I am trying to code a C program to start specific functions on the OS X El Capitan.
The code looks like this:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main()
{
char mainchoice;
printf(">>> ");
scanf("%s", &mainchoice);
if (strcmp(&mainchoice, "start ftp") == 0) {
system("ftp");
}
else if (strcmp(&mainchoice, "start say") == 0) {
system("say hello");
}
else {
system("say Error")
}
}
This is just a sample code.
When I run it, it always says error via the say command. What am I doing wrong?
Focus here:-
char mainchoice; //declared as a char
scanf("%s", &mainchoice); //using the %s placeholder which is for string
//for character it is %c
Getting the logic behind your code is you want to enter a String not a character.
Make an array of characters like this:-
char mainchoice[20]; //this can hold your string, one character at one index each of the array
Since, your are using multi word in string comparison("start say")
(strcmp(&mainchoice, "start say") == 0)
scanf does not work for multi words. scanf stops reading from the keyboard as soon as you provide a whitespace, tabs, newline.
For solving that problem, use fgets. It's the best way to read multi words or even whole sentences. Never use gets()! It is vulnerable to buffer overflow!
fgets(mainchoice, 20, stdin);
you are declaring mainchoice as a character rather then a string.
use char mainchoice[10]; to create a string.
and replace scanf("%s",&mainchoice) with fgets(mainchoice, 10, stdin);
size is 10 because you are comparing it with a string of length 9, so(9+1 null=10) 10 are enough.
Related
The Question is to take three names as input and check whether the 4th input is the first letter of one of those three names.
*It needs to be only of alpha type and no other.
Sample Input:
#ON SNOW
ARYA STARK
HODOR
#
Output:
NO
My code:
#include <stdio.h>
#include <ctype.h>
int main(){
char s1[100],s2[100],s3[100];
char ch[1];
scanf("%[^\n]s",s1);
scanf("%[^\n]s",s2);
scanf("%[^\n]s",s3);
scanf("%s",ch);
if(isalpha(ch[0]) && (s1[0]==ch[0] || s2[0]==ch[0] || s3[0]==ch[0]))
printf("yes");
else
printf("no");
}
As I figured out that gets() no longer works, I tried scanf("%s",s1);.
But since it does not store the calude after the whitespace " ", I tried this scanf("%[^\n]s",s1);. But the scanning stops at the first name itself.
John Player
NO
What is the way to store multiple strings in a char array without using loops and only using branching??
Now before getting to the specified problem lets see what the actual problem is:
Problem with scanf()
The problem with scanf() is that it is really bad at managing overflows. And in case of chars or char sequences the newline character is read on the press of enter key in the subsequent scanning. There are numerous instances on SO that concern this problem. scanf() leaves the new line char in buffer? and Problems with scanf
Problem with gets()
The biggest drawback is that here you need to know the size of input before-hand. If you know your input extremely well, you may use it (Still I won't recommend). Why is the gets function so dangerous that it should not be used?
Problem with fgets()
There are two really common problems with fgets()
The syntax of fgets() is:
char *fgets(char *restrict s, int n, FILE *restrict stream);`
and generally used like
fgets(char_array,100,stdin)
First problem arises when the input is bigger than the integer n provided in the fgets() second parameter. When the input in the buffer is bigger than n it will strip off 1st n chars and allocate it the char pointer which might be a char array. But what about remaining chars? They still are there in the input buffer and will be allocated to next fgets(). Messing things up.
Second problem is that every time a new line feed is allocated to the end of char sequence when the input is smaller than the int n-1.
But if we think for a while the problems with fgets() can be tackled with a simple trick.
Just check for the last character in the char sequence which has been recently allocated. If it is new line, replace it by NULL. Else we know that the input was more than that int provided inside fgets(). So all we have to do is eat up the remaining chars in the input buffer.
Here is an example:
char str1[5];
char str2[5];
fgets(str1,5,stdin);
if(strlen(str1)>0){//to avoid Undefined Behavior in case of null byte input
if(str1[strlen(str1)-1]=='\n'){
str1[strlen(str1)-1]='\0';
}
else{
while((getchar())!='\n');//eating the remaing chars in the buffer
}
}
fgets(str2,5,stdin);
if(strlen(str2)>0){
if(str2[strlen(str2)-1]=='\n'){
str2[strlen(str2)-1]='\0';
}
else{
while((getchar())!='\n');
}
}
printf("\n1.%s\n2.%s",str1,str2);
You could even convert strings you got using fgets to float and integers using things like strol or sscanf, but beware they may not show independent behavior.
Now coming back to the solution to your problem:
#include <stdio.h>
#include <string.h>
int main(){
char s1[100],s2[100],s3[100];
char ch[2];//make ch atleast 2 char wide
fgets(s1,100,stdin);
if(strlen(s1)>0){
if(s1[strlen(s1)-1]=='\n'){
s1[strlen(s1)-1]='\0';
}
else{
while((getchar())!='\n');
}
}
fgets(s2,100,stdin);
if(strlen(s2)>0){
if(s2[strlen(s2)-1]=='\n'){
s2[strlen(s2)-1]='\0';
}
else{
while((getchar())!='\n');
}
}
fgets(s3,100,stdin);
if(strlen(s3)>0){
if(s3[strlen(s3)-1]=='\n'){
s3[strlen(s3)-1]='\0';
}
else{
while((getchar())!='\n');
}
}
fgets(ch,2,stdin);
if(strlen(ch)>0){
if(ch[strlen(ch)-1]=='\n'){
ch[strlen(ch)-1]='\0';
}
else{
while((getchar())!='\n');
}
}
if(isalpha(ch[0]) && (s1[0]==ch[0] || s2[0]==ch[0] || s3[0]==ch[0]))
printf("yes");
else
printf("no");
return 0;
}
Try fgets.
fgets(s1,100,stdin);
fgets(s2,100,stdin);
fgets(s3,100,stdin);
#include <stdio.h>
int main(){
char s1[100],s2[100],s3[100];
char ch[1];
fgets(s1,100,stdin);// like gets but limited by length
fgets(s2,100,stdin);
fgets(s3,100,stdin);
scanf("%s",ch);
if( (s1[0]>='a' && s1[0]<='z') || (s1[0]>='A' && s1[0]<='Z') ){//ch-alpha?
if(s1[0] == ch[0]){ printf("yes"); }//s1[0] == ch[0] ??
else{ printf("no"); }
}
else
printf("no"); return 0; }
I am currently learning C, and so I wanted to make a program that asks the user to input a string and to output the number of characters that were entered, the code compiles fine, when I enter just 1 character it does fine, but when I enter 2 or more characters, no matter what number of character I enter, it will always say there is just one character and crashes after that. This is my code and I can't figure out what is wrong.
int main(void)
{
int siz;
char i[] = "";
printf("Enter a string.\n");
scanf("%s", i);
siz = sizeof(i)/sizeof(char);
printf("%d", siz);
getch();
return 0;
}
I am currently learning to program, so if there is a way to do it using the same scanf() function I will appreciate that since I haven't learned how to use any other function and probably won't understand how it works.
Please, FORGET that scanf exists. The problem you are running into, whilst caused mostly by your understandable inexperience, will continue to BITE you even when you have experience - until you stop.
Here is why:
scanf will read the input, and put the result in the char buffer you provided. However, it will make no check to make sure there is enough space. If it needs more space than you provided, it will overwrite other memory locations - often with disastrous consequences.
A safer method uses fgets - this is a function that does broadly the same thing as scanf, but it will only read in as many characters as you created space for (or: as you say you created space for).
Other observation: sizeof can only evaluate the size known at compile time : the number of bytes taken by a primitive type (int, double, etc) or size of a fixed array (like int i[100];). It cannot be used to determine the size during the program (if the "size" is a thing that changes).
Your program would look like this:
#include <stdio.h>
#include <string.h>
#define BUFLEN 100 // your buffer length
int main(void) // <<< for correctness, include 'void'
{
int siz;
char i[BUFLEN]; // <<< now you have space for a 99 character string plus the '\0'
printf("Enter a string.\n");
fgets(i, BUFLEN, stdin); // read the input, copy the first BUFLEN characters to i
siz = sizeof(i)/sizeof(char); // it turns out that this will give you the answer BUFLEN
// probably not what you wanted. 'sizeof' gives size of array in
// this case, not size of string
// also not
siz = strlen(i) - 1; // strlen is a function that is declared in string.h
// it produces the string length
// subtract 1 if you don't want to count \n
printf("The string length is %d\n", siz); // don't just print the number, say what it is
// and end with a newline: \n
printf("hit <return> to exit program\n"); // tell user what to do next!
getc(stdin);
return 0;
}
I hope this helps.
update you asked the reasonable follow-up question: "how do I know the string was too long".
See this code snippet for inspiration:
#include <stdio.h>
#include <string.h>
#define N 50
int main(void) {
char a[N];
char *b;
printf("enter a string:\n");
b = fgets(a, N, stdin);
if(b == NULL) {
printf("an error occurred reading input!\n"); // can't think how this would happen...
return 0;
}
if (strlen(a) == N-1 && a[N-2] != '\n') { // used all space, didn't get to end of line
printf("string is too long!\n");
}
else {
printf("The string is %s which is %d characters long\n", a, strlen(a)-1); // all went according to plan
}
}
Remember that when you have space for N characters, the last character (at location N-1) must be a '\0' and since fgets includes the '\n' the largest string you can input is really N-2 characters long.
This line:
char i[] = "";
is equivalent to:
char i[1] = {'\0'};
The array i has only one element, the program crashes because of buffer overflow.
I suggest you using fgets() to replace scanf() like this:
#include <stdio.h>
#define MAX_LEN 1024
int main(void)
{
char line[MAX_LEN];
if (fgets(line, sizeof(line), stdin) != NULL)
printf("%zu\n", strlen(line) - 1);
return 0;
}
The length is decremented by 1 because fgets() would store the new line character at the end.
The problem is here:
char i[] = "";
You are essentially creating a char array with a size of 1 due to setting it equal to "";
Instead, use a buffer with a larger size:
char i[128]; /* You can also malloc space if you desire. */
scanf("%s", i);
See the link below to a similar question if you want to include spaces in your input string. There is also some good input there regarding scanf alternatives.
How do you allow spaces to be entered using scanf?
That's because char i[] = ""; is actually an one element array.
Strings in C are stored as the text which ends with \0 (char of value 0). You should use bigger buffer as others said, for example:
char i[100];
scanf("%s", i);
Then, when calculating length of this string you need to search for the \0 char.
int length = 0;
while (i[length] != '\0')
{
length++;
}
After running this code length contains length of the specified input.
You need to allocate space where it will put the input data. In your program, you can allocate space like:
char i[] = " ";
Which will be ok. But, using malloc is better. Check out the man pages.
The below code tries to manipulate several lines of text one at the time.
1.My first issue is to write a loop to read several lines of text(using scanf()) and quit when the first character typed is a newline. These lines of text have some conditions: The first character must be a number between 2 and 6 followed by a space and a line of text(<80).This number will make "dance" the text.
2.My second issue is to figure out how to convert the letters from small to capital and viceversa according to the first number typed. I have to function to make these conversions but I don't know how to call them to change the text.For example: if I typed "3 apples and bananas" the correct output should be "AppLes And BanNas".As you see, the white spaces are ignored and the text always start with a capital letter.
#include <stdio.h>
#include <stdlib.h>
#include <string>
#include <ctype.h>
using namespace std;
void print_upper(string s1);
void print_lower(string s2);
void main(void)
{
char text[80];
text[0]='A';//Initialization
int count_rhythm;
while (text[0] != '\n'){//To make the loop run until a newline is typed
scanf(" %79[^\n]",text);
if(isdigit(text[0])) //To verify that the first character is a number
{
printf("\nGood");//Only to test
}
else
{
printf("\nWrong text\n");//Only to test
}
}
}
void print_upper(string s1)//Print capital letters
{
int k1;
for(k1=0; s1[k1]!='\0'; ++k1)
putchar(toupper(s1[k1]));
}
void print_lower(string s2)//Print small letters
{
int k2;
for(k2=0; s2[k2]='\0'; ++k2)
putchar(tolower(s2[k2]));
}
You could also define a function printNthUpper() which would take a string and and integer n that would specify which characters to print in upper case. The of the function would be a loop similar to the functions you already have, but with a conditional that compares the provided integer value and the index of a given letter to decide whether to
call toupper() (e.g. printf("%c", i%n == 0 ? toupper(s[i]) : s[i]);).
to write a loop to read several lines of text you can use condition based infinite loop in combination with fgets rather than using scanf.
char line[80];
char result[80]
while(1)
{
fgets(line,sizeof(line),stdin); //read line with fgets
puts(line);
if(line[0]=='\n')
break;
if((strlen(line)>=4) &&'2'< =line[0] && line[0] <= '6' && line[1]==' ')
{
strcpy(result,change_case_of_nth_char(line));// call change case of nth letter
}
else
{
//prompt user to enter input again
}
}
char *change_case_of_nth_char(char *str)
{
}
Can anyone tell me why this code crashes? It's simple, if the length of the string is > than 16, ask again for a string. It works if I write control = 1 inside the if statement, but it should work the same without it, 'cause the value of control at that point is 1, am I right?
thans (I'm learning)
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int
main(void)
{
int control = 1;
char word[16] ;
printf("Enter a word: ");
while(control == 1)
{
scanf("%s", word);
int len = strlen(word);
printf("Lenght is: %d\n", len);
if (len >= 16)
{
printf("Word lenght to long, enter a new one: ");
}
else
{
control = 0;
}
}
printf("This is the word: %s\n", word );
}
char word[16] allocates 16 bytes of store for a string.
scanf() then reads a string into that store.
If you read in more than the amount of allocated store, memory is corrupted after the end of the store.
That's why you crash.
The problem is that if the user types more than the 15 characters which you have allocated space for, then the computer will merrily write all of them in memory past the end of your array. This will result in "undefined behavior" including crashing your program.
As others have noted, your fundamental problem is that you're allocating 16 characters for the string, and scanf will happily allow you to write past those 16 characters into memory that doesn't belong to you.
Be aware that C will allow you to do this with arrays generally, and understand how standard C strings work: you need to null-terminate them, meaning that you'll always need an extra space in the array for a null-terminating character \0.
There is a way to limit scanf with respect to C strings, using a field width specifier with %s, like so:
char input[17]; // room for 16 characters plus null-terminator
// here scanf will stop after reading 16 characters:
scanf("%16s", input);
With this code, you can safely use scanf to fill your string with no more than 16 characters, and scanf will null-terminate the string for you.
But as others have also noted, scanf is pretty poor at handling user input. It's usually better to use fgets and manage the input string on your own, piece-by-piece.
This question already has answers here:
How can I read an input string of unknown length?
(11 answers)
Closed 4 years ago.
I want to read in an arbitrary number of strings, one at a time, using <stdio.h> in C.
I know that you can do this with integers using:
while (scanf("%d ", &integer))
but I cannot do:
while (scanf("%s", string))
How can I implement the above?
The input is on separate lines.
You usually want to use fgets to read input as strings, especially when you want one line of input to end up as one string.
You can also use fscanf with a scanset conversion to read a line at a time, such as:
char line[100], newline;
fscanf("%99[^\n]%c", line, &newline);
Then you can check whether newline=='\n' to determine whether you've read the entire line successfully, or the line was larger than the buffer you provided.
When you're trying to read line-oriented input, you normally want to avoid "%s" (even with a specified length) though, as this reads white-space delimited tokens, not entire lines.
Use a char array:
char charArray[100];
while (scanf("%s", &charArray))
I guess your problem is to terminate the loop. scanf returns the number of successful scanned elements. In case of a string, also the empty string is successful scanned. Thus, you need another criterion, e.g.
while(scanf("%s",string) && (strlen(string)!=0))
I did not completely understand what you were trying to do from your original question. When you said you wanted to read in an arbitrary number of strings, I took that to mean, you wanted your program to be able to read 0 to n strings. Unfortunately in C, you will either have to cap off the maximum number of strings you would ever want to read in like
#define MAX_NUMBER_OF_STRINGS_TO_READ 25, or get into some sophisticated memory allocation scheme to read a string in and then add it to dynamic memory (returned from malloc).
I took the cap the maximum number of strings approach and wrote the following snippet:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
char charArray[5][25] = {0};
int main(int argc, char *argv[])
{
int in_idx = 0;
int out_idx = 0;
printf("\n\n%s\n", "Enter no more than 5 strings, no more than 25 characters long.");
while(fgets (charArray[in_idx], 25, stdin))
{
if('\n' == charArray[in_idx][0])
{
printf("%s\n", "Entry terminated with newline.");
break;
}
in_idx++;
}
for(out_idx=0; out_idx < (in_idx + 1); out_idx++)
{
printf("%s", charArray[out_idx]);
}
printf("\n%s\n", "Program ended.");
return 0;
}
I made the termination character a newline. If I only want two strings, I press Enter when I've entered the second string. I terminated fgets by looking for a '\n' in the first position of the character array.