使用sprintf，C填写字符串

Question

I'm trying to create an array of patterns for a triangle that I'm also printing to the console. I do this by creating a 2d char array where char patterns [number_of_patterns][pattern_lengths]. I pass this to a function that takes the array patterns along with the height of the triangle I'm trying to make.

void printTriangle (int rows, char rowPatterns[][rows]) {

    int initialSpaces = rows - 1;
    int numberOfAsterisks = 1;
    int i;

    for (i = 0; i < rows; i++) {
        char temp[rows];
        int spaceCounter = 0;
        int asteriskCounter = 0;

        while (spaceCounter < initialSpaces) {
            printf(" ");
            sprintf(temp, " ");
            spaceCounter++;
        }
        while (asteriskCounter < numberOfAsterisks) {
            sprintf(temp, "*");
            printf("*");
            asteriskCounter++;
        }
        while (spaceCounter < initialSpaces) {
            spaceCounter = 0;
            sprintf(temp, " ");
            spaceCounter++;
        }


        strcpy(rowPatterns[i], temp);
        printf("\n");
        initialSpaces--;
        numberOfAsterisks+=2;
    }

}

For every row of the triangle that I'm printing, I create a string for that row called temp. At the end of the for loop that prints the row to the console and sprintf's it to the array temp, I strcpy temp into patterns[i]. Then I go back to the top of the loop, reinitialize temp to make it fresh, and loop again until I have all my rows. Except for some reason sprint won't fill in my array temp. Is this incorrect use of the function, or does it have to do w my parameter passing?

Answer 1

sprintf always writes to the start of the string. To append, you can maintain a pointer to the end of the string:

char *ptr = rowpatterns[i];

ptr += sprintf(ptr, "*");

You might also hear the suggestion to use strcat - avoid that function. When building strings, repeated strcat is very slow and is a common source of performance issues in string code.