Parse a String in C

Question

Using just C

I would like to parse a string and

a) count the occurrences of a character in a string

(so i.e. count all the e's in a passed in string)

b) Once counted (or even as I am counting) replace the e's with 3's

Thanks.

Answer 1

OK, you're either lazy, or stuck, assuming stuck.

You need a function with a signature something like

int ReplaceCharInString(char* string, char charToFind, char charThatReplaces)
{

}

Inside the function you need

1. To declare an integer to count the occurrences
2. A loop that moves from the start of the string to it's end
3. inside the loop, an if statement to check is the current char the charToFind,
4. statements to increment the count of occurrences and perform the replacement
5. After the loop, you need to return the count of occurrences

Answer 2

Here's a shell to get you started. Ask here if you need any help.

#include <string.h>
#include <stdio.h>

int main(){
    const char* string = "hello world";
    char buffer[256];
    int e_count = 0;
    char* walker;

    // Copy the string into a workable buffer
    strcpy(buffer,string);

    // Do the operations
    for(walker=buffer;*walker;++walker){
     // Use *walker to read and write the current character
    }

    // Print it out
    printf("String was %s\nNew string is %s\nThere were %d e's\n",string,buffer,e_count);
}

Answer 3

This function will take a string, replace every 'e' with '3', and return the number of times it performed the substitution. It's safe, it's clean, it's fast.

int e_to_three(char *s)
{
    char *p;
    int count = 0;
    for (p = s; *p; ++p) {
        if (*p == 'e') {
            *p = '3';
            count++;
        }
    }
    return count;
}

Answer 4

In general, it's better use a standard library function rather than rolling your own. And, as it just so happens, there is a standard library function that searches a string for a character and returns a pointer to it. (It deals with a string, so look among the functions that have the prefix "str") (The library function will almost certainly be optimized to use specialized CPU opcodes for the task, that hand written code would not)

1. Set a temp pointer (say "ptr") to the start of the string.

2. In a loop, call the function above using ptr as the parameter, and setting it to the return value.

3. Increment a counter.

4. Set the character at the pointer to "3" break when 'e' is not found.

Answer 5

Some of you guys are starting in the middle.

A better start would be

char *string = "hello world";
Assert(ReplaceCharInString(string, 'e', '3') == 1);
Assert(strcmp(string, "h3llo world") == 0);