--- title: "String; replace diagonal by minimum" subject: "Computer Programming" chapter: 7 key: "semester_1/computer_programming:string-min-matrix-diagonal" source: https://www.bctengineer.com/q/semester_1/computer_programming/string-min-matrix-diagonal site: BCT Engineering --- # String; replace diagonal by minimum Computer Programming · Chapter 7: Arrays and Strings - Asked in: 2072 Chaitra, 2+8 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_1/computer_programming/string-min-matrix-diagonal - Whole chapter: https://www.bctengineer.com/md/semester_1/computer_programming/chapter-7 ## Question What is a string? Write a program to read a 3*3 square matrix, find minimum integer value of a matrix, replace the diagonal elements by the minimum element and display it using pointer. ## Short answer A string is a character array terminated by '\0'. The program reads 9 integers into a 3x3 matrix using a pointer, finds the minimum value via pointer arithmetic, then sets all diagonal elements (positions where i=j) to that minimum, and prints the modified matrix. ## Answer #### String A string is a sequence of characters stored in a character array and ended by the null character `'\0'`. It is declared as `char name[20];` and can be initialised as `char name[] = "Nepal";` (6 bytes including `'\0'`). Strings are read with `scanf("%s")`, `gets()` or `fgets()`, printed with `%s`, and handled with `` functions such as `strlen`, `strcpy`, `strcmp` and `strcat`. #### Program A pointer `p` is set to the first element of the matrix. A 2D array is stored row by row, so element a[i][j] is `*(p + i*3 + j)`. The program finds the minimum with the pointer, puts it on the diagonal positions (i = j), and prints the matrix using the pointer. ```c #include int main(void) { int a[3][3], *p = &a[0][0], min; printf("Enter 9 integers: "); for (int i = 0; i < 9; i++) scanf("%d", p + i); /* p + i is the address of element i */ min = *p; for (int i = 1; i < 9; i++) if (*(p + i) < min) min = *(p + i); for (int i = 0; i < 3; i++) *(p + i * 3 + i) = min; /* a[i][i] = min */ printf("Minimum = %d\nMatrix after replacing the diagonal:\n", min); for (int i = 0; i < 3; i++) { for (int j = 0; j < 3; j++) printf("%4d", *(p + i * 3 + j)); printf("\n"); } return 0; } ``` **Sample run** (matrix 7 2 9 / 4 8 6 / 5 3 1) Input: `7 2 9 4 8 6 5 3 1` ```text Enter 9 integers: Minimum = 1 Matrix after replacing the diagonal: 1 2 9 4 1 6 5 3 1 ``` ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is a string in C? A. A character array ended by the null character '\0' B. An int array C. A group ended by a newline D. A char with fixed length 1 Answer: A. The null character marks the end. (Key point: String definition) 2. With p = &a[0][0], which expression is a[i][j]? A. *(p + i + j) B. *(p + j*3 + i) always C. *(p + i*3 + j) D. *p + i*3 + j Answer: C. A 2D array is stored row by row with 3 columns. (Key point: Pointer access) 3. Which elements are replaced by the minimum? A. a[i][i], at offsets i*3 + i B. a[i][2-i] C. The first row D. Every element Answer: A. The diagonal has i = j. (Key point: Diagonal) 4. For 7 2 9 / 4 8 6 / 5 3 1, what are the new diagonal elements? A. 2, 2, 2 B. 7, 8, 1 C. 1, 1, 1 D. 9, 9, 9 Answer: C. The minimum is 1. 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