
if (i==0){
if(c=='0'){
while (c=='0'){
c=getchar();
}
}
printf("%c",c);
i=i+1;
}
else if (c==' '){
printf("%c",c);
c=getchar();
if(c=='0'){
while (c=='0'){
c=getchar();
}
}
printf("%c",c);
}
else if (c=='E'){
c=getchar();
if (c=='O'){
c=getchar();
if(c=='F'){
printf("\n");
return 0;
}
}
}
else{
printf("%c",c);
}
}
}
The important stuff:
int c; // IMPORTANT, cannot be char
while (1) {
c = getchar();
if (c == EOF) break; // exit loop
// ...
}
There has to be some way to tell the program to exit.
With this, the program will exit on the letter x or two consecutive newlines or entering END.
getchar will return EOF when there is nothing left to read from a file. That can be simulated from stdin ( the keyboard) with ctrl + z on Windows or ctrl + d on Linux.
#include <stdio.h>
#include <string.h>
int main ( void) {
char done[4] = "";
int c = 0;
int prior = 0;
int reading = 0;
int zero = 1;
while ( EOF != ( c = getchar ( )) && 'x' != c) {
if ( '\n' == c && '\n' == prior) {
break;
}
if ( c >= '0' && c <= '9') {
reading = 1;
if ( '0' != c) {
zero = 0;
}
if ( ! zero) {
putchar ( c);
}
}
else {
if ( reading) {
if ( zero) {
putchar ( '0');
}
if ( ' ' == c || '\n' == c) {
putchar ( c);
}
else {
putchar ( ' ');
}
}
reading = 0;
zero = 1;
}
prior = c;
done[0] = done[1];
done[1] = done[2];
done[2] = c;
done[3] = 0;
if ( 0 == strcmp ( done, "END")) {
break;
}
}
putchar ( '\n');
return 0;
}
getchar() returns an int, not a char. If it only returned a char, there would be no way for it to return a value that indicates end of file, since all char values are valid and can’t be used for another purpose.
A motivating example in decimal system may be: A function checks the temperature returns a two-digit number. Any temperature between 0 and 99 is valid. How do you report errors when the thermometer is disconnected? You have to return a number with more digits, and use a special value like UNPLUGGED = 100.
But int is a wider type: it has many more values than char, and the “extra” values can be used to indicate some special condition that means “hey, this is not a valid character, but something else I had to tell you”.
getchar() returns the EOF constant upon failure (any failure), for example if no more input is available. There’s nothing sensible you can do even if the reason for the failure other than end of input. You should end processing at the first EOF.
Thus, change the type of c to int, and every time you call getchar(), you must check that its value is not EOF, and return when you encounter it.
The nested structure of your loops means that EOF checking has to be repeated all over the place. There are other ways to structure the code to keep this check in one place, but, admittedly, the nested loops have at least the potential to exploit the branch predictor, whereas a single getchar followed by a state-machine style switch statement will make it perform potentially worse. None of this matters in a simple homework problem, but it’s something to keep in mind. In any case, performance has to be benchmarked - no other way around it.
Try this code, I think it does what you requested:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
static int getLine(char *prmpt, char *buff, size_t sz) {
int ch, extra;
// Get line with buffer overrun protection.
if (prmpt != NULL) {
printf("%s", prmpt);
fflush(stdout);
}
if (fgets(buff, sz, stdin) == NULL)
return -2;
// If it was too long, there'll be no newline. In that case, we flush
// to end of line so that excess doesn't affect the next call.
if (buff[strlen(buff) - 1] != '\n') {
extra = 0;
while (((ch = getchar()) != '\n') && (ch != EOF))
extra = 1;
return (extra == 1) ? -1 : 0;
}
// Otherwise remove newline and give string back to caller.
buff[strlen(buff) - 1] = '\0';
return 0;
}
int* convert2numbers(char* arr, int size) {
int i;
int j;
int k;
char token[100];
int* numbers;
int last_space = 0;
int index = 1;
int amount = 1;
// Count the amount of tokens.
for (i = 0; i < size; ++i) {
if (arr[i] == ' ') {
++amount;
}
}
numbers = (int *)malloc(amount * sizeof(int));
numbers[0] = amount;
for (j = 0; j <= size; ++j) {
if (arr[j] == ' ' || arr[j] == '\0') {
// Copy token from input string.
for (k = 0; k < j; ++k) {
token[k] = arr[k + last_space];
}
token[j] = '\0';
numbers[index] = atoi(token);
// Clear the token and continue.
memset(token, '\0', sizeof(token));
last_space = j;
++index;
}
}
return numbers;
}
int main(void) {
int i;
int size;
int* numbers;
int amount;
char input[100];
char help[] = "Numbers> ";
printf("Input numbers below or press enter to exit!\n");
while (1) {
getLine(help, input, sizeof(input));
// If input is empty exit.
if (input[0] == '\0') {
break;
}
size = strlen(input);
numbers = convert2numbers(input, size);
amount = numbers[0];
for (i = 1; i < amount + 1; ++i) {
printf("%d ", numbers[i]);
}
printf("\n");
}
return 0;
}
When run with these inputs this code outputs:
Input numbers below or press enter to exit!
Numbers> 01492 102934
1492 102934
Numbers> 9312 0 01923
9312 0 1923
Numbers> 0001249 0000
1249 0
Also if you press enter in console, it exits, as to escape the while(1) loop, easily.
I wrote a program that scans an unknown amount of integers into an array but when I run it, it print the last value it has gotten an infinite amount of times.
For example for the input: 1 2 3 4 5
The output would be 55555555555555555555555...
Why does this happen and how can I fix that?
My goal here is to create a array, for an instance {1, 2, 3, 4, 5} and then print what it scanned into the array, ONLY ONCE...
int *pSet = (int*) malloc(sizeof(int)); int i; int c;
printf("Please enter a stream of numbers to make a set out of them: ");
printf("\n");
scanf("%d", &c);
pSet[0] = c;
printf("%d ", c);
for(i = 1; c != EOF; i++) {
pSet = (int*) realloc(pSet, sizeof(int)*(i+1));
if(pSet == NULL) {
return FAIL;
}
scanf("%d", &c);
pSet[i] = c;
printf("%d ", c);
}
free(pSet);
Why does this happen (?) (print ... an infinite amount of times.)
Look at the loop terminating conditions c != EOF.
int c;
scanf("%d", &c);
for(i = 1; c != EOF; i++) { // Not good code
scanf("%d", &c);
}
EOF is some negative value, often -1. scanf("%d", &c) attempts to read user input and convert to an int. scanf() returns a 1,0,EOF depending on if it 1) succeeded, 2) failed to find numeric text or 3) end-of-file or input error occurred. Unfortunately code does not use that return value. Instead code used the number read, c and checked if that number read was the same as EOF.
how can I fix that?
Only loop when the return value of scanf() is as expected (1).
for(i = 1; scanf("%d", &c) == 1; i++) {
...
}
Putting this together with some other ideas
#include <stdio.h>
#include <stdio.h>
int main(void) {
printf("Please enter a stream of numbers to make a set out of them:\n");
int *pSet = NULL; // Start with no allocation
size_t i = 0;
int c;
for (i = 0; scanf("%d", &c) == 1; i++) {
// +--------------------------- No cast needed.
// v v----------v Use sizeof de-referenced pointer
void *p = realloc(pSet, sizeof *pSet * (i + 1));
if (p == NULL) {
free(pSet);
return EXIT_FAILURE;
}
pSet = p;
pSet[i] = c;
}
for (size_t j = 0; j < i; j++) {
printf("%d ", pSet[j]);
}
free(pSet);
return 0;
}
There are a number of problems.
1) Terminate the loop when scanf fails instead of using EOF. Do that by checking that the return value is 1 (i.e. the number of input items
successfully matched)
2) Don't allocate memory until it's needed
3) Never do realloc directly into the target pointer - always use a temp variable.
Fixing this your code could be:
#include <stdio.h>
int main(void) {
int *pSet = NULL;
printf("Please enter a stream of numbers to make a set out of them: ");
printf("\n");
int i = 0;
int c;
while (1) {
if (scanf("%d", &c) != 1)
{
printf("Terminating input loop\n");
break;
}
int* tmp = realloc(pSet, sizeof(int)*(i+1));
if(tmp == NULL) {
printf("oh dear...\n");
break;
}
pSet = tmp;
pSet[i++] = c;
printf("%d ", c);
}
for (int j=0; j < i; ++j) printf("%d\n", pSet[j]);
free(pSet);
return 0;
}
Input:
1 2 3 4 5 6 7 stop
Output:
Please enter a stream of numbers to make a set out of them:
1 2 3 4 5 6 7
Terminating input loop
1
2
3
4
5
6
7
You should stop your loop when scanf fails. According to the manual:
On success, [scanf] return[s] the number of input items successfully matched and assigned; this can be fewer than provided for, or even zero, in the event of an early matching failure.
The value EOF is returned if the end of input is reached before either the first successful conversion or a matching failure occurs. EOF is also returned if a read error occurs. [...]
So you can turn your for loop into a while one.
#include <stdio.h>
#include <stdlib.h>
#define FAIL 0
int main() {
int *pSet = (int*) malloc(sizeof(int));
int c;
int i=0;
printf("Please enter a stream of numbers to make a set out of them: ");
while(scanf("%d", &c) == 1) {
pSet[i] = c;
pSetNew = (int*) realloc(pSet, sizeof(int)*(i+1));
if(pSetNew == NULL) {
free(pSet);
return FAIL;
} else {
pSet = pSetNew;
}
printf("%d ", c);
i++;
}
free(pSet);
}
But if you want a more robust piece of code, I suggest you to retrieve the answer as a string (NULL-terminated array of char), and then parse it with dedicated functions like strtol which let you check if the whole string is a valid entry, and not only the first characters.
Note: HengLi fixed a potential memory leak in the code sample above
I am creating a program where I insert a number of sentences and the program outputs them in order. I have finished the program, but when I run it it seems like the characters I input into the array aren't displayed or stored correctly, getting as a result random letters instead of the full sentence. Here is the code of the program:
char ch;
int i,j,k;
int nothing = 0;
int count = 1;
char lines[5][256];
int length[256];
int main() {
printf("Please insert up to a max of 5 lines of text (Press enter to go to next line and twice enter to stop the program):\n");
i = 0;
while (i<5){
j = 0;
ch = getche();
if (ch == '\r'){
if(i!= 0){
break;
}
printf("You have not inserted anything, please insert a line:");
i=-1;
}
if(ch != '\r'){
lines[i][j]=ch;
while (ch!='\r'){
ch = getche();
lines[i][j] = ch;
j++;
}
}
printf("\n");
i++;
}
for (k=i ; k > 0; k--){
printf("\tphrase %i :", count);
for ( j =0 ; j <= length[k]; j++){
printf("%c",lines[j][k]);
}
count++;
printf("\n");
}
return 0;
}
How can I get the characters to be stored and displayed correctly? Any help is appreciated, thank you!!
There are numerous problems with your code. I'll try and summarise here, and give you improved code.
Fist, some changes that I made to get this to compile on my system:
Changed getche() to getchar() (getche() does not appear to be available on Ubuntu).
I took out the section about re-entering a string, and just focused on the rest (since the logic there was slightly broken, and not relevant to your question). It will still check for at least one line though, before it will continue.
I had to change the check for \r to \n.
I changed your length array to size 5, since you'll only have the lengths of maximum 5 strings (not 256).
Some problems in your code:
You never updated the length[] array in the main while loop, so the program never knew how many characters to print.
Arrays are zero indexed, so your final printing loops would have skipped characters. I changed the for parameters to start at zero, and work up to k < i, since you update i after your last character in the previous loop. The same with j.
Your reference to the array in the printing loop was the wrong way around (so you would've printed from random areas in memory). Changed lines[j][k] to lines[k][j].
No need for a separate count variable - just use k. Removed count.
The nothing variable does not get used - removed it.
#include <stdlib.h>
#include <stdio.h>
char ch;
int i,j,k;
char lines[5][256];
int length[5];
int main()
{
printf("Please insert up to a max of 5 lines of text (Press enter to go to the next line and twice enter to stop the program):\n");
i = 0;
while (i<5)
{
j = 0;
ch = getchar();
if ((ch == '\n') && (j == 0) && (i > 0))
{
break;
}
if (ch != '\n')
{
while (ch != '\n')
{
lines[i][j] = ch;
j++;
ch = getchar();
}
}
length[i] = j;
printf("\n");
i++;
}
for (k = 0; k < i; k++)
{
printf("\tPhrase %i : ", k);
for (j = 0; j < length[k]; j++)
{
printf("%c", lines[k][j]);
}
printf("\n");
}
return 0;
}
I'm trying to take the input of floating numbers from a file and arrange it into an array.
The only trouble is that I don't know exactly how many floating numbers there will be each time though I do know that the max amount of floating numbers is 1000.
What I need to do is have fscanf take all the floating numbers but then stop at the next line, which is full of integers.
Here is what I have so far for this section:
for (repetition = 0; repetition <= 1000; repetition++)
{
fscanf(userFile, "%f", &itemPrice[itemNumber]);
itemNumber++;
}
But unfortunately, this continues on to assign array values to all of the other values in the next several lines.
I found another user input, auctionItems and used that to control the array length using while(itemNumber < auctionItems)
The return value of fscanf is the number of items successfully read. Use that to decide when to stop.
#include <stdio.h>
int main(int argc,char * argv[])
{
int i;
float ff[1000];
char next_text[17];
for (i=0; i < 1000; i++) {
int n_read;
n_read = fscanf(stdin, " %f", &( ff[i] ));
if (n_read < 1) {
fscanf(stdin, "%16s", next_text);
next_text[16] = (char) 0;
printf("Next text: '%s'\n", next_text);
break;
}
}
printf("Read %d items, %f .. %f\n",i,ff[0],ff[i-1]);
return 0;
}
Maybe you could try this method using fgetc and fputc:
for (repetition = 0; itemPrice <= 1000; repetition++)
{
int c = fgetc(userFile);
if (c == '\n' || c == EOF) break;
fputc(c);
fscanf(userFile, "%f", &itemPrice[itemNumber]);
itemNumber++;
}
char input[MAX];
while(fgets(input, MAX, userFile) != NULL){
sscanf(input, "%lf", &itemPrice[itemNumber++]);
}
OP is fairly close. Just stop when scanning fails.
fscanf() returns the number of fields scanned or EOF. In this case, checking for 1 is sufficient.
// for (repetition = 0; itemPrice <= 1000; repetition++) Use <, not <= #Arpit
for (i = 0; i < 1000; )
{
int retval = fscanf(userFile, "%f", &itemPrice[i]);
if (retval != 1) break;
i++;
}
A robust solution would detect unexpected data. This solution simple goes until the end-of-file or non-float text is encountered.
[Edit]
It appears OP has only 1 line of floats. In that case code could read the whole line and then parse the buffer.
#define CHAR_PER_FLOAT_MAX 20
char buf[1000*(CHAR_PER_FLOAT_MAX + 1)]; // Some large buffer
if (fgets(buf, sizeof buf, input)== NULL) return; // EOF or IO error
char *p = buf;
for (i = 0; i < 1000; ) {
int n = 0;
if (sscanf(p, "%f %n", &itemPrice[i], &n) != 1) break;
p += n;
i++;
}
if (*p) Handle_ExtraTextOnLine();
foo(itemPrice, i); // Use data
Another approach is to read direct from the file 1 float at a time and then look at the following white-space for an end-of-line. Less elegant.
for (i = 0; i < 1000; ) {
if (fscanf(input, "%f", &itemPrice[i]) != 1) {
// need to add code to consume the rest of the line if processing is to continue.
break;
}
i++;
// look for standard white-space
char buf[2];
while (fscanf(input, "%1[ \f\n\r\t\v]", buf) == 1) {
if (buf[0] == '\n') break;
}
}
foo(itemPrice, i); // Use data
My task is read two strings of digits and save them in different arrays.
I decided to use scanf function, but program can read only first string.
This is my bad-code.
int main()
{
int firstArray[50], secondArray[50], i, j;
/* fill an array with 0 */
for(i=0; i<50; ++i)
{
firstArray[i]=secondArray[i]=0;
}
i=j=0;
while((scanf("%d", &firstArray[i]))== 1) { ++i; }
while((scanf("%d", &secondArray[j]))== 1) { ++j; }
/* Print this. */
for(i = 0; i < 20; ++i)
{
printf("%d ", firstArray[i]);
}
putchar('\n');
for(j = 0; j < 20; ++j)
{
printf("%d ", secondArray[j]);
}
return 0;
}
I just don't understand how scanf function works. Can someone please explain?
scanf ignores blank characters (including new line). Thus your scan will read entire input into firstArray if you have no "non blank" separator.
If file/data has ; at end of first line it will stop the read into firstArray there, and never read anything into secondArray - as you never consume the ;.
/* This will never be 1 as ; is blocking */
while((scanf("%d", &secondArray[i])) == 1) {
So: if you separate with i.e. ; you will have to read / check for this before you read into secondArray.
You could also add something like:
char c;
/* this can be done more tidy, but only as concept */
while((scanf("%d", &firstArray[i])) == 1 && i < max) {
++i;
if ((c = getchar()) == '\n' || c == ';')
break;
}
Also instead of initializing array to 0 by loop you can say:
int firstArray[50] = {0}; /* This set every item to 0 */
Also take notice to ensure you do not go over your 50 limit.
You say strings of digits and you read %d. The format scans the input for the longest sequence representing an integer (signed) value. Two "digit strings" are consumed by the first while loop.
EDIT Instead of "strings of digits" you should say "strings of integers". In this case it is a little bit more subtle since the first while can consume all the integers, unless they are separated by something that is not a possible integer (e.g. a ;).
So, to make the following to work, you must separate the two "lines" with something that can't be parsed as integer and which is not considered "white character". Not the better solution, but one the possible.
#include <stdio.h>
#include <ctype.h>
int main()
{
int firstArray[50] = {0};
int secondArray[50] = {0};
int i, j, l1, l2;
int tmp;
i = j = 0;
// read integers, but not more than size of array
while( scanf("%d", &firstArray[i]) == 1 && i < sizeof(firstArray) ) {
++i;
}
// consume non digits
for(tmp = getchar(); tmp != EOF && !isdigit(tmp); tmp = getchar());
// on EOF you should exit and stop processing;
// we read one more char, push it back if it was a digit
if (isdigit(tmp)) ungetc(tmp, stdin);
while( scanf("%d", &secondArray[j]) == 1 && j < sizeof(secondArray) ) {
++j;
}
l1 = i; // preserve how many ints were read
l2 = j;
/* Print this. */
for(i = 0; i < l1; ++i)
{
printf("%d ", firstArray[i]);
}
putchar('\n');
for(j=0; j < l2; ++j)
{
printf("%d ", secondArray[j]);
}
return 0;
}
EDIT A solution that maybe fits your need better is to read the lines (one per time) into a buffer and sscanf the buffer.
You cannot use scanf to do that.
Read the documentation.
Observations:
with scanf if you enter a digit your loop runs forever
there is no check on size 50 limit of your arrays
if you press return then it ignores that line because does not match your pattern
if you enter a letter the pattern does not match and loop breaks
So use some other function, maybe gets, atoi or strtol. And remember to check the size 50 limit of your arrays.
Actually, there is one special point in C's arrays.
Though you declare an array's size. say int arr[5]; You can store values beyond the size of 5. It doesn't show any error but leads to undefined behavior (Might overwrite other variables).
Please Refer this question: Array size less than the no. of elements stored in it
In you case, that was your problem. The compiler had never passed beyond the first while statements. Thus, you didn't get any output. In fact, it didn't even compile the whole code yet!
while((scanf("%d", &firstArray[i]))== 1) { ++i; }
So, you could write this while statement like this:
while( scanf("%d", &firstArray[i]) ==1 && i<50 )
i++;
or else:
while(i<50 )
{
scanf("%d", &firstArray[i]);
i++;
}
or else:
for (i=0; i<50; i++)
scanf("%d", &firstArray[i]);