Parsing Integer to String C

Question

How does one parse an integer to string(char* || char[]) in C? Is there an equivalent to the Integer.parseInt(String) method from Java in C?

Answer 1

The Java parseInt() function parses a string to return an integer. An equivalent C function is atoi(). However, this doesn't seem to match the first part of your question. Do you want to convert from an integer to a string, or from a string to an integer?

Answer 2

This is discussed in Steve Summit's C FAQs.

Answer 3

This is not an optimal solution. This is my solution given multiple restrictions, so if you had limited resources based on your course/instructor's guidelines, this may be a good fit for you.

Also note that this is a fraction of my own project implement, and I had to read in operands as well as digits, so I used getchars. Otherwise, if you only need integers and no other type of characters, I like using this:

int k;
while (scanf("%d", &k) == 1)

The rules were no specific, "advanced" C concepts: no String variables, no structs, no pointers, no methods not covered, and the only include we were allowed was #include

So with no simple method calls like atoi() available or any String variables to use, I chose to just brute force it.

1: read chars in using getchar (fgets was banned). return 1 (exit status 1) if there is an invalid character. For your problem based of parseInt in Java 1 11 13 is valid but 1 11 1a is invalid, so for all values we have a valid "string" iff all chars are 0-9 ignoring whitespace.

2: convert the ASCII value of a char to its integer value (eg. 48 => 0)

3: use a variable val to store an int such that for each char in a "substring" there is a corresponding integer digit. i.e. "1234" => 1234 append this to an int array and set val to 0 for reuse.

The following code demonstrates this algorithm:

int main() {
    int i, c;
    int size = 0;
    int arr[10]; //max number of ints yours may differ
    int val = 0;
    int chars[1200]; //arbitrary size to fit largest "String"
    int len = 0;

    //Part 1: read in valid integer chars from stdin
    while((c = getchar()) != EOF && (c < 58 && c > 47)) {
        chars[len] = c;
        len++;
     }

    //Part 2: Convert atoi manually. concat digits of multi digit integers and  
    //        append to an int[]
    for(i = 0; i < len; i++){
    for(i = 0; i < len; i++){
        if(chars[i] > 47 && chars[i] < 58){
        while((chars[i] > 47 && chars[i] < 58)){
            if(chars[i] == 48)
                c = 0;
            if(chars[i] == 49)
                c = 1;
            if(chars[i] == 50)
                c = 2;
            if(chars[i] == 51)
                c = 3;
            if(chars[i] == 52)
                c = 4;
            if(chars[i] == 53)
                c = 5;
            if(chars[i] == 54)
                c = 6;
            if(chars[i] == 55)
                c = 7;
            if(chars[i] ==56)
                c = 8;
            if(chars[i] == 57)
                 c = 9;
            val = val*10 + c;
            i++;
        }
        arr[size] = val;
        size++;
        if(size > 10) //we have a check to ensure size stays in bounds
           return 1;
        val = 0;
        }
        //Print: We'll make it a clean, organized "toString" representation
        printf("[");
        for(i = 0; i < size-1; i++){
            printf("%d, ", arr[i]); 
        }
        printf("%d]", arr[i];

        return 0;

Again, this is the brute force method, but in cases like mine where you can't use the method concepts people use professionally or various C99 implements, this may be what you are looking for.