I'm working with C and I need to check that the user inputed second command line argument argv[1] is made up of only alphabetical charchaters and if not, to do what is inside the else loop. I used the is alpha function but when i compile and run the program no matter what my second command line argument is (alphabetical or otherwise), its always executing the "else loop". How do i fix this?
#include <stdio.h>
#include <cs50.h>
#include <ctype.h>
#include <string.h>
#include <stdlib.h>
int main(int argc, string argv[])
{
int a = argc;
if (a != 2)
{
return 1;
}
string b = argv [1];
int c = strlen(b);
string m;
for (int i = 0; i < c; i++)
{
if (isalpha(b[c]))
{
m = GetString();
}
else
{
printf("Please provide a valid keyword\n");
return 1;
}
}
}
Try replacing
if (isalpha(b[c]))
with
if (isalpha(b[i]))
Currently you are checking the element at the index which is the result of strlen(b) at every iteration of your loop. Because array indices are zero based in C b[strlen(b)] is referencing '\0', the null terminator.
In reference to the Keith Thompson comment below and the answer to this question you should actually be casting the value passed to isalpha to an unsigned char to ensure that undefined behaviour is not invoked.
Thus you should change your code to
if (isalpha((unsigned char)b[i]))
to ensure there is no UB
Use isalpha(b[i]) instead of isalpha(b[c])
like this:
if (isalpha(b[i]))
{
m = GetString();
}
Related
here is my code
#include <stdio.h>
#include <stdlib.h>
int main(int argc,char* argv[])
{
int a;
for(int i=1;i<=argc;i++){
a+=atoi(argv[i]);
}
printf ("%d",a);
}
I keep getting segmentation faults but i am trying to add up all elements of the command line so for example ./a.out 5 6 7 would give 18 as the output, cheers.
The problem (with the crash) is the loop itself:
for(int i=1;i<=argc;i++)
The argc argument is the number of arguments passed to the program, including the "program name" at argv[0]. So valid indexes for the actual arguments are argv[1] to argv[argc - 1].
Furthermore the argv array is terminated by a null pointer, which will be at argv[argc].
Since you include argv[argc] in your loop you pass a null pointer to atoi which leads to undefined behavior and likely crashes.
The simple solution is to use less-than < instead of less-than-or-equal as the loop condition:
for(int i=1;i<argc;i++)
You never initialized a to 0. Also, use strtol() function.
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char **argv) {
int a = 0;
for (int i = 1; i < argc; i++) {
a += strtol(argv[i], NULL, 10);
}
printf("%d\n", a);
return EXIT_SUCCESS;
}
I am working on creating a shell and I haven't use C for a while. I have the shell initizing properly but when I try to compare the user input to an array of strings I have I get a segmentation fault. I was planning on adding casce statements in a the for loop to initiate each of the processes once they are called by the user. I haven't included those since I have been trying to figure out how to get the user input to match with a value in my string array. Under debug I was only receiving the first character of the builtins[j] value which kind of makes since since it is a pointer right. However I am stuck and could use some ideas for why this isn't returning 0 when I input "exit". Thanks
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/stat.h>
#include <fcntl.h>
//This code is for creating a basic shell
void init_shell(int num, char *prompt[]){
char s1[] = "-p";
int result;
if(num>1){
result = strcmp(s1, prompt[1]);
if(result==0){
printf("%s>$", prompt[2]);
}else{
printf("308sh>$");
}
}
//printf("%s\n %s\n %s\n %d\n", prompt[0], prompt[1], prompt[2], result);
else{
printf("308sh>$");
}
}
//The infinite loop for accepting user input until it closes
int main(int argc, char *argv[]){
const char *builtins[7];
builtins[0] = "exit\n";
builtins[1] = "pid\n";
builtins[2] = "ppid\n";
builtins[3] = "cd\n";
builtins[4] = "pwd\n";
builtins[5] = "set\n";
builtins[6] = "get\n";
char usr_in[]="";
char cmp[]="";
while(1==1){
init_shell(argc, argv);//intial prompt for the shell
fgets(usr_in,100,stdin);
//Check for builtin Commands
int cmds_size = 7;
int j=0;
int res;
for(j; j<cmds_size; j++){
res=strcmp(usr_in, hold);
if(res==0){
printf("Execucting\n");
}
else{
printf("no command\n");
}
}
}
return(0);
}
The issue here is that you're writing the user's input to a buffer that isn't big enough to hold anything other than a null terminator.
char user_in[] = "";
The above line tells the C compiler that you need just enough space to store [ '\0' ], which is a single byte. The C compiler doesn't know that you may later write a 100-byte string to that buffer.
When you write to the buffer, the user's input overflows and will overwrite other values in your stack. Since the other values in your stack are pointers, what'll happen is you'll run into seg-faults, since you're writing character values into those bytes, but interpreting them as char pointers.
You are correctly limiting the size of the allowed input from the user to 100 characters, but you should make sure that your buffer is big enough to hold the value you're reading in:
char user_in[101];
for(int i = 0; i < sizeof(user_in) / sizeof(user_in[0]); i++) {
user_in[i] = 0; // Since this is allocated on the stack *in main*, this
// shouldn't be necessary
}
Here's one example of how you can rewrite your main method:
#include <stdio.h>
#include <string.h>
typedef enum { false, true } bool; // If you don't have this
// defined already
int main(int argc, char *argv[]) {
const char *builtins[7];
builtins[0] = "exit\n";
builtins[1] = "pid\n";
builtins[2] = "ppid\n";
builtins[3] = "cd\n";
builtins[4] = "pwd\n";
builtins[5] = "set\n";
builtins[6] = "get\n";
char user_in[101];
for(int i = 0; i < sizeof(user_in) / sizeof(user_in[0]); i++) {
user_in[i] = 0;
}
while(1) {
printf("Enter a command: ");
fgets(user_in, 100, stdin);
bool found = false;
for(int i = 0; i < sizeof(builtins) / sizeof(builtins[0]); i++) {
if (!strcmp(user_in, builtins[i])) {
printf("Found command %s", builtins[i]);
found = true;
break;
}
}
if (!found) {
printf("Didn't find command\n");
}
}
return 0;
}
Also, regarding your function init_shell: you're checking to see if argc is greater than 1, but that only guarantees that argv[1] is defined; it doesn't guarantee that argv[2] is defined. (Remember, argc is the size of the argv array, where the first element is the name of the program being executed). You want to make sure that argc is at least 3 before checking for the prompt flag in the way you are.
It may be overkill for your use-case, but consider using the getopt function for getting a custom prompt value from the user. See http://man7.org/linux/man-pages/man3/getopt.3.html for documentation regarding that method.
I am trying to write a small function to trim left spaces from a string, but I cannot get it right. In this version, I get the following error:
bus error: 10
Could anyone please explain to me what I am doing wrong? I am not looking so much for an alternative piece of code, but would like to understand the errors in my code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
void trim_string(char *);
int main(int argc, char *argv[]) {
char *temp = " I struggle with strings in C.\n";
trim_string(temp);
printf("%s", temp);
return 0;
}
void trim_string(char *string) {
char *string_trimmed = "";
int i=0, j=0;
while (isblank(string[i])) {
i++;
}
while (string[i] != '\0') {
string_trimmed[j] = string[i];
i++;
j++;
}
string_trimmed[j] = '\0';
strcpy(string, string_trimmed);
}
I have now found a workaround solution, shown below. But I am still not very clear about what I did wrong in the first place:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#define MAX_LENGTH 100
void trim_string(char [MAX_LENGTH]);
int main(int argc, char *argv[]) {
char temp[MAX_LENGTH] = " I struggle with strings in C.\n";
trim_string(temp);
printf("%s", temp);
return 0;
}
void trim_string(char string[MAX_LENGTH]) {
char string_trimmed[MAX_LENGTH];
int i=0, j=0;
while (isblank(string[i])) {
i++;
}
while (string[i] != '\0') {
string_trimmed[j] = string[i];
i++;
j++;
}
string_trimmed[j] = '\0';
printf("c\n");
strcpy(string, string_trimmed);
}
Both string and string_trimmed point to string literals, here in main:
char *temp = " I struggle with strings in C.\n";
^
|
This is a string literal
temp points to a string literal and the standard says you are not allowed to modify them.
In the function trim_string you are modifying a them which is undefined behavior of which a bus error is one possible result, although anything can happen.
string_trimmed either needs to be an array like this:
char string_trimmed[n] ;
where n is the size of your input using strlen(string) would probably make sense or dynamically allocated via malloc which you would need to free at the end of your function. The same things goes for your input from main, this would work as a substitute:
char temp[] = " I struggle with strings in C.\n";
For completeness sake, the draft C99 standard section 6.4.5 String literals paragraph 6 says (emphasis mine):
It is unspecified whether these arrays are distinct provided their elements have the
appropriate values. If the program attempts to modify such an array, the behavior is
undefined.
First, my objective with this code: take in a sentence into a C string. Iterate through the sentence and see how many instances of a particular letter occur.
This code is working somewhat but not giving the right number? Not sure why:
#include <stdio.h>
#include <string.h>
int tracker=0;
int letterCount (char *sentence)
{
int s=strlen(sentence);
int i=0;
for (i=0; i<s; i++){
if (sentence[i]=='h') {
tracker++;
}
}
return tracker;
}
int main(int argc, const char * argv[])
{
char *string="Hi there, what's going on? How's it going?";
letterCount(string);
printf("this sentensce has %i H's", tracker);
return 0;
}
The output I'm getting:
this sentensce has 2 H's
Not quite right. Any ideas?
This is the correct code if you mean case insensitive H:
#include <stdio.h>
#include <string.h>
int tracker=0;
int letterCount (char *sentence)
{
int s=strlen(sentence);
int i=0;
for (i=0; i<s; i++){
if (sentence[i]=='h' || sentence[i]=='H') { //'h' is not the same as 'H'
tracker++;
}
}
return tracker;
}
int main(int argc, const char * argv[])
{
char *string="Hi there, what's going on? How's it going?";
letterCount(string);
printf("this sentensce has %i H's", tracker);
return 0;
}
You have just mispelled small and the capital letter in your code.
Remember, the C language is case sensitive!
Although your label talks about the number of Hs, your letterCount looks for hs instead -- and it looks to me like the input you've provided does have two instances of lower-case h, just as it says.
If you want to count them together, you might consider filtering each input with tolower or toupper before checking what you have.
That number looks correct to me: you have 2 'h' characters in that sentence. If you want to count the 'H' characters as well, then you need a separate check.
size_t letterCount(const char* sentence, char c)
{
size_t count = 0;
while(sentence)
{
count += (*sentence == c);
++sentence;
}
return count;
}
What do we see here?
You can't have negative count, so use an unsigned type like size_t
sentence shouldn't be modified, so it should be const
pass in the char you want to match
sentence is a pointer, if it is null you are done. Don't need to call strlen.
sentence is a pointer, the actual pointer is pass by value, so you can modify it (see the increment, no need to make an extra variable)
boolean operators return 1 or 0, so no need to use the if. (Although, I haven't looked at the assembly to see if an if branch or an add 0 is cheaper. YMMV)
i have the following problems in C programming.
I have an array of strings stored as words[10][50]. I want to extract each of the string from the array and then pass it on to another function. I tried on the following:
#include "stdafx.h"
#include "stdio.h"
#include "conio.h"
#include "stdlib.h"
int Check_Anagram(char*,char*);
void main()
{
char words[10][20];
int i;
int flag;
for(i=0;i<3;i++)
{
scanf("%s\n",words[i][20]);
}
for(i=1;i<10;i++)
{
flag = Check_Anagram(words[i][20],words[i-1][20]);
}
getch();
}
int Check_Anagram(char *a,char *b)
{
printf("%s %s\n",a,b);
return 1;
}
This creates an exception during compiling.
Now i think that when i use the "printf" statement then this nomenclature works fine i.i words[i] prints the string "i" from the double dimension words array. When i try to do the same thing with the check function then the error occurs.
Can soemone point me how to do this passing ?
P.S. Please ignore any error in efficiency of program and likewise. I need your help and this is just a test program at learning string passing to a function
Thanks
You're passing words[i][20]. You need to pass words[i] instead in both loops. Try this:
for(i = 1; i < 3; i++) /* i < 3 */
{
flag = Check_Anagram(words[i], words[i-1]);
}
Another problem is that you're reading 3 strings and trying to print 10. So when you pass words[3] it contains garbage: printf tries to print garbage which need not be 0-terminated.
In the first for loop, when i is 0, you're pointing to words[-1], that's your exception.
flag = Check_Anagram(words[i][20],words[i-1][20]);
You are passing the 21st letter of each word the Check_Anagram. Instead you should pass the words themselves:
flag = Check_Anagram(words[i],words[i-1]);
You have a similar problem where you use scanf. To read a line from the console to each word you would use:
for(i=0;i<10;i++)
{
scanf("%s\n",words[i]);
}
#include "stdafx.h"
#include "stdio.h"
#include "conio.h"
#include "stdlib.h"
int Check_Anagram(char [],char []);
void main()
{
char words[10][20];
int i;
int flag;
for(i=0;i<3;i++)
{
scanf("%s\n",words[i]);
}
for(i=1;i<10;i++)
{
flag = Check_Anagram(words[i],words[i-1]);
}
getch();
}
int Check_Anagram(char a[],char b[])
{
printf("%s %s\n",a,b);
return 1;
}
I finally got it corrected thanks to the help of all users.
I have posted the corrected code for people who are struggling with passing of string extracted from an array of strings to another function. hope it helps.