I am trying to use sscanf to convert an array of char in C to a double. My issue is best described in this short minimal working example:
#include <stdio.h>
int main (int argc, char **argv)
{
char string_one[] = "1.2e-4";
char string_two[6] = "1.2e-4";
double one, two;
sscanf(string_one, "%lf", &one);
sscanf(string_two, "%lf", &two);
printf("%e, %e\n", one, two);
return 0;
}
Compiled with GCC and running the resulting executable returns 1.200000e-04, 1.200000e-41, instead of the same number twice. The issue does not appear if the floating point number I want to convert is of the form 3e4(if there is no decimal separator).
For my program I have to define an array of char with a fixed size (as I am filling the array during the course of the program), which is why I am running into this issue.
I guess the problem might be pretty obvious - but as I am a newbie in C I am out of ideas what the problem could be. Any help appreciated!
char string_two[6] = "1.2e-4"; ... sscanf(string_two, "%lf", &two); expects to scan a string. string_two[] is not a string as it lacks a null character. Result: undefined behavior (UB).
Solution: Insure string_two[] is a string.
Anyone know how to copy to strings? Cause I used the function strcpy but when I print the result it show strange characters. I want to concatenate 'name' + '#' + 'e-mail'. With scanf I have to put the character null '\0'?
#include <stdio.h>
#include <string.h>
int main (){
char message[150];
char name[150];
char mail[150];
char result[150];
printf("Introduce name: \n");
scanf("%s",message);
printf("Introduce email \n");
scanf("%s",server);
strcpy(result,message);
result[strlen(result)]='#';
strcpy(&result[strlen(result)],server);
printf("RESULT: %s\n",result);
return 0;
}
result[strlen(result)]='#'; overwrites the NUL terminator introduced into result by strcpy(result,message);. So the result of a subsequent strlen is undefined.
A better solution is to use strncat, or you could get away with writing
char result[150] = {'\0'};
which will initialise the entire array.
But you still run the risk of overflowing your result array. You could use the safer strncpy to obviate that. Better still, use snprintf and have the C standard library perform the concatenation for you.
Whenever i declare a varible as char and scan it as string "%s" my output console crashes. Here is the code
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>
int main()
{
char a[20];
int i;
printf("Enter a name ");
scanf("%s",&a);
for(i=0;i<strlen(a);i++)
{
a[i] = toupper(a[i]);
i++;
printf("%s\n",toupper(a[i]));
}
return 0;
}
The second i++; inside the for loop body may cause the index to point off-by-one then, printf("%s\n",toupper(a[i])); will be out of bound access which invokes undefined behavior.
You can remove the i++ from inside the loop body.
Next, toupper(a[i]) returns an int, which is invalid for %s format specifier, which again invokes UB.
That said,
to prevent buffer overflow from excessive long input, it's best to limit the input length with scanf(),
You don't need to pass the address of an array, just the array name will be sufficient
So, overall, you should write
scanf("%19s",a);
You have two problems in the loop where you use toupper and print.
The first is that you increment the variable i twice in the loop.
The second problem is the printf:
printf("%s\n",toupper(a[i]));
Here you ask printf to print a string, but as argument you give it a single character (actually an int, toupper returns an int). Use the "%c" format specifier to print a single character.
By the way, you don't need to call toupper when printing, as the character should already be in upper-case because of the previous assignment.
Your printing loop is mis-behaving and giving you undefined behavior:
printf("%s\n",toupper(a[i]));
is passing toupper(a[i]) to printf()'s %s format specifier, which requires a char * not a plain char. That triggers undefined behavior.
There's no point in printing each character one by one, instead convert the entire string to upper case, then print it once:
for(i = 0; a[i] != '\0'; ++i)
{
a[i] = (char) toupper((unsigned char) a[i]);
}
printf("%s\n", a);
The casts around toupper() are sometimes needed since it takes and returns int, and you want to be a bit careful about when converting to/from characters.
Notice that I factored out the call to strlen(), this might be a bit "too clever" but it's how I would expect this code to be written. If we're going to loop over the characters, there's no need to compute the length separately.
I'm relatively new to C. I wanted to lern the language a bit by solving coderbyte challenges.
But I'm stucked at the first. It is supposed to be a simple String reverse algorithm.
When I input things like "asdf" or "1234567" the output is correct ("fdsa", "7654321"). But when I type "12345678" or "thisiscool" I get "87654321▒#"/"loocsisiht#" as a result. I don't know where the # are comming from.
This is my code:
#include <stdio.h>
#include <string.h>
void FirstReverse(char str[]) {
int len = strlen(str);
char nstr[len];
int i;
for(i = 0; i < len; i++) {
nstr[i] = *(str+len-1-i);
}
printf("%s\n", nstr);
}
int main(void) {
char str[100];
FirstReverse(gets(str));
return 0;
}
Can someone please tell me where I can find the error?
Thanks in advance :)
In C, strings are zero-terminated. A string "cat", for example, has 4 characters, and is represented as ('c','a','t',(char)0). You forgot about the final 0.
Note that strlen returns the string length without the final 0, so a string foo contains strlen(foo)+1 characters. Remember this when you allocate strings.
As the other answers have mentioned, you're missing a terminator.
It should also be noted that it's bad practice to allocate strings the way you did. An array should always have a fixed size if you create it that way.
You should instead do:
char * nstr = malloc(sizeof(char) * (len+1));
Thereby allocating the size of each character (1 byte) times the lenght.
Note the +1 because you need room for the string terminator.
When you call printf(, string); , it's gonna start from the first letter and print everything up to the terminator. Since you have no terminator here, it prints random characters, such as #.
What you're gonna wanna do to fix that, is adding:
nstr[i] = '\0';
after your loop.
Also remember to free the allocated memory.
You forgot to allocate a char for the terminating '\0' in nstr[].
So, better use: char nstr[len + 1]; and set nstr[len] = 0;
Furthermore: gets() is evil: from the glibc manual page:
Never use gets(). Because it is impossible to tell without knowing the data in advance how many characters gets() will read, and because gets() will continue to store characters past the end of the buffer, it is extremely dangerous to use. It has been used to break computer security. Use fgets() instead.
Alright I've been at this all day and can't for the life of me get this down, maybe you chaps can help. I have a file that reads as follows
1301,105515018,"Boatswain","Michael R.",ABC, 123,="R01"
1301,103993269,"Castille","Michael Jr",ABC, 123,="R03"
1301,103993267,"Castille","Janice",ABC, 123,="R03"
1301,104727546,"Bonczek","Claude",ABC, 123,="R01"
1301,104731479,"Cruz","Akeem Mike",ABC, 123,="R01"
1301,105415888,"Digiacomo","Stephen",ABC, 123,="R02"
1301,106034479,"Annitto Grassis","Susan",ABC, 123,="R04"
1301,106034459,"Als","Christian",ABC, 123,="R01"
And here is my code...
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAX_NAME 15
#define MAX_SUBSEC 3
#define N 128
//void printArr(struct *students);
struct student{
int term;
int id;
char lastname[MAX_NAME];
char firstname[MAX_NAME];
char subjectname[MAX_SUBSEC];
int catalog;
char section[MAX_SUBSEC];
}students[10];
int main(){
int term;
int id;
char lastname[MAX_NAME];
char firstname[MAX_NAME];
char sub[MAX_SUBSEC];
int cat;
char sec[MAX_SUBSEC];
char fname[N];
FILE *inputf;
printf("Enter the name of the text file: ");
scanf("%123s",fname);
strcat(fname,".txt");
inputf = fopen(fname,"r");
if (inputf == NULL){
printf("I couldn't open the file for reading.\n");
exit(0);
}
//TROUBLE HERE!
fscanf(inputf, "%d,%d,%[^,]s", &students[0].term, &students[0].id,students[0].lastname);
printf("%d\n", students[0].term);
printf("%d\n", students[0].id);
printf("%s\n", students[0].lastname);
/*for (j = 1 ; j <= 10-1 ; j++){
for(k = 0 ; k <= 10-2 ; k++){
if(students[k] > students[k+1]){
temp = students[k];
students[k] = students[k+1];
students[k+1] = temp;
}
}
}*/
fclose(inputf);
system("pause");
return 0;
}
void printArr(int a[], int tally){
int i;
for(i = 0 ; i < tally ; i++){
printf("%d ", a[i]);
}
printf("\n");
}
My objective is to take each one of those values in the text file and input it to where it belongs in the struct and subsequently the struct array, but I can't get passed the first 2 ints.
Getting the lastname string, because it is a max of 15 characters, it spills over into the first name string right after it and takes what remaining characters it needs in order to fill up the lastname char array. Obviously I do not want this. As you can see I have tried strtok but it doesnt do anything, not sure what I have to do though as I have never used it before. Also have tried just including all the variables into fscanf statement, but I either get the same output, or it becomes a mess. As it is, I am extremely lost, how do I get these values into the variables they belong?!
EDIT: updated my code, I have gotten a little farther but not much. I can now print out just the last name but can not more farther from there, I cant get to the firstname string or any of the variables beyond it.
What you have there is a CSV file with quoted strings, and so I would recommend you use a CSV parser (or roll your own) rather than trying to do it all with scanf (since scanf cannot deal with quotes, e.g. commas within quoted strings). A quick Google search turns up libcsv.c which you may be able to use in your project.
With the fscanf format string "%d,%d,\"%[^\"]\",\"%[^\"]\",%[^,],%d,=\"%[^\"]\"" we can read a whole line's data. Besides, you have to define
char lastname[MAX_NAME+1];
char firstname[MAX_NAME+1];
char subjectname[MAX_SUBSEC+1];
int catalog;
char section[MAX_SUBSEC+1];
— the +1 to account for the terminating '\0' character.
I have a question for you... If you want to know how to use a diamond cutter, do you try it and see, or do you consult the manual? The problem here isn't the result of your choice, but your choice itself. Believe it or not, I have answered these questions so often that I'm tired of repeating myself. The answer is all in the manual.
Read the POSIX 2004 scanf manual — or the POSIX 2008/2013 version — and the answer this question and you'll have some idea of what you're not doing that you should be. Even fscanf code should use assert as a debugging aid to ensure the number of items read was correct.
%[^,]s It seems as though there's a mistake here. Perhaps you meant %[^,]. The %[ format specifier is a different format specifier to the %s format specifier, hence in the presumably mistaken code there are two directives: %[^,] and s. The s directive tells scanf to read an 's' and discard it.
1.There is a syntax error in
while(result != NULL){
printf(".....);
......
}
}//error
fscanf(inputf, "%s", lastname); can't read a line ,fscanf will stop when it comes across an space
fscanf reads one line at a time, and you can easily capture the contents of each line because your file is formatted pretty nicely, especially due to the comma separation (really useful if none of your separated values contain a comma).
You can pass fscanf a format like you're doing with "%d" to capture an int, "%s" to capture a string (ends at white space, be weary of this when for example trying to find a name like "Annitto Grassis, which would require 2 %s's), etc, from the currently read line of the file. You can be more advanced and use regex patterns to define the contents you want captured as chars, such as "Boatswain", a sequence comprised chars from the sets {A-Z}, {a-z}, and the {"}. You'll want to scan the file until you reach the end (signified by EOF in C) so you can do such and capture the contents of the line and appropriately assign the values to variables like so:
while( fscanf(inputf, "%d,%d,%[\"A-Za-z ],%[\"A-Za-z .]", &term, &id, lastname, firstname) != EOF) {
.... //do something with term, id, lastname, firstname - put them in a student struct
}
For more about regex, Mastering Regex by Jeff Friedl is a good book for learning about the topic.