How to convert a string value to numerical value? - c

Question

I have tried this code to separate my Str[] string into 2 string, but my problem is "I want to separate John(name) as string and 100(marks) as integer",How can I do it, any suggestion?
#include <stdio.h>
#include <string.h>
void main()
{
char Str[] = "John,100";
int i, j, xchange;
char name[50];
char marks[10];
j = 0; xchange = 0;
for(i=0; Str[i]!='\0'; i++){
if(Str[i]!=',' && xchange!=-1){
name[i] = Str[i];
}else{
xchange = -1;
}
if(xchange==-1){
marks[j++] = Str[i+1];
}
}
printf("Student name is %s\n", name);
printf("Student marks is %s", marks);
}

Answer 1

How to separate "John,100" into 2 strings?
There are three common approaches:
Use strtok() to split the string into individual tokens. This will modify the original string, but is quite simple to implement:
int main(void)
{
char line[] = "John,100;passed";
char *name, *score, *status;
/* Detach the initial part of the line,
up to the first comma, and set name
to point to that part. */
name = strtok(line, ",");
/* Detach the next part of the line,
up to the next comma or semicolon,
setting score to point to that part. */
score = strtok(NULL, ",;");
/* Detach the final part of the line,
setting status to point to it. */
status = strtok(NULL, "");
Note that if you change char line[] = "John,100"; then status will be NULL, but the code is otherwise safe to run.
So, in practice, if you required all three fields to exist in line, it would be sufficient to ensure the last one was not NULL:
if (!status) {
fprintf(stderr, "line[] did not have three fields!\n");
return EXIT_FAILURE;
}
Use sscanf() to convert the string. For example,
char line[] = "John,100";
char name[20];
int score;
if (sscanf(line, "%19[^,],%d", name, &score) != 2) {
fprintf(stderr, "Cannot parse line[] correctly.\n");
return EXIT_FAILURE;
}
Here, the 19 refers to the number of chars in name (one is always reserved for the end-of-string nul char, '\0'), and [^,] is a string conversion, consuming everything except a comma. %d converts an int. The return value is the number of successful conversions.
This approach does not modify the original string, and it allows you to try a number of different parsing patterns; as long as you try them the most complex one first, you can allow multiple input formats with very little added code. I do this regularly when taking 2D or 3D vectors as inputs.
The downside is that sscanf() (all functions in the scanf family) ignores overflow. For example, on 32-bit architectures, the largest int is 2147483647, but scanf functions will happily convert e.g. 9999999999 to 1410065407 (or some other value!) without returning an error. You can only assume the numerical inputs are sane and within the limits; you cannot verify.
Use helper functions to tokenise and/or parse the string.
Typically, the helper functions are something like
char *parse_string(char *source, char **to);
char *parse_long(char *source, long *to);
where source is a pointer to the next character in the string to be parsed, and to is a pointer to where the parsed value will be stored; or
char *next_string(char **source);
long next_long(char **source);
where source is a pointer to a pointer to the next character in the string to be parsed, and the return value is the value of the extracted token.
These tend to be longer than above, and if written by me, then quite paranoid about the inputs they accept. (I want my programs to complain if their input cannot be reliably parsed, rather than silently produce garbage.)
If the data is some variant of CSV (comma-separated values) read from a file, then the proper approach is a different one: instead of reading the file line by line, you read the file token by token.
The only "trick" is to remember the separator character that ended the token (you can use ungetc() for this), and use a different function to (read and ignore the rest of the tokens in the current record, and) consume the newline separator.